How does a catalyst work?
4 marks · NormalThe table shows energy values for a chemical reaction.
| Description | Energy (kJ/mol) |
|---|---|
| Reactants | 75 |
| Peak without catalyst | 142 |
| Peak with catalyst | 95 |
| Products | 45 |
Calculate the activation energy for the uncatalysed reaction.
Calculate the activation energy for the catalysed reaction.
Calculate the overall energy change () for the reaction.
State how the catalyst changes the reaction profile diagram.
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(a) 67 kJ/mol
(b) 20 kJ/mol
(c) -30 kJ/mol
(d) The catalyst lowers the activation energy, shown on the diagram as a lower peak for the catalysed pathway.
Common mistakes: Subtracting in wrong order for activation energy; Giving negative answer for activation energy
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Marking points
A1 (a) · 1 mark · Activation energy (without catalyst) = peak (no catalyst) - reactants (142 - 75 = 67 kJ/mol)
A2 (b) · 1 mark · Activation energy (with catalyst) = peak (with catalyst) - reactants (95 - 75 = 20 kJ/mol)
A3 (c) · 1 mark · Enthalpy change () = products - reactants (45 - 75 = -30 kJ/mol)
B1 (d) · 1 mark · States that the catalyst lowers the activation energy, which appears on the diagram as a lower peak or curve for the catalysed pathway.
Worked solution
a) Activation energy (uncatalysed) = 142 - 75 = 67 kJ/mol
b) Activation energy (catalysed) = 95 - 75 = 20 kJ/mol
c) Overall energy change () = 45 - 75 = -30 kJ/mol
d) The catalyst lowers the activation energy, so on the diagram the peak for the catalysed reaction is lower than for the reaction without a catalyst.