Problem 18 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

The process of ammonium chloride dissolving in water is an endothermic process as the solution becomes colder, indicating heat energy is absorbed from the surroundings. The solution forms because the attractive forces between the solute (ammonium chloride) and the solvent (water) are strong enough to overcome the attractive forces within the solute and the solvent, resulting in the dissociation of ammonium chloride into its individual ions (ammonium ions and chloride ions) in an aqueous solution. This process is called solvation.

Step 1: Determine endothermic or exothermic process

When a substance dissolves in water, it means that the particles of the substance are interacting with water molecules. If the solution becomes colder, it indicates that the dissolution process is absorbing heat energy from the surroundings. This is an endothermic process. In an exothermic process, the system releases heat to the surroundings, which would result in the solution becoming warmer instead. So, the process of ammonium chloride dissolving in water is an endothermic process.

Step 2: Why the solution forms

A solution forms when the solute dissolves in the solvent. In this case, ammonium chloride is the solute, and water is the solvent. The formation of a solution depends on the balance between the attractive forces within the solute, the attractive forces within the solvent, and the attractive forces between the solute and the solvent. In order for a solution to form, the attractive forces between the solute and the solvent must overcome the attractive forces within the solute and the solvent. The water molecules surround the ammonium chloride ions, creating a hydration shell around each ion, and effectively separate them. This process is called solvation. As a result, the ammonium chloride dissociates into its individual ions (ammonium ions and chloride ions), forming an aqueous solution. The process of ammonium chloride solution formation in water occurs because the attractive forces between the solute and solvent are strong enough to overcome the attractive forces within the solute and the solvent.