How does enthalpy affect the spontaneity of a reaction?

Jul 23, 2015

It doesn't directly affect it.

• Gibbs' Free Energy ($\Delta G$, not $\Delta {G}^{o}$!!!) tells us the spontaneity of the reaction.
• Enthalpy ($\Delta H$) tells us if the reaction is exothermic or endothermic.
• Entropy ($\Delta S$) 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):

$\setminus m a t h b f \left(\Delta G = \Delta H - T \Delta S\right)$

• If $\Delta G < 0$, then the reaction is spontaneous.
• If $\Delta G > 0$, then the reaction is nonspontaneous.
• If $\Delta G = 0$, the reaction is at equilibrium.

CONDITIONS FOR SPONTANEITY

Thus, we have the following conditions:

Irrespective of the temperature

• If $\Delta S < 0$, and $\Delta H > 0$, 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 $\Delta S > 0$, and $\Delta H < 0$, 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, $\Delta S > 0$, and $\Delta H > 0$, then the reaction is spontaneous.
• If the temperature is high, $\Delta S < 0$, and $\Delta H < 0$, then the reaction is nonspontaneous.

Conditional on low temperature

• If the temperature is low, $\Delta S < 0$, and $\Delta H < 0$, then it depends on the actual values of $T$ and $\Delta S$.
• If the temperature is low, $\Delta S > 0$, and $\Delta H > 0$, then it depends on the actual values of $T$ and $\Delta S$.