Activation Energy Chemistry Notes
Activation Energy :
→ We know that it is necessary to collide reactant molecules with each other for the completion of reaction. But, products are not formed as a result of all collisions taking place between reactant molecules. The collisions between reactant molecules by which products are formed, are called effective collisions.
→ The number of effective collisions is very less than number of total collisions. A certain minimum enrgy is required for effective collisions between reactant molecules in a chemical reaction. This certain minimum energy is called threshold energy.
→ Only those molecular collisions are effective whose energy is either equal to or more than threshold energy The molecules, which have energy less than thershold energy, can’t show effective collision. So for effective collision we have to provide certain extra amount of energy to them, this extra amount of energy which is provided to molecules, known as activation energy.
→ “The minimum extramount of energy which is essentially provided to molecules for effective collisions of molecules, is called activation energy (Ea).”
Activation energy = Threshold energy – Average energy
Ea = ET – ER
Here, Ea = Activation energy
ET = Threshold energy
ER = Average energy of reactants
Important Points related to Activation Energy :
→ Only those molecular collisions are effective whose energy is either equal to or more than threshold energy. The molecules, which have energy less than thershold energy, can’t show effective collision. So for effective collision we have to provide certain extramount of energy to them this extra amount of energy which is provided to molecules, known as activation energy.
- Activation energy is measured in calorie or joule per mole.
- The value of activation energy in a reaction is certain and fixed.
- Activation energy does not depend on the concentration of reactants.
- The value of activation energy decreases on adding positive catalyst.
- Activation energy of a slow reaction is high and activation energy of a fast reaction is low.
A + BC → C + AB, ∆H = -ve
Energy Profile for Exothermie Reaction
So, Threshold energy = Activation energy + Energy of Reactants
X + YZ → XY + Z, ∆H = +ve
Energy Profile for Endothermic Reaction
Here, Ea = activation energy for forward reaction
E = activation energy for backward reaction
For endothermic reaction, E’a < Ea
For exothermic reaction Ea > Ea