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#.