Chapter 15 · Chemical Equilibrium
15.12The Path of a Reaction and the Effect of a Catalyst
10 min · two checks
Predict
Does a catalyst make an endothermic reaction exothermic?
The idea
- Read an energy diagram: activation energy versus ΔH.
- State what a catalyst changes.
A reaction-energy diagram plots energy against the progress of the reaction. Reactants sit on the left, products on the right. The hump between them is the activation energy. If products lie below reactants, ΔH is negative and the reaction is exothermic. The size of the hump is not the size of ΔH. A reaction can release a lot of heat and still be slow if the hump is high, which is why a pile of wood does not burst into flame at room temperature.
A catalyst provides a different path with a lower hump. Both directions get the lower barrier, so the rate forward and the rate backward increase, and the meeting point — equilibrium — does not move. Enzymes are biological catalysts, extraordinarily specific, and they obey the same rule: they speed a reaction that thermodynamics already allows. They do not let a reaction with a tiny K produce a flood of product. Heat also speeds reactions, but heat changes K and can cook a living system. Catalysts are the more precise tool.
Keep these
- Activation energy is the hump. ΔH is the net drop or climb.
- Catalysts lower the hump for both directions. K is unchanged.
- Enzymes are catalysts. They do not change what equilibrium allows.