# What's the unit of enthalpy?

##### 2 Answers

**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

So,naturallyenthalpy 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

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"#