Heat Capacity at Constant Pressure — From Intuition to Precision
Imagine you have a pot of water on a stove. You turn the burner on, and the water gets hotter. How much heat does it take to raise its temperature by, say, 10°C? That depends on two things: how much water you have, and whether the pot is open to the air or sealed tight.
If the pot is open (constant pressure — the air above it is always at atmospheric pressure), the water can expand as it heats. Some of the energy you supply goes into pushing the atmosphere aside — doing work against the outside air. So you need to put in more heat than if the pot were sealed (constant volume), where no expansion work is possible.
That extra heat is the key idea behind heat capacity at constant pressure, denoted Cp.
The Intuition First
Heat capacity tells you: "How much heat must I add to raise the temperature of this substance by 1°C (or 1 K)?"
- At constant volume (Cv): All the heat goes into increasing the internal energy (the kinetic and potential energy of the molecules). No work is done because the volume doesn't change.
- At constant pressure (Cp): Some heat goes into internal energy, but some also goes into the work of expansion against the constant external pressure. So Cp is always larger than Cv for gases (and for most solids/liquids, the difference is tiny because they barely expand).
For an ideal gas, the difference is exactly Cp−Cv=nR, where n is the number of moles and R is the universal gas constant. This is a direct consequence of the first law of thermodynamics.
The Precise Statement
Heat capacity at constant pressure is defined as the amount of heat required to raise the temperature of a substance by 1 K (or 1°C) while keeping the pressure constant.
Mathematically:
Cp=(dTδQ)p
The subscript p means "at constant pressure." The δQ (not dQ) reminds us that heat is a path-dependent quantity, not a state function.
But we can rewrite this in terms of a state function — enthalpy (H). At constant pressure, the heat added equals the change in enthalpy:
δQp=dH
Therefore:
Cp=(∂T∂H)p
This is the working definition you'll use in problems: Cp is the partial derivative of enthalpy with respect to temperature at constant pressure.
Molar vs. Specific Heat Capacity
You'll encounter two common forms:
- Molar heat capacity at constant pressure (Cp,m): heat capacity per mole (units: J mol⁻¹ K⁻¹)
- Specific heat capacity at constant pressure (cp): heat capacity per unit mass (units: J kg⁻¹ K⁻¹)
The total heat capacity of a sample is:
Cp=n⋅Cp,m=m⋅cp
Why It Matters
In most chemical reactions and physical processes, the system is open to the atmosphere — constant pressure. So Cp is the relevant quantity for:
- Calculating enthalpy changes (ΔH=nCp,mΔT)
- Designing calorimeters (like coffee-cup calorimeters that operate at constant pressure) …