Paper 2 Markschemes

Question 1

Explain why a honeycomb structure is used in a catalytic converter in a car

Answer 1

-increases the surface area of the platinum catalyst, providing more active sits for adsorption
-the honeycomb structure allows gases to flow through the exhaust

Question 2

Why do different amino acids have different Rf values

Answer 2

-different affinity for the stationary phase
-different solubility in the mobile phase

Question 3

Why are argon and nitrogen suitable carriers for gas chromatography

Answer 3

-they do not react with the components of the mixture (inert)

Question 4

In an airbag, metal oxides are made to react with silicon dioxide. What kind of reaction is occurring and why?

Answer 4

-neutralisation reaction occurs
-this is needed because the metal oxides are corrosive, but the salts they form when reacted with SiO2 are inert and so dont cause harm

Question 5

How do anti-bumping granules prevent bumping

Answer 5

Prevent uncontrolled boiling by distributing heat more evenly and providing a surface for bubbles to form

Question 6

Why is using distillation apparatus instead of reflux not an efficient way to produce ethanoic acid from ethanol

Answer 6

-ethanol is first oxidised to ethanal
-ethanal will distil before ethanoic acid can be formed due to the low boiling temperature of ethanal

Question 7

Devise an experiment that would enable the relative rates of hydrolysis of Halogenoalkanes to be determined

Answer 7

-dissolve the same volume of each haloalkane into the Sam volume of ethanol and place into a water bath
-add an equal volume of silver nitrate in nitric acid to three other tubes and place in the water bath
-wait some time so all tubes reach the same temperature, then mix the acidified silver nitrate into one of the haloalkanes and measure the time taken for precipitate to form

Question 8

Explain the trends in rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane

Answer 8

1-iodobutane has the fastest rate of hydrolysis, then 1-bromobutane, then 1-chlorobutane has the slowest rate of hydrolysis
This is because the C-Cl bond is stronger than the C-Br bond which is stronger than the C-I bond so requires more energy to break

Question 9

What is meant by the term chiral

Answer 9

A chiral molecule forms two isomers which are non-superimposable as the chiral carbon has four different groups attached

Question 10

Compare and contrast the mechanism of hydrolysis of the primary haloalkane RCH2X with the tertiary haloalkane R3CX, using aqueous potassium hydroxide

Answer 10

-both undergo nucleophilic substitution
-hydrolysis mechanism for RCH2X is SN2 but for R3CX is SN1
-RCH2X and OH- are in the rate determining step
-only R3CX is in the rate determining step
-RCH2X forms a transition state whereas R3CX forms a carbocation intermediate (don’t forget to draw them if asked)

Question 11

Give one advantage and one disadvantage of the disposal of polystyrene by incineration

Answer 11

+ve= releases a large amount of energy that can be used to generate electricity
-ve=releases CO2 which is a greenhouse gas that will contribute to global warming (and polycyclic aromatics release toxic fumes when burned)

Question 12

Compare and contrast the bromination of phenol vs benzene

Answer 12

-both undergo electrophilic substitution
-benzene requires FeBr3 catalyst and phenol doesn’t
-the lone pair on the oxygen atom in phenol interacts with the benzene ring of delocalised electrons so electrophilic attack more likely, hence why phenol gets tri-substituted to form 2,4,6-tribromophenol but benzene only monosubstituted
-phenol down at RTP. But benzene needs to be heated under reflux

Question 13

Why would the order of reaction with respect to iodide ions not be five, even though 5 moles of iodide ions are shown in the equation for the reaction

Answer 13

The chance of five or more ions colliding in the correct orientation in the rate determining step is negligible

Question 14

Why should the total volume of an experiment in which you’re varying the volume of one reactant but keeping the concentrations of other reactants be kept the same

Answer 14

So that the volume of the thing you’re changing is proportional to its concentration (needed when calculating its order of reaction)

Question 15

What’s the purpose of starch is the iodine clock

Answer 15

Starch acts as an indicator, as the time taken for starch to turn from colourless to blue-black can be used to calculate rate of reaction

Question 16

Explain the shape of PCl3

Answer 16

-trigonal pyramidal
-the central P atom has three bond pairs and one lone pair
-electron pairs repel to positions which minimise repulsion between them

Question 17

Explain why phosphorus forms PCl5 but nitrogen doesn’t form NCl5

Answer 17

-phosphorus can expand its octet
-nitrogen can only accommodate eight electrons in its outer shell

Question 18

Explain why NCl3 has a higher boiling temperature than NF3

Answer 18

-London forces are greater in NCl3 than NF3 because the NCl3 molecule has more electrons than NF3
-although the permanent dipole-dipole interactions are stronger in NF3 because F is more electronegative than CL, London forces are more significant so more energy is required to overcome the intermolecular forces between NCl3 molecules than NF3 molecules

Question 19

What is the compound responsible for a peak at 0 on NMR and what is its purpose

Answer 19

Tetramethylsilane so that chemical shifts due to other hydrogen atoms can be compared

Question 20

What things need to mentioned if asked to justify the identification of an NMR H peak

Answer 20

-splitting pattern shows (n-1) neighbouring H atoms
-peak area of X means X H atoms in this environment

Question 21

Equation for the regeneration of the AlCl3 catalyst in friedel crafts

Answer 21

AlCL4- + H+ —> AlCl3 + HCl

Question 22

Explain whether phenol is more or less likely to be reactive than benzene with a nucleophilic addition reaction

Answer 22

-phenol more reactive because the lone pair on the oxygen of the OH group delocalises and donates into the benzene ring, increasing the electron density of the ring so that it is more susceptible to electrophilic attack

Question 23

Explain why different compounds will have different retention times in the same column in gas chromatography

Answer 23

-retention time depends on affinity of the component for the stationary phase
-the greater the affinity of the component for the stationary phase means the greater the retention time

Question 24

Explain how a catalyst increases the rate constant k

Answer 24

K increases because the catalyst provides and alternate reaction pathway with a lower activation energy, so a greater proportion of molecules have energy greater than the activation energy so more frequent successful collisions occur

Question 25

Explain how impurities in gaseous reactants could make a heterogenous catalyst less effective

Answer 25

-impurities adsorb to the catalyst surface, so fewer active sites are available for the reactants to adsorb to for a reaction to take place -impurities bond very strongly to the catalyst surface so are less likely to desorb