2 - First Law of Thermodynamics

Abstract

The first law of thermodynamics is stated in terms of the existence of an extensive function of state called the internal energy. For a chemically closed system, the internal energy changes when energy is added by heat transfer or work is done by the system. Heat and work are not state variables because they depend on a process. Reversible quasistatic work can be done by a system by using pressure to change its volume very slowly. Heat capacities are defined as the amount of energy needed to cause temperature change at constant volume or pressure. Processes are illustrated for an ideal gas whose energy depends only on temperature. Sudden volume changes can result in irreversible work during which pressure is undefined. We define an auxiliary state function known as enthalpy to relate to processes at constant pressure. Phase transformations such as melting involve enthalpy changes that liberate latent heat.