Problem: Find ΔS°rxn for the combustion of ethane (C2H6) to carbon dioxide and gaseous water. Is the sign of ΔS°rxn as expected?

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Step 1:


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

          Reactant          Product

C            2                       1 2

H            6                       2 6

O            2 7                    3 7


Balanced chemical equation

2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O (g)


9 mol gas reactant  10 mol gas reactant

increase in entropy

sign of ΔS should be positive


Step 2:

S°rxn =S°product - S°reactantS°rxn =[S°CO2 (g) +S°H2O (g) ] - [S°C2H6 (g) + S°O2 (g)]


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Problem Details

Find ΔS°rxn for the combustion of ethane (C2H6) to carbon dioxide and gaseous water. Is the sign of ΔS°rxn as expected?