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Date May 2013 Marks available 1 Reference code 13M.2.hl.TZ1.3
Level HL Paper 2 Time zone TZ1
Command term Define Question number 3 Adapted from N/A

Question

To determine the activation energy of a reaction, the rate of reaction was measured at different temperatures. The rate constant, k, was determined and lnk was plotted against the inverse of the temperature in Kelvin, T1. The following graph was obtained.

M13/4/CHEMI/HP2/ENG/TZ1/03

Define the term activation energy, Ea.

[1]
a.

Use the graph on page 8 to determine the value of the activation energy, Ea, in kJmol1.

[2]
b.

On the graph on page 8, sketch the line you would expect if a catalyst is added to the reactants.

[1]
c.

Markscheme

minimum energy needed to react/start a reaction / energy difference between reactants and transition state;

a.

gradient of the line: –63;

Accept –60 to –65.

Ea (=R×gradient)=0.52 (kJmol1);

Accept 0.50 to 0.54.

b.

gradient of the line less steep (less negative);

Accept any position as long as gradient less steep.

c.

Examiners report

The idea of activation energy being a minimum was seldom communicated. Few were able to follow through all the mathematics to find Ea by a graphical method and those that did had often omitted 102 in their calculations. The answers were often poorly set out so it was difficult to assess the award of part marks; indeed, many candidates seemed to hope that a correct answer would somehow emerge from a mass of incomprehensible figures.

a.

The idea of activation energy being a minimum was seldom communicated. Few were able to follow through all the mathematics to find Ea by a graphical method and those that did had often omitted 102 in their calculations. The answers were often poorly set out so it was difficult to assess the award of part marks; indeed, many candidates seemed to hope that a correct answer would somehow emerge from a mass of incomprehensible figures.

b.

The idea of activation energy being a minimum was seldom communicated. Few were able to follow through all the mathematics to find Ea by a graphical method and those that did had often omitted 102 in their calculations. The answers were often poorly set out so it was difficult to assess the award of part marks; indeed, many candidates seemed to hope that a correct answer would somehow emerge from a mass of incomprehensible figures. The gradient of the graph for (c) was generously marked; all candidates had to do was to realize that the catalyst would lower the activation energy and thus the gradient would be less negative. As long as a line with less negative gradient was drawn, the mark was awarded.

c.

Syllabus sections

Core » Topic 6: Chemical kinetics » 6.1 Collision theory and rates of reaction
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