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The standard entropies of C(graphite), H2(g), and CH4(g) are 5.740, 130.684, and 186.264 J/(K*mol) respectively. Calculate the standard entropy change delta_S for the reaction: C(graphite) + 2H2(g) -> CH4(g).

1. (A) -80.844 J/K
2. (B) +80.844 J/K
3. (C) -130.844 J/K
4. (D) +130.844 J/K

Correct answer: (A) -80.844 J/K

Solution

delta_S = S(CH4) - S(C, graphite) - 2*S(H2) = 186.264 - 5.740 - 261.368 = 186.264 - 267.108 = -80.844 J/(K*mol). The negative value indicates a decrease in entropy as 3 moles of reactants (1 solid + 2 gas) form 1 mole of gas.

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