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, T−1. The following graph was obtained.
Define the term activation energy, Ea.
Use the graph on page 8 to determine the value of the activation energy, Ea, in kJmol−1.
On the graph on page 8, sketch the line you would expect if a catalyst is added to the reactants.
Markscheme
minimum energy needed to react/start a reaction / energy difference between reactants and transition state;
gradient of the line: –63;
Accept –60 to –65.
Ea (=−R×gradient)=0.52 (kJmol−1);
Accept 0.50 to 0.54.
gradient of the line less steep (less negative);
Accept any position as long as gradient less steep.
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 10−2 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 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 10−2 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 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 10−2 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.