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"#