Practice Problem 2

Which of the following processes will lead to an increase in the entropy of the system?

(a) N₂(g) + 3H₂ (g) 2 NH₃(g)

(b) H₂O(l) H₂O(g)

(c) CaCO₃(s) CaO(s) + CO₂(g)

(d) NH₄NO₃(s)  + H₂O(l)    NH₄⁺ (aq) + NO₃^- (aq)

Answer

(a) Because the total number of molecules decreases in this reaction, the system becomes more ordered.  The entropy of the system therefore decreases.

(b) Gases are more disordered than the corresponding liquids, so the entropy of the system increases.

(c) Reactions in which a compound decomposes into two products lead to an increase in entropy because the system becomes more disordered.  The increase in entropy in this reaction is even larger because the starting material is a solid and one of the products is a gas.

(d) The NH₄⁺ and NO₃^-  ions are free to move in a random fashion through the aqueous solution, whereas these ions are locked into position in the crystal.  As a result, the entropy of the system increases in this reaction.