Account for the following :
Ozone is thermodynamically unstable.
Ozone decomposes into O2 with the evolution of heat, i.e. ΔH is negative (exothermic).
Since the decomposition of O3 increases the number and freedom of particles, entropy also increases.
Now, ΔG = ΔH − TΔS
Both −∆H and`-TDeltaS` (since ∆S is positive) result into large negative ∆G. Hence, O3 becomes thermodynamically unstable and decomposes into oxygen easily.