Problem 18 - Chemistry: The Central Science in SI Units, Global Edition

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

(a) The solution process of ammonium chloride in water is endothermic. (b) The solution forms because the overall energy change and the increase in entropy of the system favor the formation of solute-solvent interactions, leading to the dissolution process.

Step 1: Determine if the solution process is exothermic or endothermic

When ammonium chloride dissolves in water, the solution becomes colder. This indicates that heat is being absorbed from the surroundings, which means the process is endothermic. An endothermic process is one where energy (in the form of heat) is absorbed by the system from its surroundings.

Step 2: Answer (a)

The solution process of ammonium chloride in water is endothermic.

Step 2: Understand the forces responsible for the formation of the solution

The solution forms as a result of interactions between solute particles (ammonium chloride) and solvent particles (water). Since it is an endothermic process, it suggests that the energy required to break the solute-solute and solvent-solvent interactions is less than the energy released by forming solute-solvent interactions. As water has a high polarity, it can effectively solvate and stabilize the positive ammonium ions (NH4+) and negative chloride ions (Cl-) produced by the dissociation of ammonium chloride. Another important factor is entropy. The natural tendency of a system is to move towards a higher entropy state which means a state with higher randomness or disorder. The dissolution process increases the entropy of the system, as the ions (or molecules) become more dispersed in solution.

Step 4: Answer (b)

The solution forms because the overall energy change and the increase in entropy of the system favor the formation of solute-solvent interactions, leading to the dissolution process.

Key Concepts

Solvation

Solvation is the process through which solute particles are surrounded by solvent molecules. When attempting to dissolve ammonium chloride in water, solvation is a critical step in the dissolution process. During this interaction, the polar water molecules associate with the positively charged ammonium ions ( NH_4^+ ) and negatively charged chloride ions ( Cl^- ), effectively solvatiazing them.
This interaction helps stabilize the ions in solution.

• They cluster around the ions due to their polar nature, creating a medium that supports their dispersion.
• Solvation also helps overcome the lattice energy of the solid solute.

In the specific example of ammonium chloride dissolving in water, solvation effectively pulls the ions apart, leading to a homogeneous aqueous solution where ions are uniformly distributed.

Entropy

Entropy is a measure of disorder or randomness in a system. It's an essential concept in thermodynamics that helps predict the spontaneity of processes. When ammonium chloride dissolves in water, an increase in entropy is observed because the solid crystalline structure is disrupted, allowing ions to become dispersed throughout the solution.
This increase in entropy contributes to the dissolution process being favorable, despite the fact that it is endothermic.

• The random movement of ions in the solution represents a higher entropy state compared to the ordered crystalline structure of the solid.
• Systems naturally tend toward increased entropy, which makes the dissolution process thermodynamically favorable.

While energy is absorbed (indicating an endothermic process), the increase in entropy provides a driving force that aids in the dissolution of ammonium chloride in water.

Dissolution

Dissolution is the process where a solute interacts with a solvent to form a solution. For ammonium chloride dissolved in water, the dissolution implies a transition from a solid ionic lattice to individual ions in solution.
This involves overcome the ionic lattice energy, as the strong ionic bonds within the solid must be broken.

• Upon dissolution, the ammonium and chloride ions are separated and stabilized by solvent molecules through solvation, facilitating the dissociation process.
• This process is influenced significantly by the entropic factor, as increasing the disorder in the system aids in driving the dissolution forward.

The net result is an endothermic dissolution where the increased disorder (or entropy) aids in making the process thermodynamically favorable, even as energy is absorbed from the surroundings.