Knowledge Organiser: Reaction Profiles
Key Terms
- Reaction profile — energy diagram of a reaction
- Activation energy (Ea) — reactants to peak
- ΔH — reactants to products (overall)
- Peak — the highest-energy point on the profile
- Catalyst — lowers Ea, unchanged at end
Must-Know Facts
- Exothermic: products LOWER than reactants
- Endothermic: products HIGHER than reactants
- Ea = from reactants UP to the peak
- Catalyst: lower peak, same ΔH, same start/end
- Axes: y = Energy, x = Progress of reaction
- Ea is always required — even for exothermic reactions
Key Equations
- ΔH = energy level of products − energy level of reactants
- Exothermic: ΔH negative (products lower on diagram)
- Endothermic: ΔH positive (products higher on diagram)
- Ea = energy from reactant level to peak of profile
Common Mistakes
- Drawing Ea from the peak to the products: Activation energy is measured from the REACTANTS up to the peak — not from the peak down to products
- Saying a catalyst changes ΔH: A catalyst only lowers the activation energy (shorter peak) — the start, end and overall ΔH remain exactly the same
- Forgetting to label axes correctly: y-axis = Energy (kJ/mol); x-axis = Progress of reaction (or Reaction coordinate) — missing labels lose marks
- Drawing exothermic profiles with products higher: In an exothermic reaction, products are at a LOWER energy level than reactants — draw the right side of the curve lower than the left
Memory Aid
- 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.