MCAT General Chemistry Question 262: Answer and Explanation

Home > MCAT Test > MCAT general chemistry practice tests

Test Information

Use your browser's back button to return to your test results.
Do more MCAT general chemistry practice tests.

Question: 262

4. Which of the following is consistent with the differences in ∆H^o_diss 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⁺.

Correct Answer: D

Explanation:

D Effective dissolution involves the endothermic step of overcoming the electrostatic charges holding the solid salts together and the exothermic step of coordinating solvents to the separated ions. A negative value of ∆H^o_diss likely indicates relatively weak electrostatic forces in the solid (small endothermic step), and effective solvation by water (large exothermic step). Table 2 shows that LiCl has a much more negative value of ∆H^o_diss than NaCl. Choice A, stronger electrostatic forces in LiCl and no difference in solvation, would lead to a more negative ∆H^o_diss for NaCl. Choice B would also result in a more negative value of ∆H^o_diss for NaCl, as it indicates that electrostatics are equivalent while Na⁺ has stronger interactions with water. Choice C is incorrect because a large negative value of ∆H^o_diss would be difficult to achieve if water were unable to coordinate Li⁺. Choice D includes a viable combination of slightly weaker attractive forces in NaCl but much better solvation for Li⁺.

Previous Next