Finding Spontaneity of Reaction

[Jennifer Su 2L]

Towards the end of lecture on Friday Week 4, Lavelle talked about how just knowing delta U (or delta H) isn't enough to know if a reaction will occur and that we also need to know the entropy (S). Can someone explain how knowing delta U and S will tell us how the system will change? In other words, what values of U and S will cause the reaction to be spontaneous, and what values would cause it to not be spontaneous?

[Matthew Tran 1H]

The state function G (Gibb's free energy) allows us to determine the spontaneity of a reaction by the relation: . When is negative, the reaction is spontaneous, and when it's positive, the reaction is not spontaneous. I wouldn't expect this to be on the midterm though since it only goes up to the end of entropy.