Heat and energy

As you may recall, the enthalpy of a system is given by H = U + pV, where U is the energy, p is the pressure and V is the volume.

Thereby follows, that the changes in enthalpy and energy of a system are related by the formula

ΔH = ΔU + Δ(pV)

In a constant pressure calorimeter the pressure is constant and the last term in the equation can be written as pΔV. The change in volume for most reactions is negligible, whereby the term approaches zero and it is fair to assume that ΔH = ΔU.

In a constant volume calorimeter the volume is constant and the last term in the equation can be written as VΔp. Since the reaction vessel has been pressurized beforehand, the change in pressure is negligible, whereby the term approaches zero and we make the assumption that ΔH = ΔU.

With the assumption that ΔH = ΔU, calorimetry can be used to determine the specific energy of a compound.