Question

Select correct statement? (a) Heats of vaporisation for a pure solvent and for a solution are similar because similar intermolecular forces between solvent molecules must be overcome in both cases (b) Entropy change between solution and vapoür is smaller than the entropy change between pure solvent and vapour (c) Boiling point of the solution is larger than that of the pure solvent (d) All are correct statements

Short answer

Option (c), which states that the boiling point of the solution is larger than that of the pure solvent, is the correct statement.

Step-by-step solution

Statement Analysis - Option (a)

Analyze if the heat of vaporization for a pure solvent and for a solution being similar is a correct statement. The heat of vaporization for a pure solvent vs. a solution may differ because, in a solution, interactions between solute and solvent molecules also have to be considered. However, the fundamental intermolecular forces that must be overcome between solvent molecules are similar. Hence, while the values may be different, the underlying physical process is similar, which makes this statement contextually true.

Statement Analysis - Option (b)

Evaluate whether the entropy change between solution and vapor is smaller than that between pure solvent and vapor. The presence of a solute in a solution usually results in a decrease in entropy compared to a pure solvent due to the interaction between solute and solvent molecules. Thus, vaporizing a solution would involve a larger change in entropy than vaporizing a pure solvent, because you are starting with a lower entropy state in the case of a solution. This makes the statement false.

Statement Analysis - Option (c)

Consider if the boiling point of the solution being larger than that of the pure solvent is true. Boiling point elevation is a colligative property, which means that the addition of a solute to a solvent will increase the boiling point of the solution compared to the pure solvent. This is due to the solute particles causing a decrease in the vapor pressure of the solvent. This change requires a higher temperature to achieve the vapor pressure necessary for boiling. Therefore, this statement is correct.

Statement Analysis - Option (d)

Determine if all the given statements are correct. From the analysis above, option (b) was found to be incorrect. Hence, the statement that all are correct is false.

Key concepts

Boiling Point Elevation

Understanding the concept of boiling point elevation is crucial when studying solutions and their properties. It's a phenomenon where the boiling point of a solvent increases upon the addition of a solute. This occurs because the solute particles interfere with the solvent molecules' ability to evaporate.

For the boiling point to be reached, a liquid must attain a vapor pressure that equals the atmospheric pressure. Solutes in a solution lower the vapor pressure, which means that a higher temperature (boiling point elevation) is now needed for the liquid to turn into vapor.

Mathematically, this can be described using the formula: \(\Delta T_b = i \cdot K_b \cdot m\), where \(\Delta T_b\) is the boiling point elevation, \(i\) is the van’t Hoff factor representing the number of particles the solute provides, \(K_b\) is the ebullioscopic constant, and \(m\) is the molality of the solution. This demonstrates how the addition of a solute invariably leads to an increase in the boiling point, which can be calculated with known values.

Entropy Change in Solutions

The concept of entropy change in solutions can seem abstract at first, but it provides valuable insight into the behavior of solutions. Entropy, a measure of disorder or randomness within a system, changes when a solute is dissolved in a solvent.

When a solute is introduced into a solvent, the ordered arrangement of the solvent's molecules is disrupted, generally leading to an increase in entropy (more disorder). However, there can also be cases where the interactions between solute and solvent might result in a decrease in entropy, typically where association or strong interactions occur, creating a more ordered system.

Upon vaporization, the system's entropy increases significantly as the molecules move from a liquid state to a gaseous state, which is more disordered. However, the initial entropy of a solution is usually lower than that of a pure solvent, the vaporization of a solution results in a larger change in entropy due to the initially more ordered state. Hence, understanding these entropy changes is key for predicting the behavior of a solution during phase transitions.

Heat of Vaporization

The heat of vaporization is the energy required to transform a liquid into a gas at its boiling point. For pure substances, this is a distinct value; however, for solutions, this quantity can be influenced by the presence of a solute.

In a pure solvent, the heat of vaporization corresponds to the energy needed to overcome the intermolecular forces holding the molecules together in the liquid state. With a solution, these forces are altered by the presence of the solute, which can either strengthen or weaken the existing forces within the solvent. Therefore, this could potentially change the amount of energy needed for vaporization.

While the intermolecular forces to be overcome are similar, the presence of the solute usually requires additional energy for vaporization compared to the pure solvent. Hence, while the underlying physical process is the same, the values of the heat of vaporization for pure solvents and solutions are different, which is important in practical applications like distillation and the production of chemical products.