| Date | May 2014 | Marks available | 3 | Reference code | 14M.3.sl.TZ1.8 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | State and Suggest | Question number | 8 | Adapted from | N/A |
Question
Although crude oil is considered an extremely important energy source, it cannot be used directly as a resource.
Suggest why crude oil needs to be refined before it can be used.
Thermal cracking, catalytic cracking and steam cracking can all be used to convert molecules of alkanes into alkenes.
(i) State the type of cracking which can be used to crack ethane into ethene, the chemical equation for the process and one reaction condition required.
Type of cracking:
Chemical equation:
Reaction condition:
(ii) Suggest one use for the other product formed in this reaction in addition to ethene.
Markscheme
viscous / varied composition / complex mixture / has to be broken down into more usable substances / OWTTE;
(i) Type of cracking:
steam;
Chemical equation:
\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}} \to {{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}} + {{\text{H}}_{\text{2}}}\);
One reaction condition:
high temperature range / low pressure;
Allow any temperature if specified in the range 800–1400 °C/1073–1673 K.
Award [2] for all three correct, [1] for any two correct.
(ii) fuel (eg, in space vehicles) / to make fertilizer (on reaction with nitrogen) / margarine manufacture / reduction of metal ores;
Do not apply ECF from (i).
Accept other reasonable answers.
Examiners report
Candidates struggled with this option. The few who attempted this option had difficulties with almost all the questions. For part (a) ‘refined’ was interpreted as impurities that needed to be removed. The idea of different fractions used as fuels in the crude oil was missed by majority of the candidates. Many did not provide a complete response by comparing crude oil as a fuel and as a feedstock; many only addressed one of these two. Candidates also had difficulty providing examples for crude oil components and feedstock. Students did not score well in this part. Part (b) was also challenging for the students. Most students performed poorly unable to provide examples for feedstock and crude oil fuel fractions. About half the candidates gave thermal cracking as the response for (c)(i), and were not able to score the point. Majority of the candidates gave the correct response for the uses of the products of cracking for (c)(ii).
Candidates struggled with this option. The few who attempted this option had difficulties with almost all the questions. For part (a) ‘refined’ was interpreted as impurities that needed to be removed. The idea of different fractions used as fuels in the crude oil was missed by majority of the candidates. Many did not provide a complete response by comparing crude oil as a fuel and as a feedstock; many only addressed one of these two. Candidates also had difficulty providing examples for crude oil components and feedstock. Students did not score well in this part. Part (b) was also challenging for the students. Most students performed poorly unable to provide examples for feedstock and crude oil fuel fractions. About half the candidates gave thermal cracking as the response for (c)(i), and were not able to score the point. Majority of the candidates gave the correct response for the uses of the products of cracking for (c)(ii).
