How does enthalpy affect the spontaneity of a reaction?
1 Answer
Jul 23, 2015
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# .
