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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.
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