Question

You and a friend are studying for a chemistry

exam. What if your friend tells you, “Since exothermic

processes are favored and the sign of the enthalpy

change tells us whether or not a process is

endothermic or exothermic, the sign of Δ

H solution tells us

whether or not a solution will form”?

How would you explain to your friend that this conclusion is

not correct? What part, if any, of what your friend says is

correct?

Answer 1

for exothermic reaction ΔH is negative . all exothermic reactions are not favored . the correct term to predict spontanous of the reaction is gibbs free energy .

ΔG = ΔH - T ΔS

there are two cases for exothermic reaction :

case 1: ΔH = -ve , ΔS = +ve then ΔG = -ve . reaction is spontasous all temperatures

case 2: ΔH = -ve , ΔS = - ve then ΔG = -ve, reaction is spontaneous at only lower temperatures.

so correct parameter to predict favor of the reaction is ΔG