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

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Identify the process: Dissolution of ammonium chloride (NH_4Cl) in water is an endothermic process, meaning it absorbs heat from the surroundings.

Understand the energy changes: The dissolution involves breaking ionic bonds in NH_4Cl and forming new interactions between NH_4^+ and Cl^- ions with water molecules.

Consider entropy: The process increases the disorder or randomness (entropy) of the system, as the solid NH_4Cl becomes dispersed ions in solution.

Apply Gibbs Free Energy: Use the equation \( \Delta G = \Delta H - T\Delta S \) to determine spontaneity. Even if \( \Delta H \) is positive (endothermic), a large positive \( \Delta S \) can make \( \Delta G \) negative, favoring the formation of the solution.

Conclude with spontaneity: The increase in entropy and the resulting negative \( \Delta G \) make the dissolution of NH_4Cl in water spontaneous, despite the absorption of heat.

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Key Concepts

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

Dissolution Process

The dissolution process involves the breaking apart of solute particles and their interaction with solvent molecules. In the case of ammonium chloride (NH4Cl), when it dissolves in water, the ionic bonds between the ammonium and chloride ions are broken, allowing them to disperse throughout the water. This process requires energy, which is absorbed from the surroundings, leading to a decrease in temperature.

Endothermic Reactions

An endothermic reaction is a chemical process that absorbs heat from its surroundings. The dissolution of ammonium chloride in water is an example of this type of reaction, as it requires energy to break the ionic bonds and separate the ions. As a result, the surrounding water loses heat, causing the temperature of the solution to drop.

Thermodynamics of Solutions

Thermodynamics in the context of solutions refers to the energy changes that occur during the mixing of solutes and solvents. The overall energy change during dissolution is determined by the balance between the energy required to break solute-solute and solvent-solvent interactions and the energy released from solute-solvent interactions. In the case of ammonium chloride, the energy absorbed during dissolution exceeds the energy released, resulting in a colder solution.

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

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[Section 18.4]

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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?