What are the units for enthalpy of reaction?

Jan 4, 2016

In General Chemistry, the unstated premise is that you are doing reactions under constant pressure.

ENTHALPY IS HEAT FLOW AT CONSTANT PRESSURE

Without going into too much derivation:

$\setminus m a t h b f \left(\Delta H = {q}_{p}\right)$

where ${q}_{p}$ is the heat flow $q$ under constant pressure conditions.

Heat flow is thermal energy flow, so the units are in $\text{J}$ or $\text{kJ}$. Typical enthalpy therefore has the same units.

ENTHALPY OF REACTION

It follows from above that enthalpy of reaction, $\Delta {H}_{\text{rxn}}$, as-written, is in $\text{kJ}$.

However, you would also see enthalpies of reaction tabulated for ${25}^{\circ} \text{C}$ and (in my book) $\text{1 atm}$. That is written as $\Delta {H}_{\text{rxn}}^{\circ}$, the standard enthalpy of reaction.

The standard enthalpy of reaction, $\Delta {H}_{\text{rxn}}^{\circ}$, is tabulated such that it corresponds to the generation of $\text{1 mol}$ of a specific product. Therefore, its units are $\text{kJ/mol}$.