Chemistry: A Molecular Approach (3rd Edition)

Published by Prentice Hall
ISBN 10: 0321809246
ISBN 13: 978-0-32180-924-7

Chapter 17 - Sections 17.1-17.9 - Exercises - Problems by Topic - Page 854: 51a

Answer

$\Delta S^{\circ}_{rxn}=-120.8\,J/K$ $\Delta S^{\circ}_{rxn}$ is negative because there is a decrease in the number of moles of gas.

Work Step by Step

$\Delta S^{\circ}_{rxn}=\Sigma n_{p}S^{\circ}(products)-\Sigma n_{r}S^{\circ}(reactants)$ $=[1\,mol\times S^{\circ}(C_{2}H_{6},g)]-[1\,mol\times S^{\circ}(C_{2}H_{4},g)+1\,mol\times S^{\circ}(H_{2},g)]$ $=[1\,mol(229.2\,Jmol^{-1}K^{-1})]-[1\,mol(219.3\,Jmol^{-1}K^{-1})+1\,mol(130.7\,Jmol^{-1}K^{-1})]$ $=-120.8\,J/K$ $\Delta S^{\circ}_{rxn}$ is negative because there is a decrease in the number of moles of gas.
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