Question

Many spontaneous reactions occur very slowly. Why don’t all spontaneous reactions occur instantly?

Short answer

All spontaneous reactions do not occur instantaneously. This is because spontaneous reactions can occur even if no external energy or a catalyst is available. As a result, to cover the activation energy barrier, the reaction rate may become slow.

Step-by-step solution

Spontaneous reaction 

A reaction that has negative Gibbs free energy is known as a spontaneous reaction. It is also referred to as an exergonic reaction as it can occur without the involvement of external energy.

Therefore, the free energy of reactants is more than the products in a spontaneous reaction.

 Step 2: Definition of activation energy

The amount of energy that is required to initiate a reaction procedure is referred to as activation energy. The spontaneous reaction generally requires a low amount of activation energy to start the reaction.

Not all spontaneous reactions occur instantly

The spontaneous reaction may occur either fast or slow.The product formation occurs under a favorable condition where the reaction is occurring.

For example, two reactants A-B and B-C, combine to form the products A-C and B-D.

This spontaneous reaction occurs in the following ways-

• At first, the reactants A-B and B-C absorb energy from their surroundings.
• Then, the reactants go to an unstable state (transition state) where the bonds are mainly broken.
• After which, new bond formation occurs, resulting in products formation (A-C and B-D), and energy is released to the surroundings as a byproduct.

However, activation energy acts as a barrier determining how fast or slow the reaction will occur (reaction rate). Therefore, if a spontaneous reaction has high activation energy, the reaction will not occur instantly. Hence, all spontaneous reactions do not necessarily happen immediately.