Question

Apply the Free Energy equation to identify whether reactions with the following values of delta H and Delta S are spontaneous or nonspontaneous and whether they are exothermic or endothermic:

a. delta H = -128 kJ; delta S = 35 J/K at 500k

b. delta H = +67kJ; delta S= -140 J/K at 250k

c. delta H = +75 kJ; delta S = 95J/K at 800k

Answer 1

ΔH = -ve for exothermic reaction

ΔH = +ve for endothermic reaction

ΔG =-ve for spontaneous reaction

ΔG = +ve for non-spontaneous reaction

a.

ΔH = -ve (exothermic reaction)

ΔG = ΔH - T x ΔS

ΔG = (-128 kJ) - (500 x 0.035) = -128 - 17.5 = - 145.5 kJ (spontaneous reaction)

b.

ΔH = +ve (endothermic reaction)

ΔG = (+67 kJ) - (250 x - 0.14) = 67 + 35 = + 102 kJ (non-spontaneous reaction)

c.

ΔH = +ve (endothermic reaction)

ΔG = (+75 kJ) - (800 x - 0.095) = 75 + 76 = + 151 kJ (non-spontaneous reaction)