What is meant by heat capacity at constant pressure?

1. Feb 22, 2016

Fermion70

Generally in chemistry and physics, we learned that a material has a heat capacity at constant pressure. However, I am confused at whether this refers to a constant external pressure or a constant system internal pressure?

Thanks.

Last edited: Feb 22, 2016
2. Feb 22, 2016

BvU

Hello half integer spin,

Thermodynamics is about equilibrium. Internal pressure and external pressure are supposed to be equal.

3. Feb 23, 2016

Staff: Mentor

To elaborate on what BvU said, heat capacity is a physical property of a material, and does not depend on the process being applied to the material. Heat capacity per mole at constant pressure is defined as $$C_p=\left(\frac{\partial H}{\partial T}\right)_P$$
where H is the molar enthalpy of the material. If a process is irreversible so that the internal pressure and temperature of the material are not uniform, this equation still applies locally.

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