In this chapter of AS/A-Level Chemistry, we will learn about rate of reaction and activation energy.

We can calculate the activation energy using the Arrhenius equation:

k = Ae^_Ea/RT

Where;

k = rate constant E_A = activation energy (J)

T = temperature (K) R = gas constant (8.31 JK^-1mol^-1)

A = another constant

Some relationships to note:

1. As E_A increases, k will get smaller. Therefore, large activation energy, means a slow rate – this makes sense!
2. As T increases, k increases. Therefore at high temperatures, rate will be quicker – this makes sense too!

If we “ln” both sides of Arrhenius’ equation, we get;

ln k = – E_A/RT + ln A

This looks a bit like:

y = mx + c

If we plot ln k (y) against 1/T (x), the gradient we produce will be –E_A/R (m). Then R is just a number that we know (8.31 JK^-1mol^-1) we can rearrange and find the activation energy in AS/A-Level Chemistry.

Iodine clock reaction

e can use iodine clock reaction as an example to demonstrate the calculation under the AS/A-Level Chemistry.

S₂O₈^2- (aq) + 2I^- (aq) —> 2SO₄^2- _(aq) + I₂ (aq)

Rate of reaction is inversely proportional to the time taken for the solution to change colour

i.e. increased rate = decreased time taken

k α 1/t

We can say that 1/t is the same as k (rate constant) and we can substitute 1/t instead of k in Arrhenius’ equation and find the gradient again to find a value for E_A

Congratulations! Now you understand the calculation of activation energy in AS/A-Level Chemistry.

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