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Ch.13 - Properties of Solutions
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.13 - Properties of SolutionsProblem 21a
Chapter 13, Problem 21a

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

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Understand the concept of \( \Delta H_{\text{soln}} \): It represents the enthalpy change when a solution is formed. For nonpolar substances, the interactions are primarily dispersion forces.
Consider the nature of hexane and heptane: Both are nonpolar organic liquids, meaning they have similar types of intermolecular forces (London dispersion forces).
When similar nonpolar substances mix, the energy required to break the intermolecular forces in the pure substances is similar to the energy released when new interactions are formed in the mixture.
Since the intermolecular forces in hexane and heptane are similar, the enthalpy change \( \Delta H_{\text{soln}} \) is expected to be close to zero, as there is little net energy change.
Conclude that \( \Delta H_{\text{soln}} \) is close to zero because the energy required to separate the molecules of each liquid is approximately equal to the energy released when they mix.

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Key Concepts

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

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

The enthalpy of solution (∆H<sub>soln</sub>) refers to the heat change that occurs when a solute dissolves in a solvent. It can be positive (endothermic), negative (exothermic), or close to zero, depending on the interactions between solute and solvent molecules. In the case of mixing two nonpolar liquids, the enthalpy change is often minimal due to similar intermolecular forces.
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Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence physical properties such as boiling point, melting point, and solubility. Nonpolar molecules like hexane and heptane primarily experience London dispersion forces, which are relatively weak. When two nonpolar substances are mixed, the similar types of intermolecular forces lead to minimal energy change.
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Like Dissolves Like Principle

The 'like dissolves like' principle states that polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Since both hexane and heptane are nonpolar, they are expected to mix well without significant energy changes. This principle helps predict that the enthalpy of solution for this mixture will be close to zero, as the interactions between the two liquids are similar.
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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

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?

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. (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?