A binary solution of two volatile liquids and denote the two components as 1 and 2. When taken in a closed vessel, both the components would evaporate and eventually an equilibrium would be established between vapour phase and the liquid phase. Let the total vapour pressure at this stage be ptotal and p1 and p2 be the partial vapour pressures of the two components 1 and 2 respectively. These partial pressures are related to the mole fractions x1 and x2 of the two components 1 and 2 respectively. The French chemist, Francois Marte Raoult (1886) gave the quantitative relationship between them. The relationship is known as the Raoult’s law which states that for a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution.
Which of the following is correct about a solution showing positive deviation from Raoult's law?
Answer & explanation
Correct answer: option 4
The correct answer is option 4. Both (1) and (3).
Positive deviation from Raoult's law occurs when the interactions between unlike molecules \((A-B)\) are weaker than those between like molecules (A-A and B-B). This results in the following characteristics:
Higher Vapor Pressure: The solution has a higher vapor pressure than expected because the weaker A-B interactions make it easier for the molecules to escape into the vapor phase.
Endothermic Mixing (\(\Delta H_{\text{mix}} > 0\)): Since the interactions between unlike molecules (A-B) are weaker, energy is absorbed to break the stronger A-A and B-B interactions and form the weaker A-B interactions. This results in an overall endothermic process, meaning that the enthalpy change of mixing is positive.
Increase in Volume (\(\Delta V_{\text{mix}} > 0\)): Weaker A-B interactions mean that the molecules are less tightly packed compared to the pure components, leading to an increase in volume upon mixing.
Explanation of Each Option
1. \(\Delta H_{\text{mix}} > 0\):
This indicates that the mixing process is endothermic. In solutions showing positive deviation, energy is required to overcome the stronger A-A and B-B interactions and to form the weaker A-B interactions. Therefore, the enthalpy change of mixing (\(\Delta H_{\text{mix}}\)) is greater than zero.
2. \(\Delta H_{\text{mix}} < 0\):
This would indicate an exothermic process where the formation of A-B interactions releases energy. However, for positive deviation, the process is endothermic, so this option is incorrect.
3. \(\Delta V_{\text{mix}} > 0\):
This indicates that the volume increases upon mixing. For solutions showing positive deviation, the weaker interactions between unlike molecules cause the solution to occupy more space than the pure components, resulting in a positive volume change.
4. Both (1) and (3):
Since both \(\Delta H_{\text{mix}} > 0\) and \(\Delta V_{\text{mix}} > 0\) are true for solutions showing positive deviation from Raoult's law, this option is correct.
Conclusion: For a solution showing positive deviation from Raoult's law, both the enthalpy change of mixing is positive (\(\Delta H_{\text{mix}} > 0\)) and the volume change of mixing is positive (\(\Delta V_{\text{mix}} > 0\)). Therefore, the correct answer is: Both (1) and (3).