1.Place the following in order of decreasing standard molar entropy. H2O (l) H2O

Question

1.Place the following in order of decreasing standard molar entropy. H2O (l) H2O (g) H2O (s)

2.Calculate S°rxn for the following reaction. The absolute entropies, S°, for each of the species are shown below the reaction. C2H2(g) + 2 H2(g) C2H6(g) S°(J/mol·K) (200.9, 130.7, 229.2)

3.Which of the following processes have a Ssys > 0? A) 2 NH3(g) + CO2(g) NH2CONH2(aq) + H2O(l) B) sodium chloride dissolves in pure water C) 2 HBr(g) H2(g) + Br2(l) D) lithium fluoride forms from its elements E) All of the above processes have a S > 0.

Explanation / Answer

1) Entropy order   H2O ( g) > H2O ( l) > H2O ( s)

2)   dS rxn = dS products - dS reactansts

             = dS C2H6 - dS C2H2 - 2dS H2  

            = ( 229.2) - ( 200.9) - 2 ( 130.7) = - 233.1 J/mol

3) dS > 0 for reaction where we get more gas products than reactants or where we get more ions.

Hence   option B   i.e dissolving sodium chloride in water so that we get ions   will have dS > 0

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