ABSTRACT
The enthalpy changes associated with any reaction are dependent upon the temperature (Topic B2). They are also dependent upon the pressure, and the amounts and states of the reactants and products. For this reason, it is convenient to specify a standard state for a substance. The standard state for a substance is defined as being the pure substance at 1 atmosphere pressure, and at a specified temperature. The temperature does not form part of the definition of the standard state, but for historical reasons data are generally quoted for 298 K (25°C). For solutions, the definition of the standard state of a substance is an activity of 1 (see Topic D1).
