The Energy Hill
📖 The Energy Hill
Activation energy is like cycling over a hill! Even if you're going to end up lower (exothermic), you still need to pedal up first. That initial "push" is the activation energy — it gets the reaction started — without this initial push, the molecules cannot rearrange into products. Catalysts are like tunnels through the hill — they give you an easier, lower route to the same destination!
Chemistry glossary
- What is activation energy?
- The minimum energy particles need to react when they collide
A reaction profile (also called an energy level diagram) is a graph that shows how the energy of the reacting particles changes as they go from reactants to products. The y-axis shows the energy (in kJ/mol), and the x-axis shows the "progress of reaction".
Earn the mark scheme marks
🧠 Memory Aids
To draw the profile correctly: "Reactants UP to peak, then DOWN to products"
The curve always goes up first (activation energy required) then back down (either ending lower for exothermic or higher for endothermic).
For catalysts: "Same start, same end, lower middle"
A catalyst lowers the peak (the "middle" of the profile) but the start point (reactants) and end point (products) stay exactly the same. ΔH is unchanged.
Ea vs ΔH: "Ea is from the FLOOR, ΔH is from the START to FINISH"
Ea = from reactants level up to the peak. ΔH = from reactants level to products level. These are different measurements — do not confuse them.
Now try it yourself
Quiz · Question 1 of 28
What does activation energy represent on a reaction profile?
Tap an answer to check it
This topic in real past papers
Every real exam question we've found on reaction profiles, with a full worked answer.
AQA Paper 1
Complete or interpret a reaction profile diagram showing activation energy and overall energy change, or calculate an unknown bond energy using given bond energies and the overall energy change.
Edexcel Paper 2
Discuss what catalysts do and their effect on activation energy, using named examples