most salts are more soluble in water at higher temperatures than at lower temperatures but not all. What are two exceptions? Explain how these salts can be less soluble at higher temperatures.
Two exceptions to the general rule that most salts are more soluble in water at higher temperatures than at lower temperatures are calcium sulfate (CaSO4) and cerium(III) sulfate (Ce2(SO4)3).
To understand why these salts are less soluble at higher temperatures, we need to consider the principles of solubility and the effect of temperature on the dissolution process.
Solubility refers to the maximum amount of solute that can be dissolved in a given amount of solvent at a particular temperature. In general, as temperature increases, the solubility of solid solutes in water also increases. This is because higher temperatures provide more kinetic energy to the solvent molecules, allowing them to overcome the attractive forces between solute particles and pull them into solution.
However, for salts like calcium sulfate and cerium(III) sulfate, a different phenomenon occurs. These salts undergo an exothermic dissolution process, meaning that they release heat when dissolved in water. As a result, as the temperature increases, the excess heat causes a shift in the equilibrium towards the solid form instead of the dissolved form.
This shift occurs because the increase in temperature provides additional energy, not only to the water molecules but also to the solute particles. The increased kinetic energy of the solute particles can disrupt the hydration process, which is the interaction between the solute ions and the water molecules that stabilizes the solute in solution. As a result, the solute ions are more likely to reassemble into a solid crystal lattice instead of continuing to dissolve.
In the case of calcium sulfate and cerium(III) sulfate, the excess heat provided at higher temperatures destabilizes the solute-solvent interaction, making it energetically favorable for the salt to come out of the solution and form solid crystals. This reduces their solubility.
In summary, the exceptions of calcium sulfate and cerium(III) sulfate being less soluble at higher temperatures are due to the exothermic dissolution process, where the release of excess heat causes a shift in equilibrium towards the solid form instead of the dissolved form.