MCAT General Chemistry Question 260: Answer and Explanation
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Question: 260
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.
Correct Answer: B
Explanation:
B Similar to bond formation, forming solute-solvent interactions is exothermic, meaning energy is released, not required; similarly breaking solute-solute or solvent-solvent interactions is endothermic (requires energy), like bond breaking (eliminate choice A). In addition, sodium chloride is soluble at room temperature (eliminate choice C). Salts are held together by ionic bonds, but Table 1 shows through the small Ksp values that not all of them are substantially soluble (eliminate choice D). For the dissolution of sodium chloride to be spontaneous, the increase in entropy must outweigh the endothermic process, yielding a negative Gibbs free energy.