Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074



Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

Is the combustion of ethane, C2H6, product-favored at equilibrium at 25 °C?

C2H6(g) + 7/2 O2(g) → 2 CO2(g) + 3 H2O(g)

Answer the question by calculating the value of ΔS° (universe) at 298 K, using values of ΔfH° and S° in Appendix L. Does the answer agree with your preconceived idea of this reaction?

Interpretation Introduction


It should be predicted that if the combustion of ethane is product-favored at equlibrium using calculation of ΔS(universe).

Concept introduction:

The universe consists of two parts, system and surroundings. The entropy change for the universe is the sum of entropy change for the system and for surroundings.

ΔS(universe)= ΔS(system)+ΔS(surroundings)

The ΔS(universe) should be greater than zero for a spontaneous process.

The  ΔS(system) can be calculated by the following expression,


The ΔS(surroundings) can be calculated by the following expression,


Here, ΔrHo is the enthalpy change for the reaction.


The ΔS(universe) for the combustion of ethane is calculated below.

The Appendix L referred for the values of standard entropies and enthalpies.

The standard entropy of C2H6(g) is 229.2 J/Kmol.

The standard entropy of O2(g) is 205.07 J/Kmol.

The standard entropy of CO2(g) is 213.74 J/Kmol.

The standard entropy of H2O(g) is 188.84 J/Kmol.

The standard enthalpy of C2H6(g) is 83.85 kJ/mol.

The standard enthalpy of O2(g) is 0 kJ/mol.

The standard enthalpy of CO2(g) is 393.509 kJ/mol.

The standard enthalpy of H2O(g) is 241.83 kJ/mol.

The balanced chemical equation is:


Calculation for ΔSo(system):

The  ΔS(system) can be calculated by the following expression,

  ΔS(system)=ΔrS°=nS°(products)nS°(reactants)=[[(2 mol CO2(g)/mol-rxn)S°[CO2(g)]+(3 mol H2O(g)/mol-rxn)S°[H2O(g)]][(1 mol C2H6(g)/mol-rxn)S°[C2H6(g)]+(3.5 mol O2(g)/mol-rxn)S°[O2(g)]]]

Substituting the values,

  ΔS(system)=[[(2 mol CO2(g)/mol-rxn)(213.74 J/Kmol)+(3 mol H2O(g)/mol-rxn)(188.84 J/Kmol)][(1 mol C2H6(g)/mol-rxn)(213.74 J/Kmol)+(3.5 mol O2(g)/mol-rxn)(205.07 J/Kmol)]]=62

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