Heat of solution is defined as

Change in heat content of the system when 1 mole of the solute is dissolved in excess of water so that further dilution of solution does not bring any heat change.

The correct answer is option 3. Change in heat content of the system when 1 mole of the solute is dissolved in excess of water so that further dilution of solution does not bring any heat change.

The heat of solution is defined as the enthalpy change when one mole of a solute dissolves in a solvent to form an infinitely dilute solution under constant pressure. This means that further dilution of the solution does not result in any additional heat change.

When a solute dissolves in a solvent, there are typically two scenarios for the enthalpy change:

Endothermic Dissolution: Some solutes require energy to break their intermolecular forces and mix with the solvent molecules. This results in an endothermic process where heat is absorbed from the surroundings.

Exothermic Dissolution: Other solutes release energy when they dissolve, typically due to favorable interactions between the solute and solvent molecules. This results in an exothermic process where heat is released to the surroundings.

Enthalpy Change (\(\Delta H_{\text{sol}}\)): The enthalpy change of solution (\(\Delta H_{\text{sol}}\)) is defined per mole of solute and is generally expressed in units of energy per mole (e.g., kcal/mol or kJ/mol).

Infinitely Dilute Solution: The definition specifies that the solution formed is infinitely dilute, meaning that the solute concentration is extremely low such that further dilution does not alter the heat released or absorbed.

Sign Convention

Positive \(\Delta H_{\text{sol}}\): Indicates an endothermic process where heat is absorbed during dissolution.

Negative \(\Delta H_{\text{sol}}\): Indicates an exothermic process where heat is released during dissolution.

Example Scenarios

Endothermic Example: Dissolving ammonium nitrate (\(NH_4NO_3\)) in water requires energy and is endothermic (\(\Delta H_{\text{sol}} > 0\)).

Exothermic Example: Dissolving sodium hydroxide (\(NaOH\)) in water releases energy and is exothermic (\(\Delta H_{\text{sol}} < 0\)).

Application in Chemistry

The heat of solution is important in various fields of chemistry, including:

Thermochemistry: Studying energy changes associated with chemical processes.

Solubility and Saturation: Understanding the factors influencing solubility and saturation of solutions.

Industrial Processes: Relevant in industrial applications where precise control over energy changes during dissolution processes is necessary.

Conclusion

The heat of solution (\(\Delta H_{\text{sol}}\)) is a fundamental thermodynamic quantity that describes the enthalpy change when a solute dissolves in a solvent to form an infinitely dilute solution. It encapsulates both endothermic and exothermic dissolution processes and is a crucial concept in understanding the energetics of solutions in chemistry. Therefore, the correct definition and understanding of heat of solution aligns with option: (3) Change in heat content of the system when 1 mole of the solute is dissolved in excess of water so that further dilution of solution does not bring any heat change.