Data response Example 2

Question 2

This question is taken from the May 2009 TZ1 Paper 2 Standard Level examination. This paper is copyright IB so cannot be reproduced here but you can obtain this paper from the IB store. The same question appeared on the Higher Level May 2009 TZ1 Paper 2 examination.

Answers and comments

(The minimum answer required on the markscheme for each mark is given in the centre first.)

Vegetable oil and biodiesel both contain the –CO-OR (ester) group. What actually happens during the production of biodiesel is a transesterification reaction whereby one ester is converted into another.

Questions which involve changing amounts in mass to amounts in moles are very common in data response questions.  Once the molar mass is known then the amount in moles is simply the mass in grams divided by the molar mass in grams mol-1 to give the answers in moles.

1 .144 mol of vegetable oil would require 3 x 1.144 = 3.432 mol of methanol to react completely. Since 6.204 moles of methanol were present it is clear that the methanol is in excess by about a factor of two. One reason for this is to help to shift the position of equilibrium to the product side to increase the yield of biodiesel [see (c)(iii)].

                 (c)

It is important to distinguish between static and dynamic equilibrium as in chemistry the reaction does not stop when equilibrium is reached. At the point of equilibrium the rates of the forward and reverse reactions are equal so that the concentrations of products and reactants remain constant.

Note that since there are 3 moles each of methanol and biodiesel in the balanced equation then the concentrations of these two species are raised to the power of three in the equilibrium expression.

This is a straightforward application of Le Chatelier’s Principle. The true explanation lies in the equilibrium expression above since, if the concentration of methanol is increased, for Kc to remain constant then the concentration of products must also increase.

Catalysts have no effect on the position of equilibrium. However they do decrease the time taken for equilibrium to be achieved due to the reasons given in the markscheme.

The ester group only has very low polarity so esters such as vegetable oils and biodiesel are insoluble in water or polar solvent such as methanol.

The maximum amount of biodiesel that could be obtained from 1.144 mol of vegetable oil is 3.432 mol (see part (b) above) - that is 1018 g. The actual amount obtained is 811.0 g so the percentage yield is equal to 79.67%. Since both masses are given to four significant figures the answers should also be expressed to four significant figures.

This was set to test Aim 8 on the old programme and is probably not really on the core/AHL anymore but is covered under Option C - sub topic C.5 Environmental impact - global warming .  As a teacher you should refer to environmental and global issues such as this generally throughout the programme and give examples as the impact of chemistry on society also falls under nature of science.

General comments on Example 2

Along with the question on the comparable May 2009 TZ2 paper this was the first genuine data response question on the new programme which was first examined in 2009. After the exam a few teachers complained about the question because the production of biodiesel was not on the pre-2014 chemistry programme. This is the whole point of a data response question. Sufficient information is given which will enable students to answer the questions that follow. Students were provided with the necessary information about biodiesel as they could not be expected to know it. However all the questions that then followed were testing the principles of chemistry that clearly are on the syllabus. The question did not test just one area of the programme but included chemistry from Topics 1, 3, 4, 6,7, 10 and 11. This is a good example of the holistic nature of chemistry and shows how topics inter-relate rather than simply stand on their own.

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