MCAT General Chemistry Question 130: Answer and Explanation

Home > MCAT Test > MCAT general chemistry practice tests

Test Information

Question: 130

10. The salt KCl is dissolved in a beaker. To an observer holding the beaker, the solution begins to feel colder as the KCl dissolves. From this observation, one could conclude that:

  • A. ΔS°soln is large enough to overcome the unfavorable ΔH°soln.
  • B. KCl is mostly insoluble in water.
  • C. ΔS°soln must be negative when KCl dissolves.
  • D. boiling point depression will occur in this solution.

Correct Answer: A

Explanation:

Dissolution is governed by enthalpy and entropy, which are related by the equation ΔG°soln = ΔH°soln - TΔS°soln. The cooling of the solution indicates that heat is used up in this bond-breaking reaction. In other words, dissolution is endothermic, and ΔH is positive. The reaction is occurring spontaneously, so ΔG must be negative. The only way that a positive ΔH can result in a negative ΔG is if entropy, ΔS, is a large, positive value as in choice (A). Conceptually, that means that the only way the solid can dissolve is if the increase in entropy is great enough to overcome the increase in enthalpy. Choice (B) is incorrect because it is clearly stated in the question stem that KCl dissolves; further, all salts of Group 1 metals are soluble. Choice (C) is incorrect because ΔS°soln must be positive in order for KCl to dissolve. Finally, choice (D) is incorrect because solute dissolution would cause the boiling point to elevate, not depress. It is also not a piece of evidence that could be found simply by observing the beaker's temperature change.

Previous       Next