Uploaded June 2020 | Updated September 2026, 2 weeks ago
This video gives a complete 35 min lesson on how to answer a common A-level Chemistry question. Two methods are shown: a hand calculation and a calculation using Microsoft Excel.
0.00 = Introduction to the Arrhenius equation
8.47 = Question
9.32 = Calculation by hand
18.29 = Calculation using Microsoft Excel
The Arrhenius equation (k = Aexp(-Ea/RT)) is a formula for the temperature dependence of reaction rates. It was proposed in 1889 by the Swedish chemist Svante Arrhenius (1859-1927). He was awarded the Nobel Prize for Chemistry in 1905. It is often described as an ‘empirical equation’ because it is based on experimental evidence. The term comes from the Greek word for ‘experience’. This is in contrast to an equation based only on theory.
The exponential part of the Arrhenius equation accounts for the activation energy: reactions with a low activation energy have a high reaction rate. The fact this is an exponential is no surprise: chemical reactions are known to be particularly sensitive to temperature.
The pre-exponential factor (A) can be factorised into the product of the steric factor (P) and the frequency factor (Z). The steric factor accounts for collision geometry: reactions that require a very specific collision angle are slower. The frequency factor accounts for collision frequency: more frequent collisions lead to a higher reaction rate. The frequency factor does have a weak temperature dependence, and this can lead to a slight curve on an Arrhenius graph.
The pre-exponential factor (A) can be found from the graph by reading the y-intercept: this often requires a lengthy extrapolation.
Activation energies are always positive and normally lie in the range 50-200 kJ/mol. This is slightly lower than the strength of an average covalent bond (approximately 300 kJ/mol). This is because the peak in an energy profile diagram often corresponds to partial bond fission.
This video gives a complete 35 min lesson on how to answer a common A-level Chemistry question. Two methods are shown: a hand calculation and a calculation using Microsoft Excel.
0.00 = Introduction to the Arrhenius equation
8.47 = Question
9.32 = Calculation by hand
18.29 = Calculation using Microsoft Excel
The Arrhenius equation (k = Aexp(-Ea/RT)) is a formula for the temperature dependence of reaction rates. It was proposed in 1889 by the Swedish chemist Svante Arrhenius (1859-1927). He was awarded the Nobel Prize for Chemistry in 1905. It is often described as an ‘empirical equation’ because it is based on experimental evidence. The term comes from the Greek word for ‘experience’. This is in contrast to an equation based only on theory.
The exponential part of the Arrhenius equation accounts for the activation energy: reactions with a low activation energy have a high reaction rate. The fact this is an exponential is no surprise: chemical reactions are known to be particularly sensitive to temperature.
The pre-exponential factor (A) can be factorised into the product of the steric factor (P) and the frequency factor (Z). The steric factor accounts for collision geometry: reactions that require a very specific collision angle are slower. The frequency factor accounts for collision frequency: more frequent collisions lead to a higher reaction rate. The frequency factor does have a weak temperature dependence, and this can lead to a slight curve on an Arrhenius graph.
The pre-exponential factor (A) can be found from the graph by reading the y-intercept: this often requires a lengthy extrapolation.
Activation energies are always positive and normally lie in the range 50-200 kJ/mol. This is slightly lower than the strength of an average covalent bond (approximately 300 kJ/mol). This is because the peak in an energy profile diagram often corresponds to partial bond fission.
