# Is the synthesis of ozone, O-3, from oxygen, O_2, an example of an exothermic or endothermic reaction?

Aug 23, 2017

$\frac{3}{2} {O}_{2} + \Delta \rightarrow {O}_{3}$

#### Explanation:

The reaction as written is ENDOTHERMIC, and it is accomplished in the upper atmosphere by the impact of photons on the dioxygen molecule, i.e.

$O = O + h \nu \rightarrow 2 \dot{O}$

$\dot{O} + {O}_{2} \rightarrow O = \stackrel{+}{O} - \stackrel{-}{O}$

The first reaction is bond-breaking, and is certainly endothermic, and the second reaction, bond-making, is exothermic. The balance is towards bond-breaking, i.e. the reaction as written in the intro is endothermic.