How is the enthalpy change for a reaction influenced by pressure and volume changes?

The enthalpy change for a reaction is not directly influenced by changes in pressure or volume.

Enthalpy change, denoted as ΔH, is a measure of the total energy of a system. It is a state function, meaning its value depends only on the current state of the system and not on the path taken to reach that state. Therefore, changes in pressure or volume do not directly affect the enthalpy change of a reaction.

However, it's important to understand that while pressure and volume changes do not directly influence the enthalpy change, they can affect the conditions under which a reaction occurs, which may indirectly influence the enthalpy change. For instance, changes in pressure or volume can shift the equilibrium of a reaction, altering the concentrations of reactants and products, which could then affect the overall enthalpy change.

In the context of gases, the ideal gas law (PV=nRT) can be used to relate pressure, volume and temperature. If a reaction involves gases and results in a change in the number of moles of gas, then changes in pressure or volume can affect the reaction's enthalpy change. This is because the enthalpy change of a reaction involving gases is related to the change in the number of moles of gas.

For example, in a reaction where the number of moles of gas decreases, an increase in pressure or decrease in volume would favour the forward reaction, potentially leading to a greater enthalpy change. Conversely, if the number of moles of gas increases, a decrease in pressure or increase in volume would favour the forward reaction, potentially leading to a smaller enthalpy change.

In summary, while pressure and volume changes do not directly affect the enthalpy change of a reaction, they can influence the conditions under which a reaction occurs. This can indirectly affect the enthalpy change, particularly in reactions involving gases where changes in pressure or volume can shift the equilibrium and alter the number of moles of gas involved in the reaction.