On occasion, if one driving force is fulfilled and the other is not fulfilled, it becomes impossible to determine if the Gibbs Free Energy term is positive or negative. Under those conditions, the temperature of the process becomes an issue. At that time, a reaction may not work at one temperature, but it will work at another temperature. Such processes are referred to as temperature dependent reactions. Hess's Law says that the net change in enthalpy of a reaction will be the sum of the enthalpy changes that occur along the way.
Many reactions contain several steps.
If so, then each step will have its own enthalpy change. If all of the changes are added together, then the sum of that addition will be the net change for the system. In other words, ultimately, the only thing that matters is what the net result of a reaction is. Hess's Law is frequently stated as Hess's Law says that the net change in enthalpy of a reaction will be the sum of the enthalpy changes that occur along the way. Many reactions contain several steps.