
Using values of ΔfH° and S°, calculate the standard molar free energy of formation, ΔfG°, for each of the following compounds:
- (a) CS2(g)
- (b) NaOH(s)
- (c) ICl(g)
Compare your calculated values of ΔfG° with those listed in Appendix L Which of these formation reactions are predicted to be product-favored at equilibrium at 25 °C?
(a)

Interpretation:
The the standard molar free energy for formation of CS2(g) should be calculated and compared with the values placed in appendix L. It should be identified that whether the reaction is product favored at equilibrium.
Concept Introduction:
The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and entropy by the following expression,
ΔGo = ΔHo - TΔSo
Here, ΔHo is the change in enthalpy and ΔSo is the change in entropy.
Answer to Problem 17PS
The standard molar free energy of formation for CS2(g) is +66.6 kJ/mol-rxn.
The value of ΔG∘ is positive, hence the reaction is reactant-favored at equilibrium.
Explanation of Solution
The standard molar free energy of formation for CS2(g) is calculated below.
Given:
The Appendix L referred for the values of standard entropies and enthalpies.
C(s)+2S(s)→CS2(g)ΔfH°(kJ/mol)00+116.7So(J/K×mol)+5.6+32.1+237.8
ΔrH°= ∑nΔfH°(products)- ∑nΔfH°(reactants)= [(1 mol CS2(g)/mol-rxn)ΔfH°[CS2(g)]-[(1 mol C(s)/mol-rxn)ΔfH°[C(s)]+(2 mol S(s)/mol-rxn)ΔfH°[S(s)]] ]
Substituting the respective values,
ΔrH°= [(1 mol CS2(g)/mol-rxn)(116.7 kJ/mol)-[(1 mol C(s)/mol-rxn)(0 kJ/mol)+(2 mol S(s)/mol-rxn)(0 kJ/mol)] ]= 116.7 kJ/mol-rxn
ΔrS°= ∑nS°(products)- ∑nS°(reactants)= [(1 mol CS2(g)/mol-rxn)S°[CS2(g)]-[(1 mol C(s)/mol-rxn)S°[C(s)]+(2 mol S(s)/mol-rxn)S°[S(s)]] ]
Substituting the respective values,
ΔrS°= [(1 mol CS2(g)/mol-rxn)(237.8 J/K×mol)-[(1 mol C(s)/mol-rxn)(5.6 J/K×mol)+(2 mol S(s)/mol-rxn)(32.1 J/K×mol)] ]= 168 J/K×mol-rxn
Now, ΔGo= ΔHo-TΔSo
Substituting the value of ΔHo and ΔSo.
ΔGo= 116.7 kJ/mol-rxn-[(298K)(168 J/K×mol-rxn)](1 kJ1000 J)= 66.6 kJ/mol-rxn
The value listed in Appendix L is 66.61 kJ/mol-rxn. Thus both the calculated value and the value in Appendix L are in agreement with each other.
The value of ΔG∘ is positive, hence the reaction is reactant-favored at equilibrium.
(b)

Interpretation:
The the standard molar free energy for formation of NaOH(s) should be calculated and compared with the values placed in appendix L. It should be identified that whether the reaction is product favored at equilibrium.
Concept Introduction:
The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and entropy by the following expression,
ΔGo = ΔHo - TΔSo
Here, ΔHo is the change in enthalpy and ΔSo is the change in entropy.
Answer to Problem 17PS
The standard molar free energy of formation for NaOH(s) is −379.82 kJ/mol-rxn.
The value of ΔG∘ is negative, hence the reaction is product-favored at equilibrium.
Explanation of Solution
The standard molar free energy of formation for NaOH(s) is calculated below.
Given:
The Appendix L referred for the values of standard entropies and enthalpies.
Na(s)+12H2(g)+12O2(g)→NaOH(s)ΔfH°(kJ/mol)000- 425.93So(J/K×mol)51.21130.7205.0764.46
ΔrH°= ∑nΔfH°(products)- ∑nΔfH°(reactants)= [(1 mol NaOH(s)/mol-rxn)ΔfH°[NaOH(s)]-[(0.5 mol H2(g)/mol-rxn)ΔfH°[H2(g)]+(0.5 mol O2(g)/mol-rxn)ΔfH°[O2(g)]+(1 mol Na(s)/mol-rxn)ΔfH°[Na(s)]] ]
Substituting the values,
ΔrH°= [(1 mol NaOH(s)/mol-rxn)(-425.93 kJ/mol)[(0.5 mol H2(g)/mol-rxn)(0 kJ/mol)+(0.5 mol O2(g)/mol-rxn)(0 kJ/mol)+(1 mol Na(s)/mol-rxn)(0 kJ/mol)] ]= -425.93 kJ/mol-rxn
ΔrS°= ∑nS°(products)- ∑nS°(reactants)= [(1 mol NaOH(s)/mol-rxn)S°[NaOH(s)]-[(0.5 mol H2(g)/mol-rxn)S°[H2(g)]+(0.5 mol O2(g)/mol-rxn)S°[O2(g)]+(1 mol Na(s)/mol-rxn)S°[Na(s)]] ]
Substituting the values,
ΔrS°= [(1 mol NaOH(s)/mol-rxn)(64.46 J/K×mol)-[(0.5 mol H2(g)/mol-rxn)(130.7 J/K×mol)+(0.5 mol O2(g)/mol-rxn)(205.07 J/K×mol)+(1 mol Na(s)/mol-rxn)(51.21 J/K×mol)] ]= -154.6 J/K×mol-rxn
Now, ΔGo= ΔHo-TΔSo
Substitute the value of ΔHo and ΔSo.
ΔGo= - 425.93 kJ/mol-rxn- [(298K)(-154.6 J/K×mol-rxn)](1 kJ1000 J)= - 379.82 kJ/mol-rxn
The value listed in Appendix L is −379.75 kJ/mol-rxn. Thus both the calculated value and the value in Appendix L are in agreement to each other.
The value of ΔGo is negative, hence the reaction is product-favored at equilibrium.
(c)

Interpretation:
The the standard molar free energy for formation of ICl(g) should be calculated and compared with the values placed in appendix L. It should be identified that whether the reaction is product favored at equilibrium.
Concept Introduction:
The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and entropy by the following expression,
ΔGo = ΔHo - TΔSo
Here, ΔHo is the change in enthalpy and ΔSo is the change in entropy.
Answer to Problem 17PS
The standard molar free energy of formation for ICl(g) is −5.73 kJ/mol-rxn.
The value of ΔG∘ is negative, hence the reaction is product-favored at equilibrium.
Explanation of Solution
The standard molar free energy of formation for ICl(g) is calculated below.
Given:
The Appendix L referred for the values of standard entropies and enthalpies.
12I2(s)+12Cl2(g)→ICl(g)ΔfH°(kJ/mol)00+17.51So(J/K×mol)+116.135+223.08+247.56
ΔrH°= ∑nΔfH°(products)- ∑nΔfH°(reactants)= [(1 mol ICl(g)/mol-rxn)ΔfH°[ICl(g)]-[(0.5 mol I2(s)/mol-rxn)ΔfH°[I2(s)]+(0.5 mol Cl2(g)/mol-rxn)ΔfH°[Cl2(g)]] ]
Substituting the respective values,
ΔrH°= [(1 mol ICl(g)/mol-rxn)(17.51 kJ/mol)-[(0.5 mol I2(s)/mol-rxn)(0 kJ/mol)+(0.5 mol Cl2(g)/mol-rxn)(0 kJ/mol)] ]= 17.51 kJ/mol-rxn
ΔrS°= ∑nS°(products)- ∑nS°(reactants)= [(1 mol ICl(g)/mol-rxn)S°[ICl(g)]-[(0.5 mol I2(s)/mol-rxn)S°[I2(s)]+(0.5 mol Cl2(g)/mol-rxn)S°[Cl2(g)]] ]
Substituting the respective values,
ΔrS°= [(1 mol ICl(g)/mol-rxn)(247.56 J/K×mol)-[(0.5 mol I2(s)/mol-rxn)(116.135 J/K×mol)+(0.5 mol Cl2(g)/mol-rxn)(223.08 J/K×mol)] ] = 77.95 J/K×mol-rxn
Now, ΔGo= ΔHo-TΔSo
Substituting the value of ΔHo and ΔSo.
ΔGo= 17.51 kJ/mol-rxn-[(298K)(77.95 J/K×mol- rxn)](1 kJ1000 J)= -5.73 kJ/mol-rxn
The value listed in Appendix L is −5.73 kJ/mol-rxn. Thus both the calculated value and the value in Appendix L are in agreement to each other.
The value of ΔG∘ is negative, hence the reaction is product-favored at equilibrium.
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Chapter 18 Solutions
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