ANS:C - Activity co-efficient

In the context of vapor-liquid equilibria, especially in solutions, the activity coefficient (𝛾γ) is a measure of how the behavior of a component in a solution deviates from ideal behavior. In an ideal solution, the activity coefficient is equal to 1 for all components, indicating that the solution obeys Raoult's law. Raoult's law states that the vapor pressure of a component in a solution is directly proportional to its mole fraction in the solution and the vapor pressure of the pure component. 𝑃𝑖=𝑥𝑖⋅𝑃𝑖∗Pi​=xi​⋅Pi∗​ Where:

• 𝑃𝑖Pi​ is the vapor pressure of component 𝑖i in the solution
• 𝑥𝑖xi​ is the mole fraction of component 𝑖i in the solution
• 𝑃𝑖∗Pi∗​ is the vapor pressure of pure component 𝑖i

However, real solutions often deviate from ideal behavior due to intermolecular interactions between solute and solvent molecules. The activity coefficient corrects for these deviations by adjusting the vapor pressure of the component in the solution: 𝑃𝑖=𝑥𝑖⋅𝑃𝑖∗⋅𝛾𝑖Pi​=xi​⋅Pi∗​⋅γi​ Where 𝛾𝑖γi​ is the activity coefficient of component 𝑖i. If 𝛾𝑖γi​ is greater than 1, it indicates positive deviation from ideality, meaning that the component's vapor pressure in the solution is higher than expected based on Raoult's law. This can occur when the component experiences stronger intermolecular interactions with the solvent molecules, leading to a higher tendency to escape into the vapor phase. Conversely, if 𝛾𝑖γi​ is less than 1, it indicates negative deviation from ideality, meaning that the component's vapor pressure in the solution is lower than expected based on Raoult's law. This can occur when the component experiences weaker interactions with the solvent molecules, leading to a lower tendency to escape into the vapor phase. The activity coefficient is a crucial parameter in understanding and predicting the behavior of solutions, especially in processes like distillation, where accurate knowledge of vapor-liquid equilibria is essential for separation processes.