Question

What are the units for enthalpy of reaction?

Answer 1

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:

`\mathbf(DeltaH = q_p)`

where `q_p` is the heat flow `q` under constant pressure conditions.

Heat flow is thermal energy flow, so the units are in `"J"` or `"kJ"`. Typical enthalpy therefore has the same units.

ENTHALPY OF REACTION

It follows from above that enthalpy of reaction, `DeltaH_"rxn"`, as-written, is in `"kJ"`.

However, you would also see enthalpies of reaction tabulated for `25^@ "C"` and (in my book) `"1 atm"`. That is written as `DeltaH_"rxn"^@`, the standard enthalpy of reaction.

The standard enthalpy of reaction, `DeltaH_"rxn"^@`, is tabulated such that it corresponds to the generation of `"1 mol"` of a specific product. Therefore, its units are `"kJ/mol"`.