What's the unit of enthalpy?

Enthalpy is a measure of heat flow at a constant pressure.

In other words:

`\mathbf(DeltaH = q_p)`

You should have used this relationship before in General Chemistry. You just may not have known it at the time. For instance, dropping a piece of metal in water? That's at a constant surrounding atmospheric pressure.

We can show this by starting from the following equations:

`\mathbf(DeltaH = DeltaU + Delta(PV))` `" "bb((1))`

`Delta(PV) = PDeltaV + VDeltaP + DeltaPDeltaV` `" "bb((1.1))`
(where we have simply included very combination for how pressure and volume could change)

`\mathbf(DeltaU = q + w)` `" "bb((2))`

`w = -PDeltaV` `" "bb((2.1))`

where `H` is enthalpy, `U` is internal energy, `q` is heat flow, `w` is work, `P` is pressure, and `V` is volume.

Thus:

`DeltaH = q - cancel(PDeltaV + PDeltaV) + VDeltaP + DeltaPDeltaV`

but the pressure is assumed to be constant, so:

`DeltaH = q + cancel(VDeltaP + DeltaPDeltaV)`

As a result, we define a new relation; at a constant pressure:

`q = q_p`

So:

`color(blue)(DeltaH = q_p)`

Why does that help us? Well, we should know that heat flow is going to be in `"J"`, joules, from General Chemistry.

So naturally, enthalpy must then be in `\mathbf("J")`.

Of course, enthalpy can have other units. It's not to say it can't be converted to other units, but the basic unit for it is `"J"`, and more often, `"kJ"`.

Some other forms of enthalpy with their common units:

Molar enthalpy `DeltabarH: " kJ/mol"`
Standard enthalpy of reaction `DeltaH_"rxn"^@: " kJ/mol"`
Standard enthalpy of formation `DeltaH_f^@: " kJ/mol"`