Activation Energy

Activation energy is the minimum push needed to start a chemical reaction. Imagine it like a hill that the reactants have to climb before they can turn into products. It's the energy required to kick-start the transformation from reactants to products. Without this push, the reaction won't happen, no matter how much the reactants bump into each other.

Activation energy definition involves the minimum amount of energy needed to break the bonds in the reactant molecules so that the reaction can proceed to form the products. This energy barrier must be overcome for the reaction to occur, even if the overall reaction releases energy. The higher the activation energy, the slower the reaction rate because fewer molecules possess the necessary energy to overcome this barrier. Thus in this section, We will discuss what is activation energy and What factors can affect it, in Detail.

1.0Activation Energy Definition

To understand a chemical reaction, one must understand the meaning of the activation energy, as it is the key to predicting how the reaction will proceed under different conditions. It represents the minimal energy required to stimulate or empower molecules or atoms, enabling them to engage in a chemical reaction or undergo transformation.

2.0Activation Energy Unit

The SI unit of activation energy is typically measured in joules (J) or kilojoules per mole (kJ/mol) in the context of chemical reactions. In some cases, especially in older literature or specific fields, other units like kilocalories per mole (kcal/mol) or electron volts (eV) might also be used to express activation energies.

3.0Activation Energy Examples and Graphs 

In this section we will discuss different kinds of graph of activation energy and how we can determine it for various reactions.Further we will study examples of activation energy

Reaction Graphs

The reaction’s activation energy graph typically depicts the relationship between the progress of a chemical reaction and the energy involved.

Activation Energy graph For Exothermic reaction

(Ea)_f < (Ea)_b

(ROR)_f > (ROR)_b

△H = (Ea)_f – (Ea)_b

△H = -ve value

(Ea)_f = △H Only one can be possible for Exothermic Reaction

(Ea)_f > △H

Activation Energy Graph for Endothermic Reaction

(Ea)_f > (Ea)_b

(ROR)_f < (ROR)_b

△H = (Ea)_b – (Ea)_f

△H = +ve value

(Ea)_f > △H (Always )

1. E_a(f) = Activation energy for forward reaction
2. E_a(b) = Activation energy for backward reaction                               

If not specified in questions then consider E_a for forward reaction. 

4.0Factors Affecting Activation Energy

In Chemistry, activation energy of a reaction can be determined by some key factors, Here we will discuss about them in brief- 

1. Nature of reactant: For different reactants, the number of bonds and bond energies are different, therefore activation energy will also be different.

Note- Reactions which have less E_a, take place at a faster rate.

2. Presence of catalyst: Catalysts provide an alternative path of reaction mechanism for the reaction. Further we will discuss how catalyst and activation energy are related to each other and how catalysts affect activation energy.

Positive Catalysts- A positive catalyst speeds up a reaction by lowering the activation energy needed for it to occur, allowing it to happen more easily or quickly.

• In the presence of a positive catalyst threshold energy decreases, activation energy decreases and rate of reaction increases.

Negative Catalysts- A negative catalyst impedes a reaction by raising the activation energy needed, causing the reaction to proceed slower than usual.

• In the presence of a negative catalyst (inhibitor) threshold energy increases, activation energy increases, rate of reaction decreases.

5.0Activation Energy and Enzymes

Let’s understand the relation between activation energy and enzymes. Since we know, Enzymes are biological catalysts that speed up chemical reactions in living organisms. 

• They achieve this by lowering the activation energy required for a reaction to occur. 
• Activation energy of a reaction  is the energy needed to start a chemical reaction, and enzymes facilitate reactions by reducing this energy barrier. 
• Essentially, enzymes make it easier for reactions to happen by providing an alternate pathway that requires less energy, thus accelerating the process without being consumed or changed themselves.