# Chapter 5 Principles of Chemical Reactivity: Energy and Chemical Reactions - Study Questions - Page 217e: 69

#### Work Step by Step

Enthalpies of formation (kJ/mol) $O(g): 249.170, O_2(g): 0, O_3(g): 142.67$ Oxygen's standard state is $O_2(g)$. Formation of oxygen atoms from $O_2(g)$ is endothermic since the former has a higher enthalpy of formation than the latter. To create 1 mol of ozone from $O_2(g)$ 142.67 kJ are required.

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