MCAT General Chemistry Practice Test 3: Thermodynamics

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The extent to which a salt dissolves in water can be quantified by its solubility product constant, (Ksp) which is defined, for a hypothetical salt XaYb, as shown in Equation 2. The greater the value of Ksp, the more soluble the compound. The Ksp of a salt is related to the free energy of dissolution by the equation ∆Godiss = -RT ln(Ksp). Table 1 lists the Ksp values for some insoluble salts.

XaYb(s) a Xb+(aq) + b Ya-(aq)

Equation 1

Ksp = [Xb+]a [Ya-]b

Equation 2

SaltKsp
PbCl21.2 × 10-5
MgCO36.8 × 10-6
BaSO41.1 × 10-10
AgCl5.4 × 10-13

Table 1 Ksp values for select insoluble salts

When a solid completely dissolves, solute particles are separated and encapsulated by solvent molecules. This process requires several steps: 1) breaking all solute-solute interactions, 2) disrupting some solvent-solvent interactions, and 3) forming new solute-solvent interactions. The combination of these processes determines the overall enthalpy change for the dissolution, which can be either exothermic or endothermic regardless of the solubility of the salt. Table 2 shows the enthalpies of dissolution for several soluble salts.

Salt Hdiss (kJ/mol)
LiCl-37.03
KCH3CO2-15.33
NaCl3.87
NH4NO325.69
KClO441.38

Table 2 Dissolution enthalpies for some soluble salts

As solids are low entropy materials, their dissolution entails an increase in entropy. The size of ∆Sdiss is dependent on the organization of solvent molecules in the solvation sphere of the dissolved ions.

1. Which of the following species is isoelectronic with the silver ion in AgCl?

  • A. Rh+
  • B. Pd
  • C. Cd2+
  • D. In-

2. Given that the dissolution of sodium chloride is endothermic and spontaneous below the saturation concentration, which of the following statements must be true?

  • A. Forming solute-solvent interactions requires energy, while breaking solute-solute and solvent-solvent interactions releases energy.
  • B. The increase in entropy must outweigh the endothermic process to create a negative Gibbs free energy.
  • C. Sodium chloride is only soluble at high temperatures.
  • D. All ionically-bound materials are substantially soluble in water.

3. Which one of the salts in Table 1 has the smallest value of ∆Godiss?

  • A. PbCl2
  • B. MgCO3
  • C. BaSO4
  • D. AgCl

4. Which of the following is consistent with the differences in ∆Hodiss for NaCl and LiCl?

  • A. The electrostatic forces in solid LiCl are much stronger than in solid NaCl, while coordination of water is equivalent for both salts.
  • B. The electrostatic forces in the two solids are approximately equivalent, while water molecules coordinate much more effectively to Na+ than Li+.
  • C. The electrostatic forces in solid LiCl are weaker than in solid NaCl, while water cannot effectively coordinate to the very small Li+ cation.
  • D. The electrostatic forces in solid NaCl are slightly weaker than in solid LiCl, while water far more efficiently coordinates Li+ than Na+.

5. The transfer of heat to or from a solution changes the temperature of the solution according to the equation q = mcT where q is the heat transferred, m is the mass of solvent, and c is the specific heat of the solvent. If a 1 g sample of a salt was dissolved in 20 mL of water (specific heat = 4.18 J/g°C) in an insulated beaker and the temperature was found to decrease by 4°C, which of the following salts was used? Assume no phase change for the water.

  • A. LiCl
  • B. KCH3CO2
  • C. NH4NO3
  • D. NaCl