13.2.1 Temperature Change and Solubility

Question 1

Temperature Change and Solubility

Answer 1

• 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.
• 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

Question 2

note 1

Answer 2

• Whether a solvation process is endothermic or exothermic depends on the amount of energy that is required to break bonds between solute molecules and between solvent molecules, and on the amount of energy gained from the formation of new bonds between solute and solvent molecules.
• The dissolution of sodium acetate (NaOAc) in water is an endothermic process. As NaOAc dissolves in the water, the solution gets colder due to the input of energy in the form of heat. Since this solvation process is endothermic, it follows that the opposite process is exothermic. As NaOAc recrystallizes from a supersaturated solution, heat is released.
• The solubility of a substance is the maximum amount of solute that can be dissolved in a given solvent at a particular temperature.
• When the solute concentration is plotted against the
  temperature for an endothermic solvation process, the
  solubility increases with increasing temperature. Since
  endothermic processes need an input of energy in order to occur, adding heat shifts the equilibrium in favor of solution formation.
• In contrast, in exothermic solvation processes, the solubility decreases as the temperature is increased. Since the process is exothermic, adding heat shifts the equilibrium in favor of solute formation.

Question 3

note 2

Answer 3

• A supersaturated solution contains a solute at a concentration higher than its solubility.
• A supersaturated solution is created from a saturated solution at a higher temperature (1). The saturated solution is allowed to cool and a site of nucleation is prevented from forming (2). A site of nucleation is a place where a crystal starts to grow. By preventing a crystal from forming, the solution is kept out of equilibrium and more solute is dissolved than the solubility at that temperature.
• Once a crystal begins to form, further precipitation will result since the equilibrium is in favor of solute formation.
• Fractional crystallization is a method used to separate two or more solutes from one another based on their different solubilities.
• A solvent is chosen so that the solutes are both soluble at high temperatures, but only one is soluble at low temperatures. A solution is formed using a minimum amount of hot solvent, then allowed to cool to a temperature in which only one of the solutes is soluble. The solute with the lower solubility precipitates and is then filtered out of the solution. The solute with the higher solubility is then recovered from the solution.

Question 4

Which of the following processes does not use energy?

Answer 4

particle interactions (in the generation of a solution) that result in a buildup of attractive forces

Question 5

Which of the following steps in the supersaturation of a solution is not correct?

Answer 5

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.

Question 6

Look at the calcium sulfate solubility curve.

Which statement best explains this curve?

Answer 6

Calcium sulfate dissolves in a solution by an exothermic event. Increasing the temperature shifts the equilibrium toward the solute (precipitation).

Question 7

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?

Answer 7

Raising the temperature in an exothermic equilibrium circumstance will favor the dissolving of more solute in the solution.

Question 8

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?

Answer 8

Sodium acetate dissolves in a solution via an exothermic process.

Question 9

Which statement best defines solubility?

Answer 9

Solubility is the maximum possible concentration of solute in a solution at a given temperature.

Question 10

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?

Answer 10

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).

Question 11

A cooled, supersaturated solution of sodium acetate is poured out onto a crystal of sodium acetate. Which statement about this event is not true?

Answer 11

The process of recrystallization is endothermic.

Question 12

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?

Answer 12

When the sodium acetate (NaC2H3O2 ) is dissolved, it is an example of a chemical change, not a physical change.

Question 13

Which of the following does not describe solubility?

Answer 13

It is the amount of solute in a given solution.