Question

Which ΔS do you use to decide spontaneity?

When you're predicting spontaneity from `Delta`H and `Delta`S, which `Delta`S should you be using? `Delta`S universe? `Delta`S system? `Delta`S surrounding?

Answer 1

`DeltaS_("universe")`

Explanation:

Spontaneity can be calculated by Gibbs' free energy (`DeltaG`), which has to be negative for a reaction to be considered spontaneous, as in the reaction can go on once activated.

Gibbs' free energy can be defined as `DeltaG=DeltaH-TDeltaS_("sys")`, where `DeltaS_("sys")` is the total entropy of the products - the total entropy of the reactants.

Gibbs' free energy tells you how much energy is left over or needed for the reaction to take place, based on how much energy the system is "putting in" to the enthalpy change required.