⚡ Activation Energy & Collision Theory
Cambridge IGCSE Chemistry 0620 · Definitions, Energy Profiles & Collision Outcomes
Section A — Definitions (10 marks)
Define the term activation energy.
On an energy profile diagram, state what the activation energy represents graphically.
What is meant by the term 'transition state' in the context of activation energy?
Define collision theory and state the TWO conditions required for a successful reaction.
A catalyst is added to a reaction. Explain, in terms of activation energy, how a catalyst increases the reaction rate.
Section B — The Energy Hump (10 marks)
Many exothermic reactions require a spark or flame to begin, even though they release energy overall. Explain why, using the concept of activation energy.
Draw and label a fully annotated energy profile diagram for an EXOTHERMIC reaction, clearly indicating: (i) reactants, (ii) products, (iii) activation energy (Eₐ), and (iv) the overall enthalpy change (ΔH).
On an energy profile diagram, how does adding a catalyst change the shape of the curve? Draw the original and catalysed pathways.
An endothermic reaction has a high activation energy. Explain why this reaction proceeds very slowly at room temperature.
The activation energy of Reaction X is 50 kJ/mol and Reaction Y is 200 kJ/mol. Both are exothermic. Which reaction proceeds faster at the same temperature and concentration? Justify your answer using activation energy.
Section C — Collision Outcomes (10 marks)
What happens when two particles collide with energy LESS than the activation energy? Explain at the atomic level.
Increasing the temperature increases the rate of reaction. Explain this using BOTH collision frequency AND activation energy concepts.
Two hydrogen molecules collide head-on with sufficient energy. However, no reaction occurs. Suggest ONE reason why this collision is unsuccessful despite having enough energy.
A Maxwell-Boltzmann distribution curve shows the distribution of molecular energies in a gas at temperature T. (a) On the curve, mark where Eₐ falls. (b) Shade the area representing molecules that CAN react. (c) Explain what happens to this shaded area when temperature increases to T₂.
A student claims: 'Adding more reactant always increases the rate because particles collide more often.' Critically evaluate this statement using collision theory and activation energy.
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