Activation Energy of a reaction, Ea, is the minimum amount of energy reactant molecules must possess in order to form products.
On an Energy Profile, the activation energy is measured from the energy of the reactants to the peak of the energy profile diagram.
The lower the activation energy, the faster the reaction will proceed.
Enthalpy change, ΔH, is the amount of energy absorbed or released by the reaction.
On an Energy Profile, the enthalpy change is measured from the energy of the reactants to the energy of the products.
Exothermic reaction, energy is released by the reaction, the energy of the reactants is greater than the energy of the products. (ΔH is negative)
Endothermic reaction, energy is absorbed by the reaction, the energy of the reactants is less than the energy of the products. (ΔH is positive)
Catalysts speed up the rate of reaction by lowering the activation energy without being consumed by the reaction.
A catalyst does not alter the enthalpy change value for the reaction.
An enzyme is a biological catalyst.
It contains active sites on its surface. These sites are capable of bonding only to specific molecules called substrates.
The lock and key theory proposes that the active site is a cavity of fixed shape which only a substrate molecule can fit into properly to undergo reaction.
Inhibitors (negative catalysts) are substances which slow down a reaction.
Surface catalysed reactions can be inhibited when a foreign substance bonds at the catalyst's active sites blocking them for substrate molecules.
This type of inhibition is called poisoning and the inhibitor (negative catalyst) is called a poison.
Examples of Energy Profiles
Type of Reaction
Faster Reaction Lower activation energy
Slower Reaction Higher activation energy
Catalysed Reaction A catalyst lowers the activation energy