Name: Third Law of Thermodynamics
Uploaded: 2019-07-19
Duration: 3 min 41 s
Description: The third law of thermodynamics was developed by a chemist, Walther Nernst, and is therefore often referred to as Nernst's theorem or Nernst's postulate. Lea...

Question

Write a note on activation energy.

Accepted Answer

Activation energy can be best defined as the difference in free energy between the transition state and ground-state reactants. The conversion of reactants to products in any chemical reaction is accompanied by a continuous change in energy. The magnitude of the activation energy influences the rate of the chemical reaction. The higher the activation energy the slower the reaction rate.

The temperature dependence of the rate of a chemical reaction can be accurately explained by Arrhenius equation.

$k = {Ae}^{- E_{a} / RT}$

where $A$ is the Arrhenius factor or the frequency factor. It is also called pre-exponential factor. It is a constant specific to a particular reaction. $R$ is gas constant and $E_{a}$ is activation energy measured in joules/mole.

It can be understood clearly using the following simple reaction

$H_{2} (g) + I_{2} (g) \to 2 HI (g)$

According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate. It exists for a very short time and then breaks up to form two molecules of hydrogen iodide.

Formation of $HI$ through the intermediate.

The energy required to form this intermediate, called activated complex (C), is known as activation energy $(E_{a})$ . Figure is obtained by plotting potential energy vs reaction coordinate. Reaction coordinate represents the profile of energy change when reactants change into products. Some energy is released when the complex decomposes to form products. So, the final enthalpy of the reaction depends upon the nature of reactants and products.

Write a note on activation energy.

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