chem - stuff i got wrong

reasons for difference between electron affinities

1st electron affinity is exothermic as negatively charged electron is attracted to positively charged nucleus

2nd electron affinity endothermic as negatively charged electron repels negatively charged charged ion

why enthalpy of neutralisation is more exothermic for weak acids

weak acids only partially dissociate in solution so some energy is required to break the C-H bonds completely

why ligands with water are slightly acidic

metal cation polarises the O-H bonds so that some H+ ions are released into solution

why free radical substitution is not efficient

mixture of organic products formed

need to separate desired product from mixture

why bromine attacks before chlorine in electrophilic addition

bromine less electronegative so attacks first, forms secondary carbocation with more electronegative electrophile as this is more stable

why you dry hydrated crystals in a warm oven

to ensure water of crystallisation is not removed

why filter paper and funnel warmed before carrying out gravity filtration

so that solution doesn’t cool down significantly and cause premature crystallisation

ionic equation for quenching agent for kinetics core practical

NaHCO3 + H+ → CO2 + H2O + Na+

why test tube clamped at an angle

ethanol could leak down the tube

when bunsen valve used

to prevent suck back

experimental procedure to determine enthalpy change of solution for hydrated salts

weigh out hydrated salt accurately

weight out water mass of distilled/deionised water accurately

add water to polystyrene cup and measure initial temp

add hydrated salt and put insulating lid on container, measure temp of reaction mixture at intervals

plot graph of temperature against time and extrapolate it to find maximum temperature reached

how chromatography separates compnents of a mixture

each component in mixture is attracted to mobile and stationary phases but more strongly to one than the other

why pH of pure water is not always 7

As the temperature increases, equilibrium shifts to the right as the forward reaction is endothermic so Kw gets bigger

H+ ion concentration is increased so that pH is less than 7

why solutions act as buffers at beginning of titration curve

Both HA and A- present

On addition of OH- ions equilibrium shifts right so that H+ ions react with added OH- ions

why heat and reflux is used

heat - so that rate of reaction is sped up

reflux - to prevent escape of products/reactants

acidic hydrogen oxygen fuel cell

anode - H2 → 2H+ + 2e-

cathode - 1/2O2 + 2H+ +2e- → H2O

alkaline hydrogen oxygen fuel cell

anode - H2 + 2OH- → 2H2O + 2e-

cathode - O2 + 2H2O + 4e- → 4OH-

what happens if products formed in gaseous state rather than liquid

enthalpy change less exothermic as energy is taken into to form a gas

chromatography

mobile phase - molecules move, always liquid/gas

stationary phase - molecules don’t move, solid or liquid on solid support

mobile phase moves through stationary phase, distance substance moves depends on solubility in mobile phase and retention in stationary phase

HPLC

stationary phase - small particles of a solid packed into a column

liquid mobile phase - polar mixture, forced through column under high pressure

mixture is separated as different parts attracted by different amounts, UV light passed through which is absorbed by parts of the mixture as they come through and UV detector measures UV light absorbed - chromatogram produced shows retention times of components of mixture

gas chromatography

sample being analysed injected through stream of gas, gas carries it through coiled column which is coated with viscous liquid or solid

components of mixture constantly dissolve in oil/adsorb onto solid and evaporate back into gas, different components identifed by retention times