Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 8
SEC TION 14.3 ? Entropy Changes in a System
The standard entropy values of a large number of substances have been measured in J/K ? mol. To
calculate the standard entropy change for a reaction (?S° ), we look up the standard entropies of
rxn
the products and reactants and use Equation 14.5.
Worked Example 14.2 demonstrates this approach.
577
Student Annotation: The reaction generally is the system. Therefore, ?S° is ?S° .
rxn
sys
Worked Example 14.2
From the standard entropy values in Appendix 2, calculate the standard entropy changes for the
following reactions at 25°C:
(a) CaCO3(s)
(c) H2(g) + Cl2(g)
CaO(s) + CO2(g)??(b) N2(g) + 3H2(g)
2NH3(g)
2HCl(g)
Strategy Look up standard entropy values and use Equation 14.5 to calculate ?S rxn. Just as we did
°
when we calculated standard enthalpies of reaction, we consider stoichiometric coefficients to be
dimensionless—giving ?S ° units of J/K ? mol.
rxn
Setup From Appendix 2, S°[CaCO3(s)] = 92.9 J/K ? mol, S°[CaO(s)] = 39.8 J/K ? mol,
S°[CO2(g)]?= 213.6 J/K ? mol, S°[N2(g)] = 191.5 J/K ? mol, S°[H2(g)] = 131.0 J/K ? mol,
S°[NH3(g)] = 193.0 J/K ? mol, S°[Cl2(g)] = 223.0 J/K ? mol, and S°[HCl(g)] = 187.0 J/K ? mol.
Student Annotation: Recall that here per
mole means per mole of reaction as written
[9 Section 10.3].
Solution?
(a) ?S° = [S°(CaO) + S°(CO2)] ? [S°(CaCO3)]
rxn
= [(39.8 J/K ? mol) + (213.6 J/K ? mol)] ? (92.9 J/K ? mol)
= 160.5 J/K ? mol
(b) ?S ° = [2S°(NH3)] ? [S°(N2) + 3S°(H2)]
rxn
= (2)(193.0 J/K ? mol) ? [(191.5 J/K ? mol) + (3)(131.0 J/K ? mol)]
= –198.5 J/K ? mol
(c) ?S rxn = [2S°(HCl)] ? [S°(H2) + S°(Cl2)]
°
= (2)(187.0 J/K ? mol) ? [(131.0 J/K ? mol) + (223.0 J/K ? mol)]
= 20.0 J/K ? mol
Think About It?
Remember to multiply each standard entropy value by the correct stoichiometric coefficient. Like
Equation 10.18, Equation 14.5 can only ??????????????????????????????????????)A??????A??????????????????
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