Pick out the correct statement from among the following

When a liquid boils at its normal boiling point, \(Q = \Delta H\).

The correct answer is option 2. When a liquid boils at its normal boiling point, \(Q = \Delta H\).

Let us delve into each of the statements in more detail to understand the concepts behind them and why they are correct or incorrect.

1. In the process of boiling of a liquid, work is done on the system.

Boiling Process: Boiling is the phase transition from liquid to vapor at a specific temperature and pressure (boiling point). During boiling, the liquid molecules gain enough energy to overcome intermolecular forces and enter the vapor phase.

Work Done: When a liquid boils, the volume of the system increases as liquid converts to gas. In a typical scenario, such as boiling water in an open container, the system (liquid) expands against the atmospheric pressure. This means the system does work on the surroundings, not the other way around.

Conclusion: Therefore, during boiling, work is done by the system on the surroundings. This makes the statement incorrect.

2. When a liquid boils at its normal boiling point, \( Q = \Delta H \).

Boiling at Normal Boiling Point: The normal boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals atmospheric pressure.

Heat Added (Q): The heat added to the liquid at its boiling point is used to convert the liquid into vapor without changing the temperature. This process is isothermal for a pure substance at constant pressure.

Enthalpy Change (\(\Delta H\)): The enthalpy change associated with this phase transition is the enthalpy of vaporization (\(\Delta H_{vap}\)).

Energy Relationship: At the boiling point, the heat added to the system (Q) is directly equal to the enthalpy change of vaporization (\(\Delta H_{vap}\)), so \( Q = \Delta H_{vap} \).

Conclusion: This statement is correct.

3. Joule–Thomson effect is an isochoric process.

Joule–Thomson Effect: This effect describes the temperature change of a real gas when it is forced through a porous plug or valve while kept insulated so that no heat is exchanged with the environment. This process is adiabatic (no heat exchange) and isenthalpic (constant enthalpy).

Isochoric Process: An isochoric process is one that occurs at constant volume.

Difference: In the Joule–Thomson process, the volume of the gas changes as it expands or contracts, thus it is not an isochoric process.

Conclusion: The statement is incorrect because the Joule–Thomson effect is an isenthalpic process, not isochoric.

4. Molar heat of fusion is an extensive property.

Molar Heat of Fusion: This is the amount of energy required to melt one mole of a solid at its melting point. It is usually expressed in units of energy per mole (e.g., kJ/mol).

Intensive vs. Extensive Properties: Intensive properties do not depend on the amount of substance (e.g., temperature, pressure, molar heat of fusion), while extensive properties depend on the amount of substance (e.g., total heat, mass, volume).

Nature of Molar Heat of Fusion: Since molar heat of fusion is defined per mole of substance, it is an intensive property.

Conclusion: The statement is incorrect because molar heat of fusion is an intensive property.

Summary: Based on the analysis, the correct statement is: (2) When a liquid boils at its normal boiling point, \( Q = \Delta H \).