When is a reaction spontaneous?

The spontaneity of a reaction can be explained with Gibbs’s free energy.

Gibbs free energy is a thermodynamic property equal to the enthalpy minus product of absolute temperature and entropy.

G = H – TS

The energy change that occurs while going from reactants to products is called Gibbs’s free energy change.

ΔG = ΔH – TΔS

Where,

Change in Gibbs’s free energy = Free energy of products – Free energy of reactants

Spontaneous processes

When the change in Gibbs’s free energy (ΔG) is negative or less than zero, it means that the reactants have more free energy than products i.e. the reaction has released energy while occurring. These types of reactions occur on their own and are called spontaneous reactions.

Why are reactions spontaneous

The examples of spontaneous reactions are:

  1. Alkali metals added to water.
  2. Burning or oxide formation.
  3. Rusting.
  4. Dissolution of salt, sugar in water.

Almost all exothermic reactions are spontaneous along with some endothermic processes as well.

Non-spontaneous processes

When the change in Gibbs’s free energy (ΔG) is positive or more than zero, it means that the products have more free energy than reactants i.e. the reaction has absorbed energy to proceed. These types of reactions cannot occur on their own and are called non-spontaneous reactions.

When are reactions spontaneous

The examples of non-spontaneous reactions are:

  1. Metallic rust turning to metal and oxygen.
  2. Breakdown of water into hydrogen and oxygen gas.
  3. The reactions that liberate metals from oxide and chloride ores.
  4. Conversion of oxygen to ozone.
  5. De-polymerization i.e. degradation of plastic bags etc.

All endothermic reactions are non-spontaneous with some exceptions.

The relationship between the type of reactions, the enthalpy change (ΔH), Gibbs free energy (ΔG), and spontaneity can be well explained by the following table.

spontaneous vs non-spontaneous