Experimental work Question 8

Paper 3 Section A Experimental work Question 8

 
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This page of questions on experimental work in preparation for Section A of Paper 3 can be marked as direct student access either for assigning as a test or for students to work on in their own time. If you do not wish to use student access, links to downloadable versions of the questions and, separately the worked answers, can be found at Printable versions of written tasks.

Note that this question relates to 6.1 “Investigation of rates of reaction experimentally and evaluation of the results”, but is HL only as Rate equations and reaction mechanisms are covered in AHL Topic 16.1.

A class of students wanted to find a possible mechanism for the reaction between propanone and iodine in the presence of an acid catalyst.

Three different burettes were set up in the laboratory. One contained an aqueous solution of 2.0 mol dm-3 propanone, CH3COCH3, another contained an aqueous solution of 0.010 mol dm-3 iodine, I2(aq), and the third contained 2.0 mol dm-3 hydrochloric acid solution, HCl(aq).

Students were instructed to place the required volumes of the propanone and iodine solution in a test-tube together with the required volume of water. They then added the specified volume of hydrochloric acid, mixed the contents and timed how long it took for the yellow colour of the iodine to disappear.

Table showing volumes to be added

The following results show the initial concentrations of the three reagents and the average time taken for the iodine colour to disappear.

(a) Explain how the value for the initial concentration of the iodine solution in Experiment number 3 was calculated. [1]

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(b) Explain why the colour of the iodine disappears as the reaction proceeds. [1]

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(c) Determine the order of reaction with respect to iodine. [1]

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(d) Determine the rate equation for the reaction. [1]

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(e) State what can be deduced about the slow step of the reaction in terms of the reactants. [1]

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(f) Use your knowledge of chemistry to suggest a possible first step in the mechanism of the reaction if it is assumed that the first step is the slowest step. [1]

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