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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 17.19

For each of the following reactions, indicate whether you would expect the entropy of the system to increase or decrease. If you cannot tell by inspecting the equation, explain why.

(a) CH3OH(l) CH3OH(g)

(b) N2O4(g) 2 NO2(g)

(c) CO(g) + H2O(g) CO2(g) + H2(g)

(d) 2 KClO3(s) 2 KCl(s) + 3 O2(g)

(e) CH3COOH(l)CH3COOH(s)

(f) N2(g) + O2(g)2 NO(g)

(g) N2H4(l)N2(g) + 2 H2(g)

(h) 2 NH3(g) + H2SO4(aq) (NH4)2SO4(aq)






Answer:

(a) CH3OH(l) CH3OH(g)

The entropy, S, increases because there are more moles of gaseous products.

(b) N2O4(g) 2 NO2(g)

The entropy, S, increases because there are more moles of gaseous products.

(c) CO(g) + H2O(g) CO2(g) + H2(g)

Can not predict the entropy change because there are the same number of moles of gaseous products and reactants.

(d) 2 KClO3(s) 2 KCl(s) + 3 O2(g)

The entropy, S, increases because there are more moles of gaseous products.

(e) CH3COOH(l)CH3COOH(s)

So < 0 (decrease in entropy) since the reaction goes from a liquid to a solid.


(f) N2(g) + O2(g)2 NO(g)

So ~ 0 (little change in entropy) since the reaction goes from 2 moles of gases to 2 moles of gases.


(g) N2H4(l)N2(g) + 2 H2(g)

So > 0 (increase in entropy) since the reaction goes from a liquid to a 3 moles of gases.


(h) 2 NH3(g) + H2SO4(aq) (NH4)2SO4(aq)

So < 0 (decrease in entropy) since the reaction goes from a liquid + 2 moles of gases to an all liquid state.