Enthalpy

This chapter focuses on the concept of enthalpy. This property is used to track the heat output (or requirements) of physical processes and chemical reactions taking place at constant pressure. Changes in internal energy or enthalpy are measured by techniques known collectively as ‘calorimetry’. A calorimeter for studying processes at constant pressure is called an isobaric calorimeter. For a combustion reaction, an adiabatic flame calorimeter may be used. The most sophisticated way to measure enthalpy changes, however, is to use a differential scanning calorimeter (DSC). The chapter then looks at the variation of enthalpy with temperature. The enthalpy of a substance increases as its temperature is raised. The reason is the same as for the internal energy: molecules are excited to states of higher energy so their total energy increases.