Entropy Changes ΔS and Thermodynamic Equilibrium – Solved Examples

 

In a previous post entitled “Entropy, Free Energy and Thermodynamic Equilibrium” the Boltzmann definition of entropy was given and how entropy changes ΔS are associated with chemical processes was discussed. Below, some examples are given regarding entropy changes ΔS and chemical reactions.

Example #1

Choose the reaction expected to have the greatest increase in entropy:

a)  N₂(g) + O₂(g) ———› 2 NO(g)

b)  H₂O (l)    ———›   H₂O (g)

c)  2 XeO₃(s)  ———›   2 Xe(g) + 3 O₂(g)

d) C(s) + O₂(g)   ———›  CO₂ (g)

 

Answer:

The reaction with the greatest increase in the moles of gas will have the greatest increase in entropy. Answer (c) is correct. In general, when a reaction involves gaseous molecules and the number of gaseous products (or moles of gaseous products) is greater than the number of molecules of gaseous reactants (or moles of gaseous reactants) the entropy change ΔS increases (ΔS > 0).

More gaseous molecules means more possible configurations and therefore a greater probability to occur.

 

Example #2

Predict the sign of ΔS° for the oxidation of SO2 in air:

2 SO₂(g)  +  O₂(g)  ———›    2 SO₃(g)

 

Answer:

Three molecules of gaseous reactants produce 2 molecules of gaseous products. The number of gaseous products is less than the number of the reactants and therefore ΔS < 0

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Relevant Posts

Entropy, Free Energy and Thermodynamic Equilibrium

Phase Changes - Energy Changes - Heating Curves

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References

1. P. Atkins, J. de Paula, “Physical Chemistry”, 9th Edition, W. H. Freeman (2009)
2. I. N. Levine, “Physical Chemistry”, 6th Edition, McGraw-Hill (2008)
3. S. S. Zumdahl, “Chemical Principles”, 6th Edition, Houghton Mifflin Company (2009)
4. A. W. Adamson, A. P. Gast, “Physical Chemistry of Surfaces”, John Wiley & Sons (1997

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Key Terms

entropy changes examples, thermodynamic equilibrium,, ΔS, entropy increase and chemical reactions,

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