CHEMISTRY COMMUNITY

So a system is spontaneous when DeltaG is negative and also when Delta S total is positive. But why can't we tell the spontaneity of a system by looking at Delta H? I know when Delta H is negative, it's exothermic, but why can't we tell if the reaction is spontaneous just by looking at Delta H?

spontaneity is a concept that does't just depend on deltaH. It is when delta G is negative. Delta G depends on delta H, temperature, and delta S. Just because a reaction is exothermic, it is not necessarily spontaneous.

While exothermic reactions release energy, they can't be assumed to be spontaneous because a low entropy value could counter a negative enthalpy value ad make delta G positive, making it non-spontaneous.

Spontaneity is dependent on delta S and delta H so you cant assume the rxn is spontaneous

Here is a chart that helps understand the relationship

in general, you shouldn't assume anything in terms of spontaneity since it fluctuates with each reaction

Spontaneity is associated with the delta G values. If you are given delta H and you want to find if something is spontaneous or not, plug in values for T and delta S.

Because if delta S is small enough (negative enough = large negative number), then even if the reaction was slightly exothermic (delta H < 0), then delta G could be greater than 0, making it nonspontaneous.