Enthalpy and Entropy There are two factors that combine to give the free energy change for a reaction:Enthalpy(H)and Entropy(S). △G°=△H°.T△S° AH=change in enthalpy=(enthalpy of prods)-(enthalpy of reactants) AS=change in entropy =(entropy of prods)-(entropy of reactants) (Typically the AH term>>TAS term) Enthalpy The change in enthalpy is the heat evolved or consumed in a reaction,normally in kcal/mol. Reactions tend to favor products with stronger bonds(i.e.lower enthalpy). AH is-ve,exothermic reaction,heat is evolved,stronger bonds are formed. AH is +ve,endothermic reaction,heat is consumed,weaker bonds are formed. A-ve AH is a favorable contribution to the free energy change. A +ve AH is an unfavorable contribution to the free energy change The AH for chlorination of methane is-25kcal/mol.(Very exothermic) Ch04 Rates Kinetics (landscape) Page 9Ch04 Rates + Kinetics (landscape) Page 9 Enthalpy and Entropy There are two factors that combine to give the free energy change for a reaction: Enthalpy (H) and Entropy (S). G° = H° - TS° H° = change in enthalpy = (enthalpy of prods) – (enthalpy of reactants) S° = change in entropy = (entropy of prods) – (entropy of reactants) (Typically the H term >> TS term) Enthalpy The change in enthalpy is the heat evolved or consumed in a reaction, normally in kcal/mol. Reactions tend to favor products with stronger bonds (i.e. lower enthalpy). H is –ve, exothermic reaction, heat is evolved, stronger bonds are formed. H is +ve, endothermic reaction, heat is consumed, weaker bonds are formed. A –ve H is a favorable contribution to the free energy change. A +ve H is an unfavorable contribution to the free energy change. The H° for chlorination of methane is –25kcal/mol. (Very exothermic)