Rates of Reaction

What activation energy is

• Activation energy, E_a, is the minimum energy that colliding reactant particles must have in order to react
• Below E_a, even a head-on collision just bounces apart; above E_a, the existing bonds can start to break and new bonds form
• Different reactions have different activation energies: rusting has a small E_a (rust forms slowly at room temperature) while burning natural gas has a large E_a (needs a spark or flame to start)

Reaction profiles

• A reaction profile plots the energy of the reactants, the products, and the transition state in between as the reaction progresses
• The horizontal axis is "progress of reaction"; the vertical axis is "energy"
• Two key features:
  • The vertical distance from the reactants line up to the peak of the profile is the activation energy E_a
  • The vertical distance from the reactants line to the products line is the overall enthalpy change ΔH
• For an exothermic reaction, products sit lower than reactants and ΔH is negative
• For an endothermic reaction, products sit higher than reactants and ΔH is positive
• In both cases the activation-energy hump rises above the starting energy of the reactants

Catalysts on the reaction profile

• A catalyst lowers the activation-energy hump without changing the energies of the reactants or the products
• On the reaction profile, the curve has a lower peak but the same starting and ending levels
• The overall ΔH is unchanged — a catalyst does not change how exothermic or endothermic a reaction is, only how fast it gets there