How does enthalpy affect the spontaneity of a reaction?

It doesn't directly affect it.

• Gibbs' Free Energy (`DeltaG`, not `DeltaG^o`!!!) tells us the spontaneity of the reaction.
• Enthalpy (`DeltaH`) tells us if the reaction is exothermic or endothermic.
• Entropy (`DeltaS`) tells us the amount of energy dispersal in a reaction or system. You may have read it as "disorder" as well.

GIBBS' FREE ENERGY DETERMINES THE SPONTANEITY

This is a general Thermodynamics formula (generic entropy, enthalpy, and Gibbs' Free Energy. This means that entropy can be negative, provided it's not the total entropy of the universe):

`\mathbf(DeltaG = DeltaH - TDeltaS)`

• If `DeltaG < 0`, then the reaction is spontaneous.
• If `DeltaG > 0`, then the reaction is nonspontaneous.
• If `DeltaG = 0`, the reaction is at equilibrium.

CONDITIONS FOR SPONTANEITY

Thus, we have the following conditions:

Irrespective of the temperature

• If `DeltaS < 0`, and `DeltaH > 0`, then the reaction is nonspontaneous, independent of the magnitude of the temperature, because temperature has always been known to be positive on the Kelvin scale.
• If `DeltaS > 0`, and `DeltaH < 0`, then the reaction is spontaneous, independent of the magnitude of the temperature, because temperature has always been known to be positive on the Kelvin scale.

Conditional on high temperature

• If the temperature is high, `DeltaS > 0`, and `DeltaH > 0`, then the reaction is spontaneous.
• If the temperature is high, `DeltaS < 0`, and `DeltaH < 0`, then the reaction is nonspontaneous.

Conditional on low temperature

• If the temperature is low, `DeltaS < 0`, and `DeltaH < 0`, then it depends on the actual values of `T` and `DeltaS`.
• If the temperature is low, `DeltaS > 0`, and `DeltaH > 0`, then it depends on the actual values of `T` and `DeltaS`.