| Date | November 2010 | Marks available | 10 | Reference code | 10N.2.HL.TZ0.B3 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 0 |
| Command term | Describe, Discuss, and State | Question number | B3 | Adapted from | N/A |
Question
This question is about nuclear fission.
Some nuclear reactors have a heat exchanger that uses a gas that is kept at constant volume. The first law of thermodynamics can be represented as \(Q = \Delta U + W\).
(i) State the meanings of \(Q\) and \(W\).
\(Q\):
\(W\):
(ii) Describe how the first law of thermodynamics applies in the operation of the heat exchanger.
(iii) Discuss the entropy changes that take place in the gas and in the surroundings.
Markscheme
(i) \(Q\): is the energy transferred between the system and surroundings;
\(W\): work done on/by system;
(ii) \(Q\) transferred from reactor to gas;
no change in volume therefore \(W = 0\);
internal energy of gas increases;
\(Q\) transferred from gas to surroundings therefore internal energy of gas
decreases;
(iii) entropy of the gas initially increases as energy transferred from the reactor;
entropy of the surroundings increases as energy transferred (from the gas);
entropy of gas decreases on cooling;
overall the entropy of the total system increases;
Examiners report
(i) The meanings of \(Q\) and \(W\) were often expressed poorly and incompletely.
(ii) Some candidates are confused about the statement of the first law of thermodynamics and quoted the second. Others gave vague and uncreditworthy accounts that showed that they were not answering in the context of the reactor heat exchanger. There was no real attempt by most candidates to arrive at four points in the answer, despite the fact that there are two exchanges going on in the reactor.
(iii) As in part (ii) some candidates did not focus on the context intended. The failure to identify all exchanges and entropy changes was common as in part (ii).
