Question

When ammonium chloride is dissolved in water the solution becomes cold because (1) Heat of solution of ammonium chloride is positive. (2) Heat of solution of ammonium chloride is negative. (3) Heat of dilution of ammonium chloride is positive. (4) Heat of formation of ammonium chloride is positive.

Short answer

The correct answer is (1) Heat of solution of ammonium chloride is positive.

Step-by-step solution

Understand the Concept

When a substance is dissolved in water, heat can be either absorbed or released. If the solution becomes cold, it indicates that heat is absorbed. This process is called an endothermic reaction.

Identify What Heat of Solution Means

The 'heat of solution' refers to the amount of heat energy absorbed or released when a solute dissolves in a solvent. A positive heat of solution means that the process absorbs heat from the surroundings.

Apply to Ammonium Chloride

Since the solution of ammonium chloride becomes cold, it means that heat is being absorbed from the surroundings. Therefore, the heat of solution for ammonium chloride must be positive.

Eliminate Other Options

Options 2, 3, and 4 can now be eliminated because they do not describe the endothermic nature correctly. Option (2) implies release of heat, Option (3) concerns dilution, and Option (4) is about formation which is not relevant here.

Key concepts

Endothermic Reaction

An endothermic reaction is a chemical reaction in which heat is absorbed from the surroundings. This is the opposite of an exothermic reaction, where heat is released. When a reaction is endothermic, you will often notice a temperature drop in the environment where the reaction occurs. For example, if you touch the container where an endothermic reaction is taking place, it might feel cold because the reaction is pulling heat from the container and the surrounding environment.
This concept is vital to understand why the solution becomes cold when ammonium chloride is dissolved in water. Ammonium chloride's dissolution process absorbs heat, making the solution feel cold to the touch.
Remember:

• Endothermic = Heat absorbed = Temperature drop
• Exothermic = Heat released = Temperature increase

Ammonium Chloride

Ammonium chloride (NH4Cl) is a white crystalline salt that is highly soluble in water. It is commonly used in various applications, including in dry cell batteries, metalwork, and as a food additive. When ammonium chloride is added to water, it undergoes a dissolution process that influences its thermal properties.
Its behavior during dissolution is characterized by an endothermic reaction, which means it absorbs heat from its surroundings, making the solution colder. This is an essential factor to consider when understanding the thermal dynamics of dissolving ammonium chloride in water.
Key points:

• White crystalline solid
• Highly soluble in water
• Undergoes endothermic dissolution

Dissolution Process

The dissolution process involves breaking the ionic bonds in the solid solute and forming new interactions between the solute and the solvent molecules. For ammonium chloride, the process can be described as NH4Cl (s) → NH4+ (aq) + Cl- (aq). This dissociation requires energy to break the ionic bonds, and this energy comes from the surroundings, manifested as an absorption of heat.
Consequently, during the dissolution of ammonium chloride, the system absorbs heat, and this is why you observe a cooling effect. It's important to note that not all substances have an endothermic dissolution process; some release heat and warm up the solution. The endothermic nature of ammonium chloride's dissolution is specifically due to the energetics of the reaction.
Steps in dissolution:

• Breaking ionic bonds in the solid solute
• Formation of interactions with solvent molecules
• Absorption of heat from surroundings if endothermic

Thermodynamics

Thermodynamics is the branch of physics that studies the relationships and conversions between heat and other forms of energy. In the context of ammonium chloride's dissolution, thermodynamics helps explain the absorption of heat. According to the first law of thermodynamics (energy conservation), energy absorbed in the form of heat is transformed into the energy required to break the ionic bonds in ammonium chloride.
The second law of thermodynamics, which discusses entropy, also plays a role. As ammonium chloride dissolves, the system's disorder increases, explaining the natural tendency towards the dissolution process.
Key principles:

• First law: Energy conservation
• Second law: Increased entropy
• Endothermic reactions require an input of heat