Question

Given the values of ΔH∘rxn, ΔS∘rxn, and Tbelow, determine ΔSuniv.

A. ΔH∘rxn=− 118 kJ , ΔS∘rxn= 258 J/K , T= 294 K

B. ΔH∘rxn= 118 kJ , ΔS∘rxn=− 258 J/K , T= 294 K

C. ΔH∘rxn=− 118 kJ , ΔS∘rxn=− 258 J/K , T= 294 K .

D. ΔH∘rxn=− 118 kJ , ΔS∘rxn=− 258 J/K , T= 545 K .

Predict whether or not the reaction in part A will be spontaneous.

Predict whether or not the reaction in part B will be spontaneous.

Predict whether or not the reaction in part C will be spontaneous.

Predict whether or not the reaction in part D will be spontaneous.

Answer 1

We know that

ΔG^∘_rxn =ΔH^∘_rxn - TΔS^∘_rxn

A reaction is spontaneous, when ΔG^∘_rxn has a negative value.

1 kJ = 1000J

A. ΔH^∘_rxn =− 118 kJ , ΔS^∘_rxn = 258 J/K , T= 294 K

ΔG^∘_rxn =ΔH^∘_rxn - TΔS^∘_rxn

ΔG^∘_rxn =-118000 J- (294 K x 258 J/K)

ΔG^∘_rxn =-118000 J- 75852 J

ΔG^∘_rxn =-193852 J

Hence the reaction is spontaneous.

B. ΔH∘rxn= 118 kJ , ΔS∘rxn=− 258 J/K , T= 294 K

ΔG^∘_rxn =ΔH^∘_rxn - TΔS^∘_rxn

ΔG^∘_rxn =118000 J- [294 K x (-258 J/K)]

ΔG^∘_rxn =118000 J+ 75852 J

ΔG^∘_rxn =193852 J

Hence the reaction is non-spontaneous.

C. ΔH∘rxn=− 118 kJ , ΔS∘rxn=− 258 J/K , T= 294 K .

ΔG^∘_rxn =ΔH^∘_rxn - TΔS^∘_rxn

ΔG^∘_rxn =- 118000 J- [294 K x (- 258 J/K)]

ΔG^∘_rxn =- 118000 J+ 75852 J

ΔG^∘_rxn =- 42148 J

Hence the reaction is spontaneous.

D. ΔH∘rxn=− 118 kJ , ΔS∘rxn=− 258 J/K , T= 545 K

ΔG^∘_rxn =ΔH^∘_rxn - TΔS^∘_rxn

ΔG^∘_rxn =-118000 J- [545 K x (- 258 J/K)]

ΔG^∘_rxn =-118000 J+ 140610 J

ΔG^∘_rxn =  22610 J

Hence the reaction is non-spontaneous.