Which is more stable: Ortho or Para Hydrogen?
At room temperature, about $75$% of $\ce{H2}$ is $\ce{o-H2}$ and $25$% is $\ce{p-H2}$. As the temperature drops, the relative amounts of $\ce{p-H2}$ increases. The two forms are in a temperature-dependent equilibrium $\ce{o-H2&lt;=&gt; p-H2}$, $\Delta H = \pu{-1.66 kJ/mol}$.
Ref. M. Eagleson, Concise Encyclopedia Chemistry, Walter de Gruyter Berlin, New York $1994$, 1202 p. ISBN 0-89925-4547-8.