So when we look at the equation for change in enthalpy, the PV work term disappears. And that means that our equation becomes change in enthalpy. The surroundings is equal to change in energy or heat. So this becomes this. Now, step three remember when we talk about Gibbs free energy, we talk about a reversible reaction. And because enthalpy is a state function, according to Hess's law, hess's law states that the entropy change of the forward reaction is equal to the negative entropy change of the reverse reaction.