Question

Explain why ozone is thermodynamically less stable than oxygen.

Answer 1

The ozone on decomposition leads to an exothermic process which means heat is released during the reaction forming a diatomic oxygen and a monatomic oxygen. The instability of ozone is related to the Gibbs free energy.

Complete step by step answer: The decomposition reaction of ozone is shown below.
${O_3} \to {O_2} + O$
In this reaction, ozone dissociates into diatomic oxygen and mono atomic oxygen.
The ozone decomposes into oxygen by releasing heat.
As heat is released in this reaction, it is an exothermic process. The value of enthalpy is negative $\Delta H = 0$.
In the decomposition reaction of ozone, the degree of freedom increases. Therefore, the entropy value of the reaction increases. The $\Delta S$ value is positive.
Therefore, at constant temperature the Gibbs free energy is given as shown below.
$\Delta G = \Delta H - T\Delta S$
The $- \Delta H$ and $- T\Delta S$ (as entropy is positive) results in formation of large negative Gibbs free energy. Due to large negative Gibbs free energy, the ozone is thermodynamically unstable as compared to oxygen. Therefore, ozone decomposes to form oxygen easily.
To attain stability, the ozone decomposes to give diatomic oxygen.
The oxygen in the diatomic state is stable as the oxygen atom is small in size therefore the bond length between the two oxygen atoms is stronger as p-p bonding takes place. Therefore, a high amount of energy is required to break the bond.

Note: When the Gibbs free energy is negative, the reaction is said to be a spontaneous process, when the Gibbs free energy is positive then the reaction is said to be non-spontaneous. At equilibrium condition, the Gibbs free energy is zero.