[soupastupid]

Why does enthalpy equal heat at constant P??

dH = dq
constant P

I know enthalpy
H = U + PV

and deltaU = q + w

am i missing an equation?

[rabolisk]

am i missing an equation?

No. Those equations are enough to "prove" that dH = dq at constant P.

[rabolisk]

I should add, that there is one assumption you have to make. Also, pressure-volume work = PextdV could be considered the third "equation" you have to use.

[climbthewall]

It equals heat when the system cannot do any pv work

[rabolisk]

It equals heat when the system cannot do any pv work

That is not true. If anything, it is the opposite...

[climbthewall]

whoops. Oh really? i just assumed as enthaply is equal to work when Q=0, and the system is adiabatic

[rabolisk]

Let me be more clear. Enthalpy can equal heat when the system does not do any pv work. But the fact that the system does not do any pv work in itself is not a condition for enthalpy equaling heat. Enthalpy change of a system that expands (PV work) can equal the heat transfer between the system and the surroundings.

I don't know what you mean when you say the system is adiabatic. Adiabatic describes processes, not states. Regardless, enthalpy does not necessarily equal work when Q = 0 either.