Enthalpy

This chapter addresses enthalpy, which plays a role wherever processes taking place at constant pressure, a common condition in chemistry, are studied. The enthalpy of a system, a state function, is introduced in order to take energy changes arising from expansion work into account automatically. Energy transferred as heat transferred at constant pressure is equal to the change in enthalpy of the system. The chapter then considers the temperature dependence of the enthalpy; the enthalpy of a system increases when the temperature is increased. The change can be measured by monitoring the energy that must be supplied as heat to raise the temperature when the sample is open to the atmosphere (or subjected to some other constant pressure). The heat capacity at constant pressure is the slope of a graph of the enthalpy with respect to temperature.