Question 17.1: Predict the sign of ΔS in the system for each of the followi......
Predict the sign of ΔS in the system for each of the following processes:
(a) CO_{2}(s)\longrightarrow CO_{2}(g) (sublimation of dry ice)
(b) CaSO_{4}(s)\longrightarrow CaO(s) + SO_{3}(g)
(c) N_{2}(g) + 3 H_{2}(g)\longrightarrow 2 NH_{3}(g)
(d) I_{2}(s)\longrightarrow I_{2}(aq) (dissolution of iodine in water)
STRATEGY
To predict the sign of ΔS, look to see whether the process involves a phase change, a change in the number of gaseous molecules, or the dissolution (or precipitation) of a solid. Entropy generally increases for phase transitions that convert a solid to a liquid or a liquid to a gas, for reactions that increase the number of gaseous molecules, and for the dissolution of molecular solids or salts with +1 cations and -1 anions.
(a) The molecules in a gas are free to move about randomly, whereas the molecules in a solid are tightly held in a highly ordered arrangement. Therefore, randomness increases when a solid sublimes and ΔS is positive.
(b) One mole of gaseous molecules appears on the product side of the equation and none appears on the reactant side. Because the reaction increases the number of gaseous molecules, the entropy change is positive.
(c) The entropy change is negative because the reaction decreases the number of gaseous molecules from 4 mol to 2 mol. Fewer particles move independently after reaction than before.
(d) Iodine molecules are electrically neutral and form a molecular solid. The dissolution process destroys the order of the crystal and enables the iodine molecules to move about randomly in the liquid. Therefore, ΔS is positive.