Why are standard enthalpies measured at constant pressure?

Standard enthalpies are measured at constant pressure because most chemical reactions occur at constant atmospheric pressure.

In more detail, the standard enthalpy change of a reaction is defined as the heat energy transferred during a reaction at constant pressure. This is because most chemical reactions, particularly those in laboratories or industrial processes, occur at constant atmospheric pressure. Therefore, it is practical and relevant to measure the enthalpy changes under these conditions.

The term 'standard' in standard enthalpy refers to the standard state, which is defined as the most stable state of a substance at 1 bar pressure and a specified temperature, usually 298K. The standard state is used as a reference point to calculate the enthalpy changes for all substances involved in a chemical reaction.

When a reaction occurs at constant pressure, the heat change is directly equal to the change in enthalpy. This is due to the first law of thermodynamics, which states that the energy of an isolated system is constant. Therefore, any heat energy absorbed or released by the system during a reaction is equal to the change in the system's internal energy, which is the definition of enthalpy.

In contrast, if a reaction were to occur at constant volume, the heat change would be equal to the change in internal energy, not the change in enthalpy. This is because at constant volume, there is no work done by or on the system, so the only energy change is the internal energy. However, in real-world conditions, reactions rarely occur at constant volume, making constant pressure a more relevant condition for measuring enthalpy changes.

In summary, standard enthalpies are measured at constant pressure because this reflects the conditions under which most chemical reactions occur. This allows for a practical and relevant measurement of the heat energy changes that occur during these reactions.