Thermodynamics:
Entropy
Thermodynamics Gateway Page
In this module:
Introduction
Disorder in Atoms
Disorder in Energy
Measuring Entropy
Entropy of Phase Changes
Patterns in the Entropies of Substances
Entropy in Thermochemical Equations
The Second Law of Thermodynamics
The Effect of Temperature
Predicting How Reactions will Go
Two Examples

Entropy in Thermochemical Equations

Entropy can be used in thermochemical equations in just the same way as enthalpy. Thus, all the rules discussed in Thermochemical Equations 3, 4, and 5 (reversing signs when reversing equations, multiplying by constants, applies as written, Hess's Law) also apply to equations with entropy. This means that you can use absolute entropies of substances to determine the entropy changes in new equations just the same as you can with enthalpy.

Use the Thermodynamics Table to calculate the entropy changes for the following reactions. Were your predictions from the previous page correct?

CaCO3 (s) CaO (s) + CO2 (g)

Correct!
See the solution below.

That is incorrect.
See the solution below.

Ag+ (aq) + Cl- (aq) AgCl (s)
N2O4 (g) 2 NO2 (g)
CO2 (aq) CO2 (g)
DSº = 1 mol • Sº(CaO(s)) + 1 mol • Sº(CO2(g)) - 1 mol • Sº(CaCO3(s))
DSº = 1 mol • 39.75 J/mol•K + 1 mol • 213.74 J/mol•K - 1 mol •92.9 J/mol•K = 160.6 J/K
DSº = 1 mol • Sº(AgCl(s)) - 1 mol • Sº(Ag+(aq)) + 1 mol • Sº(Cl-(aq))
DSº = 1 mol • 96.2 J/mol•K - 1 mol • 72.68 J/mol•K - 1 mol • 56.5 J/mol•K = -33.0 J/K
DSº = 2 mol • Sº(NO2(g)) - 1 mol • Sº(N2O4(g))
DSº = 2 mol • 240.06 J/mol•K - 1 mol • 304.29 J/mol•K = 175.83 J/K
DSº = 1 mol • Sº(CO2(g)) - 1 mol • Sº(CO2(aq))
DSº = 1 mol • 213.74 J/mol•K - 1 mol • 117.6 J/mol•K = 96.1 J/K

Entropy in Thermochemical Equations