AQA A2 Physics Unit 4 definitions

Posted by Alex on 02:34 comments (0)

  • Newton's Second Law – The rate of change of momentum is directly proportional to the resultant force upon it
  • Newton – The unit of force where one kilogram will be accelerated one metre per second
  • Conservation of momentum – In a collision, in the absence of external forces the total momentum remains constant
  • Impulse – change of momentum
  • Elastic collision – no momentum or KE is lost
  • Inelastic collision – momentum is conserved but KE is lost
  • Totally inelastic collision – momentum is conserved but KE is gained
  • Cantripetal force – the force, acting towards the centre of the circle and perpendicular to the motion of an object, responsible for the circular motion of that object
  • Simple harmonic motion – Motion where the acceleration of an object is proportional to its displacement from a central point and directed towards the mid point
  • Damping – loss of energy from an oscillating system leading to a reduction of amplitude of oscillation
  • Resonance – the situation which occurs when the driving force of a forced oscillation has the same frequency as the natural frequency of the material
  • Gravitational Field Strength – the force acting on a 1kg unit mass at any point in a gravitational field
  • Newton's Law of Gravitation – the force of attraction between two masses is directly proportional to the product of their masses and inversely proportional to the square of the distance between them
  • Gravitational potential – The work done against the field in moving a unit mass from infinity to its current point
  • Electrical field strength – the force acting on a +1C charge at a point in an electric field
  • Electric potential – the work done against the field in moving a +1C charge from infinity to its current point
  • Capacitance – the amount of charge stored by a capacitor per volt PD across the capacitor
  • Magnetic field strength – the force on 1m of wire carrying a current of 1A at right angles to the magnetic field
  • Flux density – amount of flux (unit Wb) in an area (unit m^2) (total unit Wb m^-2 = T)
  • Flux linkage – amount of flux in an entire coil
  • Faraday's law – the emf produced by movement of a coil is directly proportional to the rate of change of flux passing through the coil
  • Lenz's law – the induced emf produced by movement of a coil always acts in the direction that opposes the force which made it

Edexcel A2 Chemistry Unit 4

Posted by Alex on 06:54 comments (0)

Rates of Reactions

  • Rate = k[A][B]
  • Observed by measuring the change in some physical quantity over time
    • Eg. Charge, colour, pH
  • Orders of rates depend on the relative effect each reagent's concentration has on the overall rate
  • Main example is propanone and iodine
    • First order in respect to propanone
    • First order in respect to H+
    • Zero order in respect to iodine
      • Unexpected because iodine present in the reaction but not in rate equation
      • Zero order means the reagent is involved in a fast step of the reaction which is not rate determining
  • By looking at the rate equation, we can see what reagents are in the rate determining step, as all reagents involved in the rate equation must be in the rate determining step of the reaction

Halogenoalkanes

  • Reaction by nucleophillic substitution to form alcohols
  • Reagent is NaOH
  • Heated under reflux
  • SN1
    • Tertiary halogenoalkanes
    • Halogen attracts electrons from the C-X bond, therefore breaking the bond
    • Forms carbocation plus halogen ion
    • Only halogenoalkane in rate determining step
    • OH group attracted to C+ so alcohol forms
  • SN2
    • Primary halogenoalkanes
    • OH group attracted to slight positive charge on central carbon due to polarity of C-X bond
    • 5 co-ordinate transition state
    • OH group pushes halogen ion away

Activation energy/catalysts

  • Activation energy is the energy needed for a reaction to occur
  • Higher activation energy means there is a lower rate as less particles have enough energy
  • Catalysts decrease activation energy required and so increase the rate
  • Homogeneous catalysts are in the same phase as the reactants
  • Heterogeneous catalysts are in a different phase to the reactants
    • Work by allowing the reactants to adsorb to the surface of the catalyst
    • Bonding with the catalyst allows the reactants' bonds to be broken more easily so the reaction happens faster
    • Also concentrates the reactant in a small area so there is more chance of collision
    • The part of the reactant molecule which will react will be properly oriented for the reaction to occur
    • The product molecules then desorb

Entropy

  • Measure of how much disorder there is in a substance
  • 0 at 0K in a perfect solid crystal
  • Energetically more stable the greater the disorder is hence why gases diffuse out
  • If total entropy is positive a reaction is spontaneous and therefore feasible, otherwise no reaction
  • Total entropy = RlnK
    • R = gas constant
    • lnK = natural log of equilibrium constant
      • This is why equilibrium constants change with temperature
  • Thermodynamically stable when entropy is less than 0 for a reaction
  • Kinetically inert when entropy is positive but temperature does not give a fast rate due to a high activation energy
  • Lattice enthalpies
    • Not sure why they're in this section but:
    • Greater charge/smaller ionic radius = larger charge density
    • Larger charge density = stronger force of attraction between ions in the lattice
    • Stronger bonds = more exothermic (making bonds is exothermic)
    • More exothermic = more negative lattice enthalpy

Equilibria

  • Dynamic equilibrium is when the rate of the forward and reverse reaction are equal
  • At a certain temperature, one particular ratio of products to reactants will always be reached
    • Therefore products/reactants = constant
      • This constant is the equilibrium constant
        • It doesn't change unless you change the temperature
      • Changing the concentration of products/reactants by removing/adding substances will cause more or less of the products and reactants to be products to maintain the equilibrium at the same point
  • Acid base equilibria
    • Strong acids dissociate fully
    • Weak acids will not fully lose all of their protons
      • This is an equilibrium between protonated and deprotonated forms
      • The constant is Ka
    • Water is a special case
      • As the concentration of water is so high in any calculation, we take it as 1
      • Therefore Ka for water = [H+] [OH-]
        • This has the special name Kw
        • Value 1*10^14 mol^2 dm^-6
      • This can be used to calculate the pH of alkaline solutions
    • pH = -log[H+]
    • In most calculations [H+] = [OH-]
      • So write it as H+ squared
    • pH curves are drawn by sketching a graph of pH as you add an acid/alkali to a volume of acid/alkali
      • Mark on the known points eg. Equivalence points, pH with no alkali etc.
      • Sketch the rest according to whether it's a strong base/weak base strong acid/weak acid
      • At half the equivalence point, pKa = pH
    • Indicators should be picked so that they have their pKa in the middle of the steep region of the pH curve
    • Buffers
      • Resist changes in pH
      • Made from weak acids/bases and their salts
      • Some pH change still happens but mostly the H+ concentration is kept constant as the buffer salt/bases will react

Organic

  • Isomerism
    • Optical isomerism
      • Caused by carbons with 4 different groups attached
      • Creates two possible shapes which are non-superimposable mirror images of each other
      • Distinguished by testing to see if they will rotate the plane of plane polarised light left or right
    • Geometric isomerism
      • E/Z or Cis/trans
        • Z for zimilar
      • Caused by the restricted rotation of groups around a C=C double bond
        • Or in a cyclic group
  • Aldehydes and ketones
    • Lower boiling point than alcohols and limited solubility
    • This is because while the carbonyl oxygen can form hydrogen bonds using its lone pair, it has no hydrogens that are polarised enough to form bonds
    • In alcohols the hydrogen in the hydroxyl group can be used to form hydrogen bonds as well as the oxygen using its lone pair
  • Nucleophillic addition using HCN
    • HCN and NaOH or KCN and sulphuric acid
    • Carbon in CN has a lone pair and attacks the positive side of the polar C=O bond
    • Electrons from bond given to carbonyl oxygen
    • Carbonyl oxygen then give electrons to H-CN to regenerate CN- nucleophile
  • Chemical tests
    • Brady's/2,4-DNPH – aldehyde or ketone
    • Tollen's reagent/Benedict's reagents – aldehyde
    • Iodine + NaOH – Iodoform, methyl ketone
    • Oxidation – Potassium dichromate (VI)
      • Orange to green
      • Manganate (VII) can also be used (purple to colourless)
    • Calcium carbonate – carboxylic acid
  • Reduction
    • Lithium aluminium hydride in dry ether
    • Carboxylic acid to alcohol/aldehyde, ketone to alcohol, aldehyde to alcohol
  • Hydrolysis of nitriles
    • Nitrile refluxed with HCl
    • Forms carboxylic acid
  • Formation of esters
    • Carboxylic acid + Alcohol
    • Reflux under dilute acidic conditions
    • Distil the ester and neutralise
    • Alternatively form by reacting acyl chloride with alcohol to give a greater yield as it goes to completion
  • Acyl chlorides
    • Formed by reacting carboxylic acids with PCl5
    • React with water to form carboxylic acid
    • React with alcohol to form ester
    • React with amines/ammonia to form amides (N substituted)
  • Reactions of esters
    • Acid hydrolysis – reverse of making
    • Base hydrolysis – reflux with dilute NaOH to form a salt and alcohol
      • Used to break down triglycerides to glycerol + soaps
      • Soponification is this
    • Trans-esterification
      • Ester + Alcohol -> New ester + alcohol (refluxed, acidic)

Spectroscopy

  • Mass spec
  • NMR
    • Low-res just measures number of hydrogens in each environment
    • High-res measures spin-spin coupling so follow n+1 rule to determine how many sub peaks each peak will be split into
  • IR
    • Bonds absortb IR radiation and so dips in the spectrum can be seen
    • Known fingerprint region can be used to positively identify a chemical else the bonds in the functional groups will be used to identify functional groups
  • Best to use data from multiple sources to confirm information
  • Chromotography
    • Change in retention time/movement of a substance depending on the force of attraction between it and the stationary phase
    • Gas chromatography
      • Stationary phase is a tube of oil
      • Vaporised sample is pumped through tube, depending on force between the sample and oil the retention time will be longer or shorter
      • Compare to a library of known retention times to identify sample
    • High performance liquid chromatography
      • Stationary phase is a tube of silica pellets
      • Tube not heated, pressure used to force sample through
      • Can be used if heat sensitive sample

AQA A2 Physics Unit 4

Posted by Alex on 09:07 comments (0)

Further Mechanics

  • Momentum
    • Momentum is always conserved in any collision
    • Kinetic energy, however, can be transferred into/from other forms in collisions
      • Elastic collisions are ones where KE is conserved
      • Inelastic collisions are ones where KE is not conserved
      • Totally inelastic collisions are ones where the two object stick together
    • Impulse is the change in momentum
      • Force * Time
    • Unit is Ns (newton-seconds)
  • Circular motion
    • Linear velocity is the usual distance/time
      • This means it's the proportion of the circumference of the circle moved through in a second
    • Angular velocity is the angle moved/time
      • Angle in radians so 2*pi radians in a circle
    • Centripetal force is a constant force which acts perpendicular to the movement of the object towards the centre of the circle
  • Simple harmonic motion
    • Motion where the acceleration of the object is proportional to its displacement from the equilibrium position, and that acceleration always acts towards the equilibrium position
    • Amplitude = maximum displacement from equilibrium position
    • In the formulas for SHM, T = 0 when x = max and then increments from there
    • KE is max when displacement = 0
    • Potential energy is max when displacement = max
    • Damping
      • Means the amplitude decreases as KE is lost
      • Time period of oscillations does not change
      • Critical damping is the amount of damping which causes the system to return to equilibrium quickest
      • Overdamping will lead to it taking longer to return to equilibrium
    • Natural frequency and resonance
      • When objects are allowed to oscillate of their own accord they will at the natural frequency
      • When objects are forced to oscillate, any damping felt will decrease around the natural frequency
        • This means that the amplitude increases closer to the natural frequency

Gravitation

  • Newton's law of gravitation
    • The force due to gravity felt by two masses is proportional to the sizes of the two masses and inversely proportional to the square of the distance between them
  • Field strength = force felt by a test mass
  • On diagrams the arrows point in the direction the test mass would go
  • Potential
    • Work done against the field to bring an object from infinity to that point
    • As the field is doing the work, negative value
    • Gradient of a potential/distance graph is field strength
  • Orbits
    • Gravity acting as centripetal force
    • Circular orbits have constant speed and distance from earth
    • Elliptical orbits have higher speeds as they are closer to earth, and slower speeds further away, meaning KE + GPE remains constant
    • Geosynchronous orbits take 24 hours

Electric Fields

  • Coulomb's law
    • The force between point charges is proportional to the size of the charges and inversely proportional to the square of the distance between them
  • Field strength = force felt by +1C charge
  • On diagrams the arrows show the direction a positive charge would move
  • Parallel plates of opposite charge will have straight field lines, creating an area of uniform field strength
  • Potential
    • Work done against the field moving a charge from infinity to that point
    • For positive charges this is positive, as work must be done as positive charges will be repelled
    • Negative charges this is negative as the field will be doing the work
  • Charged particles moving through electric fields will have a parabolic path
  • Capacitors
    • Capacitors charge quickly at first and then more slowly
    • This is all given by the equation involving natural logs
    • T = RC (time taken for charge to drop to 37% or rise to 63%)

Magnetic fields

  • Force = Flux Density * Current * Length
  • Flux density is the measure of how many lines of flux there are in an area
  • This all depends on the force, current and magnetic field being at right angles according to the left hand rule
  • Flux and flux linkage
    • Flux is measured in Wb
    • Flux linkage is the total flux in an entire coil, so number of turns * flux
    • When the coil is not perpendicular to the field, multiply the flux/flux linkage by cos(angle) where the angle is drawn from a normal perpendicular to the side of the coil compared to the direction of the magnetic field
      • Basically, draw it so the angle is 0 when the coil is perpendicular to the field and you know it's right because cos(0) = 1 and so that's when the flux linkage will be highest
  • Electromagnetic induction
    • Faraday's law
      • The induced EMF is directly proportional to the rate of change of flux linkage
    • Lenz's law
      • The induced EMF is always in the direction that will oppose the force that created it
    • Relative motion between a conductor and a magnet will create an EMF in the conductor, as the electrons in the conductor feel a force from the magnetic field and so become concentrated in one part of the conductor
  • Transformers
    • Made from two coils with a core of soft iron
    • The AC in one coil means that the flux produced is always changing, so there is an EMF induced in the second coil as there is a relative change in flux linkage
    • The ratio of the voltages is the same as the ratio of turns on either coil
    • Generally highly efficient
      • Some losses due to eddy currents induced in the core
        • (reduced by laminating the core)
      • Some losses due to changing the polarity of the core

AQA A2 Biology – Unit 4

Posted by Alex on 02:21 comments (0)

Ecology and biochemistry, because they go together right?

Ecology

  • Study of living organisms and environment
  • Terms
    • Ecosystems – self contained area containing living organisms linked together by nutrient and energy flow
    • Habitats are abiotic part of organism
    • Community is the term for all organisms of all species in an ecosystem
    • Population is for all organisms in one species
  • Fieldwork
    • Sampling
      • Random sampling – measuring tapes placed along sides like axes of graph and random numbers are used to determine co-ordinates for a quadrat where you count the species present.
        • Preferable 2% area coverage
        • Keep sampling until means remain constant so sample is representative
      • Systemetric sampling - choosing where to take samples to investigate a pattern
        • Basically, belt transects
        • Count the organisms in a quadrat placed at regular intervals along the line to investigate how samples change
      • While sampling measure abiotic factors to understand how visible differences in the ecosystem affect the local conditions
        • Soil temp
        • Light intensity at ground level
        • Soil pH
        • Soil depth
        • Altitude
        • Water flow rate
        • Water oxygen content
          • And many more...
      • Repeats for reliability, over long time periods to account for daily and seasonal variations
      • Pick your quadrat so you identify as many species as possible without wasting effort
        • Point quadrats can be used as a more accurate and less subjective measure of percentage cover
      • Quadrats are good for plants and slow animals, but otherwise use nets or traps
    • Capture-Mark-Release
      • Method of estimating the population of a species in an area (only to within 50%)
      • Capture a large number of animals, then mark them
        • Eg. Leg rings on birds
        • DNA fingerprinting can also be used
      • Release the animals so they mix with the general population
      • Capture more animals
      • The proportion of captured animals that are already marked is can be used to work out the total number of animals in the area
        • N = Sample size ^2 / Recaptures
      • Limitations
        • Marking cannot affect survival
        • There must be time for the marked animals to mix with the population
        • The population must be constant
    • Stats tests
      • GCSE stats in action
      • Key thing is that we want a 95% confidence value before we reject the null hypothesis
        • This means error bars representing 2SD must not overlap on a bar chart else we cannot assume there is a difference between the two values in reality
      • Qualitative data can be compared to expected values for frequency (eg from genotype calculations)
  • Populations
    • Affected by abiotic factors – density independent as they will limit growth regardless of competition - usually harsh environments
    • Also affected by biotic factors – these often lead to competition – usually milder temperate climates
      • Interspecific – between species
      • Intraspecific – within a species
        • Significant as they have the same niche and so are after the exact same resources
        • Stabilises population as population growth increases competition, leading to population decrease, decreasing competition and so on
        • Leads to natural selection as individuals within a species with better suited genes will be more likely to survive competition and pass those genes on
    • Predation
      • When one species uses another species as food
      • As prey increases in population size, competition in predators reduces
      • Predator population growth results in more prey being eaten so numbers reduces
      • More predator completion so predator numbers decrease
      • Prey numbers then recover and cycle restarts
    • Parasitism
      • Not in the book but ok
      • Parasites kill their hosts, so as parasite numbers increase host numbers decrease
      • Parasites then die due to lack of hosts
      • Hosts then increase again in number as there are less parasites killing them off
      • Cycle restarts
    • Each population has a niche, which is the conditions it is best adapted to surviving
      • Populations can coexist when they have different niches, but not when their niches are too similar that one species is able to outcompete the other
      • Specialists are species with a narrow niche that outcompete any other species attempting to occupy the niche
  • Succession
    • The idea that ecosystems change over time
    • It's self-perpetuating because the growth of organisms in an ecosystem changes the environment and so leads to new species gaining niches
    • Goes from pioneer species which can grow quickly through seral stages until reaching a climax community, where succession stops due to the large and complex food web being self sustaining
    • Two kinds:
      • Primary – from bare rocks or sand eg after a volcanic eruption
      • Secondary – from empty soil but no species eg after a forest fire
    • Humans have destroyed climax communities and prevent areas from reaching them again, through interventions necessary for agriculture – eg. Plowing, weeding, mowing, grazing animals
      • Grazing leads to a climax community of grasslands, as grazing animals such as sheep will eat the tips of shoots which kills dicotyledonous plants but not monocotyledonous plants such as grasses
  • Conservation
    • Management to maintain biodiversity
      • This means that species diversity should be increased, and prevention of climax communities actually helps with this
      • Interventions which prevent succession, but also recognition that different areas need to be kept at different seral stages to produce maximum biodiversity
  • Food Chains
    • Show the relationships of energy flow between organisms
    • Start with a photosynthetic producer which can use energy from the sun to convert abiotic molecules to biomass
      • (Or other autotroph)
    • As you go along the chain, you go up trophic levels
    • Detritus, which is waste biomass from dead organisms/excretion, is used as a food source by saprobiotic organisms
    • Very small amounts of energy pass from one trophic level to another, roughly 10%, the rest is all lost via respiration, undigestible biomass etc.
  • Nutrient cycles
    • Inorganic molecules used in biological processes are known as nutrients
    • Nutrients are absorbed and converted into biomass by producers, where they are passed up food chains
    • Decomposers then return nutrients in biomass to the inorganic environment to be reused by producers once the organisms have died
    • Carbon cycle
      • Photosynthesis is the process by which carbon is fixed from CO2 in the atmosphere to biomass
      • Respiration then returns that CO2 to the atmosphere, as the organic molecules are passed along food chains
      • When organisms die the carbon compounds in them are broken down by decomposers and returned to the atmosphere via respiration of those decomposers
      • Some carbon compounds in detritus are prevented from being broken down by decomposers due to geological processes, leading to fossil fuels
    • Saprobiotic organisms secrete digestive enzymes onto their food and then absorb the soluble products
    • Detritivores are small animals which feed on detritus, which start the process of breaking down detritus into inorganic molecules. They physically break down dead biomass so there is more surface area for saprobiotic organisms to feed on
    • Nitrogen cycle
      • Nitrogen in the atmosphere is converted into ammonia and then ammonium ions by nitrogen fixation (by bacteria in the soil)
      • The ammonia is then converted into nitrite, and then nitrate, in a process called nitrification (again by bacteria)
      • The nitrates are absorbed by plants and used to construct proteins
      • The proteins are eaten by animals and broken down into amino acids, then used to construct new proteins
      • When organisms die, the saprobiotic organisms turn the nitrogen-containing compounds into ammonia again
      • In anaerobic conditions, bacteria convert nitrate to nitrogen, in a process called denitrification
  • Productivity
    • Energy fixed by producers is the gross primary productivity
    • Energy absorbed by consumers is the gross secondary productivity
    • Net productivity = Gross – losses due to respiration, lack of digestion etc.
    • Different ecosystems have different productivities, as most areas have a similar amount of solar energy on them but cannot utilise it to the same extent. Rainforests are among the most productive areas.
    • Farmers want to increase the productivity as much as possible, as the greater the productivity the greater the biomass fixed into their products, which is the yield they can sell at market
      • They do this by increasing gross productivity eg. Through using fertilisers
      • They can also do it by reducing respiratory losses as seen in factory farming, where animals are restrained so they lose less energy for movement
    • Fertilisers
      • Fertilisation is when nutrients are added to the soil so plants can use them to grow more efficiently as the lack of that nutrient is reduced as a limiting factor
      • Crops called legumes have mutualistic bacteria which carry out nitrogen fixation for them, by growing these crops and then ploughing them back into the soil the nitrogen fixed by the bacteria can be used to provide for future crops
      • Inorganic fertilisers produced using the Haber Process can be used to quickly and easily add Nitrate, Potassium and Phosphate ions to the soil, which are what most plants need to grow
        • This can cause problems such as eutrophication
          • This is where addition of nutrients due to human activity leads to algal blooms in ponds, which block the light of plants on the bottom and stop oxygen diffusing in to the pond, meaning that decomposers use up all of the oxygen to break down the dead plants and eventually only anaerobic organisms can survive, which produce toxic waste products
      • Organic fertilisers are basically detritus from farms which contain the nutrients needed for new plant growth stored in biological molecules, after the detritus is broken down by decomposers the plants can use the nutrients
        • It is however slower, as the decay can take a while
    • Pest control
      • Chemical and biological pest control
        • Chemicals are pesticides/herbicides which directly kill pests
        • Biological pest controls are species which are predators to pests, so they are introduced to the area to kill off pest populations
      • Integrated pest management is when multiple methods are used together
      • Pest population must be reduced below the economic threshold where it does little harm
  • Global Warming/Climate Change
    • Caused by the greenhouse effect
      • Heat reflected by certain gases called greenhouse gases
      • The concentration of these gases is increasing so the amount of heat reflected increases
    • Humans have caused it
      • Burning of fossil fuels, deforestation of rainforest and heavy industry have increased CO2 concentration in the atmosphere
    • Effects will be mostly negative
      • Sea level rise
      • Extinction of animal species as their climates change beyond the range they are adapted for
      • Pests will be more able to spread as climates warm to pests preferred conditions
  • Human Populations
    • Human growth rate has increased exponentially in the past 100 years or so
    • Mainly due to decreased death rate meaning people live to reproduce and have kids more often
    • Democratic transition model
      • Stage 1 – High birth and high death rate – balanced
        • (Invention of better medicine)
      • Stage 2 – High birth rate and falling death rate – population growth
      • Stage 3 – Birth rate decreases, death rate remains low – slow growth
      • Stage 4 – Low birth rate, low death rate – balanced/decrease

Respiration


 

  • Glucose + Oxygen -> Carbon Dioxide + Water
  • The reaction is exothermic, and this energy is coupled to the synthesis of ATP from ADP and inorganic phosphate groups
  • ATP
    • ATP is the energy carrier molecule used in biological processes
    • Very soluble
    • Remains inside cells
    • Bond between phosphate group can be easily broken to release a useful amount of energy, returning it to ADP and Pi (inorganic phosphate)
  • Glycolysis
    • Glucose in the cytoplasm is the starting point
    • Phosphorylated using 2 ATP molecules
    • Breaks down at this point into 2 separate triose phosphate molecules (TP)
    • Each triose phosphate releases 2 ATP and reduces one NAD to become pyruvate
  • Link reaction (aerobic)
    • Inside mitochondrial matrix
    • Pyruvate -> Acetyl Coenzyme A (Acetyl CoA)
    • Releases CO2 at this point and another NAD becomes reduced
  • Anaerobic respiration
    • Happens after glycolysis, no link reaction
    • In cytoplasm still
    • In animals and bacteria
      • Pyruvate + reduced NAD -> Lactate (lactic acid) + NAD
      • Recycles reduced NAD to NAD for more glycolysis, but the lactic acid is toxic
    • In plants and fungi (fermentation)
      • Pyruvate + reduced NAD -> Ethanol + CO2 + NAD
  • Krebs cycle (aerobic)
    • Mitochondrial matrix again
    • Acetyl CoA (2-carbon) combines with 4-carbon intermediate to form citrate (6-carbon molecule)
    • This 6 carbon molecule breaks down to form 2 CO2 molecules, reduce one NAD and reduce one FAD, and produce one ATP from ADP
    • Back to a 4 carbon molecule again so the cycle turns twice
    • 2x Acetyl CoA are produced from each glucose, so it turns twice per glucose
  • Sum total of products at this point:
    • 2x ATP from glycolysis (2 used, 2 released each TP molecule)
    • 4 NAD reduced from glycolysis and link reaction
    • 6NAD and 2 FAD reduced from krebs cycle
    • 2 ATP from krebs cycle
    • 3 CO2
  • Electron transport chain
    • Occurs on the inner membrane of the mitochondria, on the cristae
    • Reduced carrier molecules are oxidised
    • This means that hydrogen atoms on the carrier molecules are stripped of their electrons and removed from the carrier
    • Energy from the electrons is used to pump the H+ ions into the intermembrane space
    • As the electrons pass down the chain of proteins more H+ ions are pumped into the intermembrane space
    • Finally the chain ends when molecular oxygen is reduced into water, accepting H+ ions and electrons
    • The H+ ions in the intermembrane space travel down a concentration gradient into the matrix of the mitochondria. This is coupled to an ATP synthesase enzyme on the end on the H+ transport protein, so the gradient of H+ ion concentration powers synthesis of ATP
  • Fats and glycogen can also be broken down by respiration, as well as proteins in extreme circumstances

Photosynthesis

  • Light dependant reactions
    • Happen on thylakoid membrane in chloroplasts
    • Chlorophyll pigments absorb light, exciting some of the electrons in Photosystem II to a higher energy level
    • This allows water to be broken down by photolysis to oxygen, H+ and electrons inside the lumen of the thylakoid
    • H+ ions build up causing a concentration gradient
    • Electrons replace lost electrons from the photosystem
    • High energy electrons are passed down a chain of proteins, pumping protons from the stroma into the lumen of the thylakoid
    • In Photosystem I, more light energy is absorbed and the electron is increased to a higher energy level again
    • This high energy electron is used to reduced NADP to NADPH by combining it with a H+ ion
    • The concentration gradient of H+ ions is again used to power the formation of ATP from ADP using an ATP synthesase enzyme
  • Light independant reactions
    • Called the Calvin cycle
    • RuBP (5 carbon) + CO2 -> 2x GP (3 carbon)
    • Relies on ATP and NADPH from the light dependant stage
    • 2x ATP + 2x NADPH + 2x GP -> 2x TP
    • Only one carbon from both of the two TP molecules combined is then removed from the cycle, to return to the original 5-carbon RuBP
    • The carbon removed is then used to create glucose, which is a 6 carbon molecule
    • 6 turns of the calvin cycle are needed for one glucose
  • Factors affecting photosynthesis
    • Temperature
      • Higher temperature = higher rate
      • Until you reach the point where the enzymes denature
    • Carbon dioxide concentration
      • Higher concentration = higher rate
      • Until something else becomes limiting
    • Light intensity
      • Higher light intensity = higher rate
      • Until something else becomes limiting
      • Until light intensity increases the temperature so much the plant gets too hot

Genetics

  • Define homozygous/heterozygous
  • Draw punnett squares
  • Dominant and recessive genes
  • Sex determination
    • This is because the egg always contains an X chromosome
    • Sperm can contain either an X or a Y depending on what they picked up in meiosis
    • Y chromosomes contain dominant genes for hormones which convert embryos to males
  • Sex linked genes
    • Y chromosomes only code for sex determination
    • X chromosomes contain other genes
    • Therefore males only have one set of X chromosome alleles (from the mother) and whatever genotype is present will be expressed
      • Colour blindness is the usual example, which is a recessive allele and so not expressed in females, but males will always express the colourblindness gene if they inherit it
  • Codominance
    • Not every gene has only two phenotypes, sometimes heterozygous individuals will display a third phenotype
    • This is due to incomplete dominance of one allele over another, leading to both being partially expressed
      • Sickle cell anaemia is the usual example
        • This has the interesting effect that heterozygous individuals have a competitive advantage in malaria-prone areas as the malaria parasite cannot infect them as easily
  • Multiple alleles
    • When there are more than 2 types of allele in the gene pool for a population
    • Only 2 can be present in one individual, but it means there can be a variety of crosses
      • Blood typing using the ABO system is an example of this
        • A and B are also codominant, to give an AB phenotype
  • Population genetics
    • Hardy Weinberg principle
      • Frequencies of dominant and recessive alleles will remain constant as long as:
        • There is no mutation
        • There is no migration
        • There is no selection of different alleles over each other
        • Random mating is observed
        • There is a large population
      • If these are true there is genetic equilibrium
    • P + q = 1
    • P^2 + pq + q^2 = 1
  • Natural selection
    • Based on 4 observations
      • There is variation within a species
      • Characteristics are inherited
      • Most organisms die before they can reproduce
      • Populations remain constant in size
    • Over time, individuals who are better adapted to the conditions they live in will be able to survive for longer and so have more offspring
    • This means that the genes which created that better adaptation make up a larger proportion of the gene pool
    • Eventually the entire population changes as a result of the changed gene pool
    • Types
      • Directional – when there is a change in environment favouring one extreme of phenotype
      • Stabilising – when the environment is stable and the average is well adapted
      • Disruptive – when there is a sudden change in environment and opposite extremes are favoured
    • Speciation
      • Formation of new species by isolation of groups of current species
      • Isolated groups will adapt to their new environments
      • When the isolated groups then meet again, the adaptations mean they are no longer able to breed
      • Therefore different species
         


  

Edexcel AS Chemistry–Unit 2

Posted by Alex on 12:19 comments (0)

Bonding and intermolecular forces

  • Electron pair repulsion theory
    • Bond angles
      • Linear 180°
      • Triganol Planar 120°
      • Tetrahedral 109.5°
      • Triganol Bipyramidal 90° and 120°
      • Octahedral 90°
      • Lone pairs of electrons count toward an area of electron density but as they repel to a much greater extent they reduce the bond angle by 2.5°
    • Bond length – the stronger the bond the shorter the length
  • Electronegativity
    • The ability to attract the electrons in a covalent bond
      • Ionic property of covalent bonds
        • Same applies the other way around
    • Differences in electronegativity cause permanent dipoles
      • Polar substances dissolve in water as they disrupt hydrogen bonding lattice
        • Non-polar substances will not dissolve in water
          • This is because there is not enough energy to make the non-polar substances break the bonds within the polar water molecules
      • Polar bonds break first in organic chemistry
        • Polar bonds are always heterolytic fission
          • Produces one electrophile and one nucleophile
  • London forces
    • Exist between all atoms
    • Weak force relative to size of electron cloud
    • Random fluctuation will induce instantaneous dipoles
  • Hydrogen bonding
    • Formed when hydrogen bonds to a highly electronegative atom with an unpaired electron
    • Formed at 180°
    • Strongest intermolecular bonds
  • Physical properties from bonding
    • Allotropes of carbon
      • Diamond – 4 bonds, giant covalent structure, requires high energy to break as there are numerous strong bonds. No conduction of electricity
      • Graphite – 3 bonds in flat sheets, makes it very slippery and useful as a lubricant. Weak bonds between flat layers are london forces. Delocalised electron so conducts electricity
      • Fullerenes – Ball shaped (Bucky ball) so strong due to distribution of bonds, conducts also
      • Nanotubes – Hollow tubes of carbon atoms, very strong due to rigid shape and conducts electricity
    • Organic chemistry
      • Longer chains have more electrons so greater london forces
      • More branching reduces boiling points as chains can’t get as close to each other so london forces are weaker

Inorganic Chemistry

  • Ionisation energies decrease down group 2
    • This is because the atomic radii increase and the shielding stays the same so the effective nuclear charge is the same but further away
  • Group 2 reactions:
    • Group 2 + Oxygen –> Metal Oxide (solid)
    • Group 2 + Water –> Hydroxide + Hydrogen gas
    • Group 2 + Chlorine –> Metal Chloride (solid)
  • Group 2 Oxide reactions
    • Oxide + Water –> Hydroxide
    • Oxide + Acid –> Chloride/Nitrate + H2O
      • Group 2 Hydroxides will also do this, 2H2O results
  • Group 2 sulphates
    • Solubility decreases down group
    • Test for sulphates: add to Barium Chloride, will give white precipitate
  • Group 2 hydroxides become less soluble down the group
  • Thermal stability of group 1 and 2 carbonates
    • Group 1 – stable, they don’t decompose from heating
    • Group 2 carbonates + heat –> Group 2 oxide + CO2
      • Stability of nitrates and carbonates increase down the group
        • This is because oxides are more stable, and the first elements are more polarising so they attract the oxygen away from the carbonate
    • Group 1 and 2 nitrates:
      • Group 1 decompose to nitrite and oxygen
      • Group 2 decompose to oxygen, nitrogen and oxide
  • Flame tests
    • Caused by electron being excited to a higher energy level, when it decreases energy level the extra energy is released as a photon
      • (Physics!)
  • Halogens are oxidising agents; their strength decreases down the group
    • Fluorine is the strongest oxidising agent as it has the strongest attraction from the nuclear charge where it is so small
  • Disproportionation with alkalis
    • Cold will lead to something like 2NaOH + Cl2 –> NaCl + NaClO + H2O
    • Hot forms the halate: 2NaOH + Cl2 –> NaCl + NaClO3 + H2O
      • So for bromine: cold leads to bromide, but if it’s hot you get a bromate
  • Iodine thiosulphate titrations
    • An oxidising agent will react with iodine ions
    • These iodine ions will then react with thiosulphate
    • Colour goes from yellow to colourless with just thiosulphate
      • Adding starch makes it go black as there are still iodine ions, as they react with thiosulphate it goes colourless
  • Test for halides
    • Add silver nitrate and colour of precipitate
      • White = Cl
      • Cream = Br
      • Yellow = I
    • Confirmation test using ammonia
      • Cl will dissolve with dilute ammonia
      • Br will dissolve with concentrated ammonia
      • I will not dissolve

Reaction rates

  • Increased by increasing:
    • Concentration
    • Pressure
    • Surface Area
    • Temperature
    • Alternative reaction routes – catalyst
  • All based on collision theory
  • Maxwell-boltzmann distribution curves
    • Total area under the graph must remain constant
    • Increasing rate of reaction means that more particles will be past eA on the energy scale
  • Equilibria
    • Dynamic state in a closed system due to reversible reactions
    • Le Chatellier’s principle says that if you change the environment, the position of equilibria will change to compensate for the environment
      • So increasing the pressure moves towards the less dense side etc etc.

Reactions of Alcohols

  • The more methyl groups the C in R-C-OH is bonded to, the more stable the molecule is
  • Combustion
  • Alcohols and sodium –> H2 and white precipitate (sodium propanoxide)
  • Substitution with a halogen produces halogenoalkanes
    • This is electrophilic substitution
    • Iodo/bromoalkanes need acid catalyst
  • Oxidisation – goes from orange to green
    • Only primary and secondary
    • Primary – aldehyde and then carboxylic acid
      • Aldehyde + Benedict’s –> Red precipitate
    • Secondary – ketone
      • Ketone + Brady’s reagent –> Yellow/orange precipitate

Halogenoalkanes

  • The more reactive the halogen, the less reactive the halogenoalkane
  • More methyl groups attached to the C in R-C-X make it more reactive
    • Electron density pulled away from the halogen
  • Strength of bond can be determined using aqueous silver nitrate solution (hot)
    • Faster precipitate = weaker bond
  • Free radical substitution
    • Green chemistry bit, with UV light
  • Reactions summarised:
    • + Hot aqueous alkali –> Alcohol (Nucleophilic substitution)
    • + Concentrated alcoholic alkali (reflux and heat) –> Alkene (Elimination)
    • + Alcoholic ammonia (heat under pressure) –> Amine

AQA AS Biology Unit 2

Posted by Alex on 03:43 comments (0)

This unit doesn’t have the same structure as unit 1, there are loose categories based on the notes I’m using to write this… but mostly each topic is on it’s own. The key idea overall is adaptation and diversity – this is the more ecology and genetics (and adaptations therein) based part.

Polysaccharides

  • Starch
    • Plant storage polysaccharide
    • Forms insoluble granules inside cells
      • Does not change water potential
        • (We’ve already been tested on this in the unit 1 exam I wouldn’t worry about explaining it)
    • Mixture of amylose and amylopectin
      • (Explains the name Amylase for the enzyme!)
    • Forms a helix shape held together by hydrogen bonds
    • Broken down into maltose
  • Glycogen
    • “Animal starch”
    • Found in muscle and liver cells
    • Highly branched
      • Lots of ends for enzymes to work on
      • Therefore converted to energy easily and quickly
    • Enzyme called Glycogen phosphorylase
  • Cellulose
    • Main component of cell walls
    • Only in plants
    • b-glucose instead of a-glucose
      • Alternate molecules are inverted
      • Straight chains!
      • Hydrogen bonds link chains
        • Strong cell walls!
        • These linked chains are called microfibrils
      • (Bit brief again because I already did this)
    • Cellulase enzyme
      • Only produced by bacteria
      • Humans don’t carry this bacteria but ruminants do
        • So it’s called “fibre”

Diffusion and the Problem of Size

  • The larger an organism, the smaller its surface area to volume ratio
    • Therefore diffusion is slower into it
      • Above a certain size, diffusion is too slow to support life
  • Maximum size of a normal cell: 100um
  • Heat exchange the same way so larger animals are better adapted for cold environments

Gas Exchange

  • Small organisms
    • Large SA:V ratio, therefore simple diffusion
    • Low metabolic rates
  • Insects
    • Waterproof exoskeleton
      • Prevents drying out and supports organs
    • Spiracles, pores in the exoskeleton, allow air in and out
      • These pores lead to tracheae and tracheoles strengthened by chitin
      • Air pores go directly to the cellular level
    • Actively respiring cells produce lactic acid, lowering the water potential and so causing water to go into the cells
      • This then means the air can reach closer to the cell as the water layer around it is thinner
        • Therefore, rate of diffusion is faster
  • Fish
    • Concentration of oxygen in water is low
    • Gills are exchange organs
      • Composed of filaments with lamellae on
        • Lamellae only a few cells thick with capillaries inside
          • Large surface area and small thickness
            • High rate of diffusion
          • If a fish is taken out of water the gills collapse and water is needed to cushion each individual lamellae
    • Ventilation
      • One way ventilation
        • In through the mouth, out through the fin valves
      • The water flows in the opposite direction to the blood in the gills so a constant concentration gradient remains
      • 80% of oxygen is extracted
  • Plants
    • Gases enter the leaf through the stomata
    • The spongy mesophyll cells then transport gas in and out of the plant
    • Water loss is the main problem by transpiration as the mesophyll cells must be damp to dissolve O2
      • This is reduced as there is a waterproof cuticle on the upper cuticle
      • Pitted stomata trap a space of moist air, reducing the concentration gradient
      • Guard cells can close stopping water loss and gas exchange
    • No ventilation as leaves are highly exposed

Human Circulatory System

  • Humans have a double circulatory system
    • This means we have two sides to our heart which pump blood to different places
  • The sequence of blood vessels is as follows:
    • Heart > Aorta > Arteries > Arterioles > Capillaries (outward journey)
    • Capillaries > Venules > Veins > Vena Cava > Heart (return journey)
  • The purpose of the blood vessels is to deliver blood to capillaries, which are spaced every 100um in the body at least
    • There is not enough blood to supply every capillary, so 20% of capillaries are closed off at any time
      • This is obvious when you consider your skin going red/pink when you exercise – heat exchange has become a priority so capillaries that were closed off mostly open up and others (in the gut) close down
  • Structures of blood vessels
    • Arteries
      • Thick walls of elastic tissue and muscle fibres
        • Allows artery to expand without bursting and resist high pressures
        • Expansion and contractions even out pressures in the blood flow
    • Capillaries
      • Very narrow
      • Walls are single endothelial cells
      • Gaps in wall allowing substances to dissolve out freely
      • Huge surface area in total
    • Veins
      • Thin inelastic walls
      • Thicker lumen
      • Valves
        • These prevent blood flowing backwards as blood in the veins is at low pressures
      • Contractions of nearby muscles help push blood up from legs etc.
        • Hence not moving your legs can cause DVT as there are no contractions to pump blood
  • Tissue fluid
    • Bathes all cells
    • Final site of transport
    • Formed from blood plasma
      • High pressure in arterial end of capillary forces blood plasma out into tissue fluid
      • Materials transfer between tissue fluid and cells by all 4 methods of transport
      • At venous end of capillary there is low pressure so blood plasma is reabsorbed
      • Some plasma is not reabsorbed, this drains into the lymph system
  • The lymphatic system
    • A secondary system for exchange and transport in the body
    • It reabsorbs into the blood near the vena cava
    • Has three main roles:
      • Drains excess tissue fluid
      • Absorbs fats from the small intestine
      • Forms part of the immune system, allowing white blood cells to develop in lymph nodes

Transport of Oxygen

  • Haemoglobin
    • Protein molecule that allows red blood cells to carry oxygen more efficiently
      • 20% concentration rather than 1%
    • One haemoglobin can carry up to 4 oxygen molecules
    • Picks up and releases oxygen according to the oxygen dissociation curve
      • This is a graph showing the percentage saturation of oxygen in haemoglobin depending on the partial pressure of O2 in the environment
      • This explains why the haemoglobin becomes saturated in the lungs, as there is a high pressure of oxygen
      • In respiring tissues there is less oxygen so it unloads from the haemoglobin
      • Respiring tissues also form CO2, which then dissolves to form carbonic acid, moving the curve to the right and so meaning more O2 unloads

Water transport in plants

  • Water is transported by xylem vessels
    • Xylem vessels are dead cells that merge together to form long hollow tubes
    • Rings of lignin form in the walls to allow the xylem to withstand pressure
  • Transport in the roots
    • There are two pathways for water to travel through the roots
      • The symplast pathway – through the cytoplasm
      • The apoplast pathway – through the cell walls
        • This stops at the casparian strip, forcing all water into the cytoplasm inside the endodermis
    • This active absorption of water means that a pressure is produced in the xylem – root pressure
      • This is quite weak however
  • Mass flow in the stem
    • As water leaves the leaves there is a tension to pull more water up to replace it
    • The thin tubes of the xylem mean that capillary action pulls the water up as it is stretched
    • Strong walls is needed to stop the xylem collapsing under the pressure
  • Diffusion through the leaves
    • There is a water potential gradient in the leaves because of transpiration so water leaves the xylem into the cells
  • Factors affecting transpiration
    • Light – makes the stomata open so gas exchange is needed and transpiration must occur
    • Temperature – water will have more kinetic energy so it’s more likely to evaporate
    • Humidity – high humidity means there is less of a concentration gradient so water leaves less quickly
    • Air movement – wind will blow saturated air so the concentration gradient remains constant
  • Examples of adaptations
    • Thick cuticle – prevents evaporation through top of leaf
    • Small leaf surface area – cactuses and conifer needles
    • Sunken stomata/hairy stomata – trap pocket of saturated air
    • Succulent leaves and stem – water storage
    • Extensive roots – collects more water

DNA

  • Nucleotides
    • Contain elements CHONP
    • Three part structure
      • Phosphate connected to a pentose sugar, connected to an organic base
        • 5 different bases, Adenine, Cytosine, Guanine, Thymine and Uracil
          • Thymine in DNA = Uracil in RNA
        • Each base joins to one other base to form a base pair
          • Guanine and Cytosine (3 hydrogen bonds)
          • Adenine and Thymine (2 hydrogen bonds)
    • Join together to form polynucleotides by condensation reactions
  • DNA structure
    • Double stranded
    • Antiparallel strands
    • Double helix structure
    • Hydrogen bonds hold strands together
    • Benefits of structure:
      • Makes it strong so it can’t be damaged easily
      • Bases can be used to encode information
      • DNA can be very long so lots of information stored
      • Complementary strands means there are two copies of information
        • Errors can be spotted easily
  • Replication
    • Used to produce two copies of DNA for cell division
    • One strand conserved in each replication, one new strand assembled
      • Remember the experiment with heavy nitrogen
        • (I’m not writing about it because it’s boring and I know it)
    • Process:
      • DNA Helicase unwinds and separates two strands of DNA
      • Free nucleotides attach to exposed bases (following base pairing rule)
      • DNA Polymerase joins the nucleotides together
      • Another enzyme winds the new strands into helixes
    • DNA replication speed is the limiting factor of cell division
  • Function
    • DNA determines the sequence of amino acids in a polypeptide
      • This in turn determines the shape of the protein
        • This in turn determines the functionality of the protein
          • This in turn determines how cells work
      • A gene is the DNA sequence for one polypeptide
    • The triplet code
      • Every three combinations of base pairs codes for one amino acid
      • 64 possible combinations, most amino acids have more than one code
    • Control genes
      • Some genes do not code for proteins but instead determine what other genes activate at particular times
        • This is important because all body cells have the full genetic code, control genes determine what parts of the genetic code are expressed
  • Mutations
    • These are small errors in replication of DNA
    • Most of the time they either have no effect or stop a gene from functioning
    • Occasionally they will have a positive effect and this is what causes evolution
  • Chromosomes
    • Chromosomes are formed of two sets of the same chromatid (a section of the total DNA)
    • They are joined at the centromere
    • They form because there is temporarily twice the usual amount of DNA in the cell before replication, so it must be wound up tighter than usual to fit within the nucleus – this makes complexes big enough to see
    • Humans have 46 chromosomes in 23 pairs
      • (and don’t forget each chromosome is double the usual amount of DNA needed)
    • Chromosome pairs are not identical, but they do contain the same type of genes
      • One of each pair comes from each parent, and so can contain different alleles of each gene

Mitosis and the Cell Cycle

  • Mitosis is needed to produce new body cells – either to grow, replace or repair
  • It is also used to produce new organisms for asexual reproduction
  • The cell cycle:
    • Interphase
      • 90% of cell cycle
      • Spent performing usual cell functions
      • Includes 3 phases:
        • G1 – Cell regrowing and recovering organelles after mitosis
        • S – DNA replicated
        • G2 – Spindle proteins replicated
    • Mitosis
      • Prophase
        • Chromosomes condensed and visible
        • Centrioles form at opposite poles
        • Nucleolus vanishes
      • Metaphase
        • Nuclear envelope vanishes
        • Chromosomes align along centre of cell
        • Spindle fibres form between centrioles and chromosomes
      • Anaphase
        • Centromeres split
        • Chromosomes start to diverge
        • Centromeres travel along spindle fibres to opposite sides of cell
      • Telophase
        • Spindle fibres disperse
        • Nuclear envelope reappears around new sets of chromosomes
        • Chromatids uncoil
      • Cytokinesis
        • The cytoplasm splits and the cells become seperate
  • Usually there is control over how often cells can divide
    • If these controls stop working, this is cancer

Meiosis and Sexual Reproduction

  • Meiosis differs from mitosis because it results in 4 cells rather than 2, each with half the usual number of chromosomes
  • This is because the sex cells combine with another sex cell, so the zygote will have the normal number once they have joined together
  • Chromosomes are re-arranged to form new combinations of genes in order to create genetic variation in the zygote
    • This is achieved in three ways:
      • Independent assortment – The homologous chromosomes pair up independently of each other, so the final meiosis cells can have any combination of maternal and paternal chromatids
      • Crossing over – When the two homologous chromosomes pair up, they become tangled together and then cut of when separated with part of the paired chromosome on one chromatid each swapped between the two, so the final meiosis cells will have combination maternal-paternal alleles on each chromatid
      • Random fertilisation – The sperm that reaches the egg could contain any combination of alleles, it is more or less random what sperm and what egg will fuse together

Antibiotic resistance

  • Antibiotics kill bacteria, often by weakening their cell walls so they burst under pressure
    • 70S ribosomes are also a common target of antibiotics
  • Mutations in bacteria can make them resistant to antibiotics
    • When the antibiotics are then used, the bacteria which are resistant will survive and pass on this mutation to their offspring
      • This is because they have a selective advantage – the environment contains antibiotics so being resistant to them gives them the change to survive
    • Mutations can also be passed on through genetic conjugation, where one bacteria donates a gene to another

Classification

  • Defining a species:
    • Organisms that are similar in appearance, biochemistry and fulfil the same ecological niche
    • Organisms that can breed to produce fertile offspring
    • Organisms that share a common ancestor
  • Species are formed when two isolated populations of the same species diverge from each other in terms of evolution
    • Eventually they will become too genetically different to breed
  • Species are the bottom rung of taxonomic hierarchy:
    • Kingdom
    • Phylum
    • Class
    • Order
    • Family
    • Genus
    • Species
  • The further down the hierarchy the more closely related two organisms are
  • Names of species are binomial, with the genus and species given to identify positively one species within the entire tree of species
  • Phylogenetic trees are also used to classify species, this is according to evolutionary relationships rather than groupings
    • The earlier a path splits on a phylogenetic tree, the more distantly related the groups are, as the point of splitting shows the last common ancestor
  • DNA Classification
    • This is where you create a hybrid DNA molecule from one strand each of two species DNA
    • The closer the species are related, the more of the bases will match up
    • Therefore, closer species will form more hydrogen bonds and require higher temperatures to break apart
  • Protein immunology classification
    • This is where you inject an animal (eg a rabbit) with protein from a species
    • The rabbit will produce antibodies against that protein
    • Collect these antibodies, then react them with proteins from different species
    • The closer related a species is, the more antibodies will react with it and so the greater the volume of sediment produced
  • Courtship behaviours
    • Contrary to human belief, courtship behaviours are intrinsic
      • This means they are genetically coded into species and aren’t a matter of choice
    • The purpose of courtship behaviours is to ensure that individuals only attempt to mate with compatible mates
      • This means they’re of the same species, sexually mature and fertile

Genetic Diversity

  • Genetic diversity is important as it means species are more likely to have the tools needed to survive a change in the environment
  • Genetic diversity is however lowered in three main ways:
    • Genetic Bottlenecks – When some event occurs that wipes out a large number of a species, therefore meaning the remaining individuals will have a reduced range of alleles
    • The Founder Effect – When only a small number of settlers become isolated from the main population, the effect of every allele carried by these settlers is increased, as there are few settlers and so few alleles
      • Not just geographically isolated either: Amish people only get married to other Amish people as a rule and nobody joins the community, so there are only a few alleles in circulation
    • Selective Breeding – When people choose to control the individuals allowed to breed in a species based on desirable characteristics, individuals with other characteristics die out as they are not able to pass these on
  • Measuring species diversity
    • Some maths or something we didn’t do so I don’t think we need
      • It’s called the Simpson Diversity Index, look it up if you care

Edexcel AS Chemistry–Unit 1

Posted by Alex on 08:20 comments (2)

Chemical quantities and formulae

Definitions:
  • Elements are substances which cannot be broken down by chemical means into other substances
  • Atoms are the smallest part of any elements that can take part in chemical reactions
  • Ions are formed when an atom gains or loses electrons, in other words a charged atom
  • Compounds are formed when two or more elements bond together
  • A molecule is a covalent compound
  • Empirical formulae are the simplest ratios of atoms possible for a given compound
    • Three steps to working these out:
      • Divide the mass (or percentage, assume it’s 100 total mass) by the molar mass of the element
      • Divide the results of the first step by the smallest result of the first step
      • This is the ratio of elements (to the nearest whole number)
  • Molecular formulae are the formulas as found in reality ie. multiplied to fit the actual mass value recorded
Ionic equations:
  • Include something if it changes state
  • Make sure both sides balance charges and all other ions
    • Change in charge and no balanced change in charge? Add an electron to balance!
      • (This is more for half equations)
    • The quick way to do half equations when you’re including acidic/alkaline conditions as well is to balance the H+ or OH- from the amount of water produced and then balance the charge with electrons
Concentrations and calculations:
  • Concentration = moles/amount of solute
    • (Alternatively can be done with grams)
  • Three steps to working out the numbers in a chemistry paper:
    • Get a balanced reaction written down
    • Figure out the mole ratio of what you’re calculating
    • Times the number of moles by the mole ratio
  • Avogadro’s constant
    • It’s written on the sheet or so I’m told, but what it tells you is the value of a mole
      • That rhymes kill me now
        • There' is a number, oh haven’t you heard? It’s six point oh-two to the twenty third
      • Avogadro’s constant = the amount of a substance particles required to form one mole of a substance
        • 6.02 x 1023
  • Gas volumes
    • In standard conditions: 1 mol of a gas fills 24dm3 of space
      • Use this value instead of the mass in grams to work out the number of moles in a gas
  • Percentage yield
    • Get the theoretical yield (from a standard calculation)
      • The actual yield by the theoretical yield and times it by 100
        • If you get more than 100%, you screwed up!
        • If you get 100% and there’s more than one product? You screwed up!
  • Atom economy
    • All theoretical
    • It works out how effective your reaction is in terms of atoms in to useful atoms out
    • RFM of product/RFM of reactants x 100

Enthalpy changes and kinetics

Enthalpy level diagrams:
  • X axis = reaction stage (roughly time)
  • Y axis = energy level
    • Exothermic reactions have energy exit the system, so the energy level decreases
    • Endothermic reactions have energy enter the system, so the energy level increases
Definitions:
  • Standard conditions – 298K and 100kPa pressure (or 1Atm pressure)
    • Remember all standard enthalpy changes will be based on reactants in the states they are at in these conditions
  • Standard enthalpy change of reaction – The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state
  • Enthalpy change of formation – The standard enthalpy change of formation of a compound is the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions, and with everything in its standard state.
    • Formed from elements! So 1/2 O2 not 1 O
    • 1 mole of the product formed
  • Enthalpy change of combustion - The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
  • Bond enthalpy – Energy required on average to break 1 mole of a type of bond
  • Relative atomic mass – Mass of a particle relative to 1/12 of C12
    • Also the average mass of an element’s isotopes worked out by the ratios of their relative abundance
Calorimeter experiments:
  • Assumptions:
    • All energy is transferred into water (no loss into environment)
    • Calorimeter does not absorb any heat
    • Fuel has undergone complete combustion
  • Calculation to be done:
    • Mass x specific heat capacity x change in temp = change in energy
      • Then work this out per mole: energy produced/moles of fuel
  • Always negative enthalpy
Hess’s Law:
  • The total enthalpy change of a reaction is the same no matter what route you take
    • Therefore, using data values you can work out the enthalpy change of a reaction by going an alternate route
  • Draw the arrows on the hess’s law diagram according to the route they reaction would take – eg if you’re given combustion data then the lines are from the reactants and products to the combustion products below
Mass spectrometry:
  • Works by ionising the molecule to be tested through bombardment of electrons
  • Ions accelerated with an electric field and deflected with magnets
  • These ions then hit the detectors at different points as different masses will be moved more/less by the same magnetic field
  • The greatest mass is always the molecular ion – RFM of the original molecule
  • Other peaks are caused by fragmentation – splitting of molecule into two ionised chunks

Ionisation energy and atomic structure

Definition:
  • Ionisation energy – the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms
Evidence for electron shells:
  • Ionisation energy has steep jumps in it at point where shells change
    • These jumps mean that there must be a large difference in energy level
      • Energy level = shell
  • Subshells can be explained in a similar way
    • s orbitals – 1 pair (2 electrons)
      • Li and Be fill this, so B has a lower energy as it has started the p ortbital
    • p orbitals – 3 pairs (6 electrons)
      • N has a slightly higher ionisation energy than O, as it has got one electron in each of the 3 orbitals, and O has a second in one orbital which feels repulsion
    • d orbitals – 5 pairs (10 electrons)
      • Don’t worry about this
Melting and boiling points:
  • Giant molecular structures
    • C and Si
    • High melting points as there are lots of strong bonds so a high lattice energy
  • Metals
    • Li, Mg, Na, etc.
    • Sea of delocalised electrons create lots of bonds so high melting point
  • Simple covalent bonds
    • Small enough to melt unless held together by intermolecular forces
      • eg. London forces and hydrogen bonding
    • Intermolecular forces are weak so low melting point
Ionic bonding:
  • Evidence for ions:
    • Electrolysis allows splitting of particles by charge
    • Conduct charge in solution/when molten only
    • Electron density maps with 0 density in between
  • Ions will attempt to gain noble gas configuration
    • Anions are negatively charged  - they gain electrons
    • Cations are positively charged – they lose electrons
  • Ionic lattices are formed when oppositely charged ions mix, so they each try and get as far away from similarly charged particles as possible while also being attracted to oppositely charged ions
  • Trends in ionic radii:
    • Increase down groups (more electron shells)
    • Cations are smaller than anions
    • Isoelectronic ions depend on the number of electrons gained or lost:
      • N3- > O2- > F- > Ne > Na+ > Mg2+ > Al3+ 
      • The stronger the nuclear charge, the smaller the ionic radius
  • Born-Haber cycles are complicated Hess’s law cycles used to determine the reaction energy from formation of a lattice
    • Latice energy is always exothermic
      • That means it’s a negative number
  • Ions may be polarised
    • This is where their electron cloud is distorted from spherical due to additional charges acting on the ion
Covalent bonding:
  • Involves sharing a pair of electrons between two atoms
  • Dative bonds are when both electrons come from one atom
  • Can form double/triple bonds with multiple shared pairs
  • Lone pairs can also affect the shape of covalently bonded molecules as they contribute areas of electron density
Metallic bonding:
  • The attraction between metal ions and a sea of delocalised electrons
  • Electrical and thermal conductivity because of movement of electrons throughout the sea
  • Strong attraction due to the amount of electrons
  • Malleable as atoms can slide along and remain surrounded by electrons

Organic Chemistry

  • Alkanes
    • Saturated hydrocarbons
    • CnH2n+2
    • Fuels separated out by fractional distillation
      • Cracking and reforming used to change lengths of alkanes
        • Cracking results in a smaller chain and an alkene
          • Carried out by heating over a catalyst of porous pot
        • Reforming is also breakdown but increases the proportion of aromatic compounds
    • Reaction mechanism: Free radical substitution
      • Initiation step: Cl2  -> 2 Cl∙
      • Propagation step:
        • CH4 + Cl∙ → ∙CH3 + HCl
        • ∙CH3 + Cl2 → CH3Cl + Cl∙
        • Must maintain one free radical on either side at all times
      • Termination: Two free radicals combine
  • Alkenes
    • Unsaturated hydrocarbons
    • Contain a double bond
      • One sigma bond
      • One pi bond
        • Pi bonds are non-rotational so you get stereoisomerism
          • If you have matching functional groups on the top, it’s a cis-whatever-ene, else it’s trans
          • If you have no matching functional groups, the largest molecular mass is used to decide if something is z-whatever-ene or e-whatever ene
            • Z = zimmilar (how I remember it) – both sides big
    • Reactions:
      • Hydrogenation – forms an alkane
      • Halogenation – di-substituted halogenoalkane
        • (Yeah what?)
      • Addition of hydrogen halide (ie HCl, HBr) – Halogenoalkane
        • Gotta know the mechanism of this
        • Oxidation with acidified KMnO4  - Diol
      • Polymerisation