Ch.13 - Properties of Solutions

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.13 - Properties of Solutions

Problem 20

Chapter 13, Problem 20

For the dissolution of LiCl in water, ∆H_soln = -37 kJ/mol. Which term would you expect to be the largest negative number: ∆H_solvent, ∆H_solute, or ∆H_mix?

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Understand the process of dissolution: When a solute dissolves in a solvent, three main enthalpy changes occur: ∆Hsolute (energy required to break apart the solute), ∆Hsolvent (energy required to separate the solvent molecules), and ∆Hmix (energy released when solute and solvent interact).

Recognize that ∆Hsoln is the overall enthalpy change for the dissolution process, given as -37 kJ/mol. This indicates that the process is exothermic, meaning it releases energy.

Consider the nature of each term: ∆Hsolute is typically positive because energy is required to break the solute into individual ions or molecules. ∆Hsolvent is also usually positive as energy is needed to disrupt the solvent structure.

Analyze ∆Hmix: This term is often negative because energy is released when the solute and solvent interact and form new bonds or attractions. Since the overall process is exothermic, ∆Hmix must be significantly negative to offset the positive values of ∆Hsolute and ∆Hsolvent.

Conclude that ∆Hmix is expected to be the largest negative number, as it must be sufficiently negative to result in the overall negative ∆Hsoln value of -37 kJ/mol.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy of Solution (∆Hsoln)

The enthalpy of solution (∆Hsoln) refers to the heat change that occurs when a solute dissolves in a solvent. A negative value indicates that the process is exothermic, meaning heat is released into the surroundings. In this case, the dissolution of LiCl in water releases -37 kJ/mol, suggesting that the interactions between the solute and solvent are energetically favorable.

Enthalpy Changes in Dissolution

The dissolution process can be broken down into three components: ∆Hsolute (energy required to separate solute particles), ∆Hsolvent (energy required to separate solvent molecules), and ∆Hmix (energy released when solute and solvent interact). The overall enthalpy change (∆Hsoln) is the sum of these three components, which helps in understanding the energetics of the dissolution process.

Thermodynamic Favorability

In thermodynamics, a process is considered favorable if it results in a decrease in enthalpy (exothermic) or an increase in entropy. For the dissolution of LiCl, the negative ∆Hsoln suggests that the process is thermodynamically favorable. The largest negative term among ∆Hsolvent, ∆Hsolute, or ∆Hmix would likely be ∆Hmix, as it represents the energy released when the solute and solvent interact, contributing significantly to the overall exothermic nature of the solution process.

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Related Practice

Textbook Question

KBr is relatively soluble in water, yet its enthalpy of solution is + 19.8 kJ/mol. Which of the following statements provides the best explanation for this behavior? (a) Potassium salts are always soluble in water. (b) The entropy of mixing must be unfavorable. (c) The enthalpy of mixing must be small compared to the enthalpies for breaking up water–water interactions and K–Br ionic interactions. (d) KBr has a high molar mass compared to other salts like NaCl.

Textbook Question

When ammonium chloride dissolves in water, the solution becomes colder. (a) Is the solution process exothermic or endothermic?

Textbook Question

When ammonium chloride dissolves in water, the solution becomes colder. (b) Why does the solution form?

Textbook Question

Two nonpolar organic liquids, hexane (C₆H₁₄) and heptane (C₇H₁₆), are mixed. (a) Do you expect ∆H_soln to be a large positive number, a large negative number, or close to zero? Explain.

Textbook Question

Two nonpolar organic liquids, hexane (C₆H₁₄) and heptane (C₇H₁₆), are mixed. (b) Hexane and heptane are miscible with each other in all proportions. In making a solution of them, is the entropy of the system increased, decreased, or close to zero, compared to the separate pure liquids?

For the dissolution of LiCl in water, ∆Hsoln = -37 kJ/mol. Which term would you expect to be the largest negative number: ∆Hsolvent, ∆Hsolute, or ∆Hmix?

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Key Concepts

Enthalpy of Solution (∆H<sub>soln</sub>)

Enthalpy Changes in Dissolution

Thermodynamic Favorability

For the dissolution of LiCl in water, ∆H_soln = -37 kJ/mol. Which term would you expect to be the largest negative number: ∆H_solvent, ∆H_solute, or ∆H_mix?