In turn, a substitution for work from equation (5) yields
Equation (7) can then be rearranged to give equation (3), as desired.
Since internal energy is a state function, as is volume, the change in enthalpy is independent of the process used to make the reaction proceed. If the products of a reaction have more energy than the reactants, energy must be added to the reaction in order for the reaction to proceed. If the products have lower energy than the reactants, the reaction releases heat as it proceeds. In exothermic reactions, the change in enthalpy is negative since heat is being released. Endothermic reactions have a positive enthalpy change.
Example:
When carbon is burned to form carbon dioxide under constant volume and standard temperature, the heat released is 393.5 kJ per mole of carbon dioxide created.
The reaction is then repeated at constant standard pressure.
