Temperature Change and Solubility (13.2.1) Flashcards
• In endothermic solvation processes, solubility increases with increasing temperatures, while in exothermic solvation processes solubility decreases with increasing temperature.
• In endothermic solvation processes, solubility increases with increasing temperatures, while in exothermic solvation processes solubility decreases with increasing temperature.
• Supersaturated solutions contain more dissolved solute than their solubility at a particular temperature.
• Supersaturated solutions contain more dissolved solute than their solubility at a particular temperature.
• Two solutes dissolved in the same solvent can be separated by taking advantage of their different solubilities at different temperatures in a process called fractional crystallization.
• Two solutes dissolved in the same solvent can be separated by taking advantage of their different solubilities at different temperatures in a process called fractional crystallization.
Look at the calcium sulfate solubility curve.
Which statement best explains this curve?
Calcium sulfate dissolves in a solution by an exothermic event. Increasing the temperature shifts the equilibrium toward the solute (precipitation). (A)
If we introduce heat energy (i.e., raise the temperature), the equilibrium will shift to the left (toward the solute). That means that if we raise the temperature of a solution of calcium sulfate, there is a greater probability of a precipitation event occurring.
Which statement best defines solubility?
Solubility is the maximum possible concentration of solute in a solution at a given temperature. (C)
At any given temperature, there is only one maximum amount of solute that a solution can tolerate or support. This defines the solubility. It is a temperature-dependent property, so as the temperature changes the solubility will change.
Which of the following does not describe solubility?
It is the amount of solute in a given solution. (B)
This is not correct for a very important reason—the solution has to be saturated. The solubility describes the concentration of a solute in a saturated solution.
When sodium acetate (NaC2H3O2 ) is dissolved in water it is an endothermic process in which the temperature of the solution decreases. Which statement about this (or a similar) event is not true?
When the sodium acetate (NaC2H3O2 ) is dissolved, it is an example of a chemical change, not a physical change. (C)
Dissolving sodium acetate is a physical change.
A cooled, supersaturated solution of sodium acetate is poured out onto a crystal of sodium acetate. Which statement about this event is not true?
The process of recrystallization is endothermic. (C)
It is an exothermic event. A lot of heat is liberated during the crystallization of sodium acetate. This makes sense. Remember, in the endothermic process, sodium acetate was dissolved into a solution. Now that we are precipitating sodium acetate from a solution, the reverse event must be occurring.
Look at the two solubility curves in the diagram.
The solubility of calcium sulfate decreases with increasing temperature. The opposite is true for sodium acetate and sodium chloride. Which statement about these solutions is not true?
Sodium acetate dissolves in a solution via an exothermic process. (C)
For an endothermic process, if you increase temperature, the equilibrium will shift toward the solution (dissolving more solute). This is exactly what happens with sodium acetate, not the opposite.
The diagram shows how a solution becomes supersaturated.
Basically, the solution, at the temperature indicated by Point A, is heated to temperature B. Then the solution is cooled to temperature C (which equals temperature A). By not allowing nucleation sites, the solution is not allowed to yield a precipitate. Thus, it is supersaturated. By nature, a supersaturated solution is very unstable. Which statement best explains why this is true?
The solution is highly unstable because it only wants to tolerate (or support) a solubility defined by Point A. The solution is essentially storing heat energy when it is supersaturated (at Point C). (C)
This is a very good explanation of the solution’s instability. At Point C, you have a supersaturated solution. The normal amount of solute in the solution at this temperature is indicated by Point A, so there is extra solute dissolved. As soon as any nucleation site is introduced, the solution will quickly yield solute (as precipitate).
Which of the following steps in the supersaturation of a solution is not correct?
Step 4: At Point 4, the solution either remains as it is or begins to yield precipitate (as you go down the line between Point 4 and Point 1) as the temperature is decreased. (D)
This statement is almost correct. At Point 4, you have a supersaturated solution because the solution has a higher solubility (Point 4) than expected (Point 1). The last part of the answer—about decreasing temperature—is incorrect. From Point 4 to Point 1, there is no change in temperature, only a change in solubility. This can occur if a nucleation site is suddenly introduced.
Which of the following processes does not use energy?
particle interactions (in the generation of a solution) that result in a buildup of attractive forces (B)
Energy is gained by how particles interact with one another via the attractive forces that build up as a solution is generated.
Look carefully at the two equations for endothermic and exothermic reactions.
solute + solvent solution + heat (exothermic)
solute + solvent + heat solution (endothermic)
Which statement is not true relative to these two equations?
Raising the temperature in an exothermic equilibrium circumstance will favor the dissolving of more solute in the solution. (C)
The exothermic equation shows that by adding heat, we are promoting the precipitation of more solute. We are not promoting the dissolving of more solute into the solution.
