What is heat capacity and what is the difference between Cp and Cv?

Heat capacity is the amount of heat required to raise the temperature of a substance by one degree. Cp (constant pressure): C = dH/dT, heat required at constant pressure - energy goes into both internal energy and PV work. Cv (constant volume): C = dU/dT, heat required at constant volume - all energy goes to internal energy. For gases: Cp - Cv = R (ideal gas), Cp/Cv = gamma (heat capacity ratio, ~1.4 for air). For liquids/solids, Cp approximately equals Cv. Cp is used in most process calculations (constant pressure operations).