IB syllabus > kinetics (hl) > 16.3 

These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

16.3 - Activation energy

16.3.1: Describe qualitatively the relationship between the rate constant (k) and temperature (T).

Effect of temperature on the reaction rate

Direct observations make it clear that increasing the temperature increases the rate of a chemical reaction. In approximate terms, most reactions double in rate for a ten degree increase in temperature.

This effect was first quantified by Arrhenius who produced the equation:

rate constant k = Ae-Ea/RT
quantity description meaning
A - the Arrhenius constant This is a measure of the proportion of molecules that collide with enough energy to react AND which have the correct orientation for successful collision.
e - the natural number on which the natural logarithms is based 2.303
Ea - the activation energy This is the minimum energy that a molecular collision must have before it can be successful and lead to reaction - units kJ mol-1
R - the universal gas constant 8.314 in SI units
T - the Absolute temperature in Kelvin (K) Equal to the temperature in Celsius + 273

16.3.2 Determine activation energy values from the Arrhenius equation by a graphical method.
The Arrhenius equation: and its logarithmic form are provided in the chemistry data booklet . Use of simultaneous equations will not be assessed.

Using the Arrhenius equation

Although it is not easy to see the relationship between the rate constant and the absolute temperature from the equation, if we break it down into steps perhaps it will help.

Example: Calculate the rate constant when T = 300K (A = 0.3, Ea = 50kJ mol-1)

k = Ae-Ea/RT

Ea/RT = 50000/(8.314 x 300) = 20.05

e-Ea/RT = 1.97 x 10-9

k = Ae-Ea/RT

k = 5.90 x 10-10