Final answer:
The solubility of cerium sulfate (Ce2(SO4)3) decreases with increasing temperature due to the larger entropy decrease caused by the ordering of water molecules around the Ce3+ ions, which is an exothermic dissolution process where equilibrium shifts towards the reactants when temperature is increased.
Step-by-step explanation:
The question asks why the curve for the solubility of cerium sulfate (Ce2(SO4)3) trends downward as the temperature increases. Generally, for many solids in water, solubility increases with an increase in temperature. However, for Ce2(SO4)3, solubility decreases with increasing temperature because of two competing entropy effects. Although the dispersal of Ce3+ and SO42− ions in solution usually results in an entropy increase, this is offset by a larger entropy decrease as a result of the ordering of water molecules around the Ce3+ ions as they become hydrated. This 'ordering' effect can sometimes cause the solubility to decrease as temperature increases, which reflects an exothermic dissolution process.
In an exothermic dissolution process, according to Le Châtelier's principle, the addition of heat is like adding a product to the reaction. This causes the equilibrium to shift to the reactants' side to counter the added stress, resulting in a decrease in solubility at higher temperatures.