How are enthalpy changes characterized in terms of process direction?

In thermodynamics, the relationship between the enthalpy change of a forward reaction and its reverse reaction is essential in understanding the properties of chemical processes. When a chemical reaction occurs in a forward direction, it has a certain enthalpy change represented as ΔforwardH. When the same reaction is considered in reverse, the enthalpy change is denoted as ΔreverseH.

The key principle here is that the enthalpy change for the forward reaction is equal in magnitude but opposite in sign to that of the reverse reaction. This means that if the forward reaction releases heat (exothermic, with a negative ΔforwardH), the reverse reaction will absorb that same heat (endothermic, with a positive ΔreverseH), and vice versa. Consequently, we can express this relationship mathematically as ΔforwardH = −ΔreverseH. This fundamental principle arises from the conservation of energy and helps in calculating the energy changes associated with chemical reactions, making it crucial for thermodynamic studies and applications in physical chemistry.