Heat Capacity at Constant Pressure
The heat needed to raise a system's temperature by one kelvin while pressure is held constant.
Formula / equation
C_p = C_V + R (ideal gas)
Unit
J/(mol·K)
In depth
At constant pressure some of the energy supplied is spent doing expansion work, so more heat is needed for the same temperature rise and C_p always exceeds C_V. For an ideal gas the difference is exactly the gas constant R, a relation known as Mayer's formula.
Examples in the real world
Argon's C_p is 20.8 J/(mol·K) against a C_V of 12.5, differing by 8.314 as predicted.