Question

The activity coefficient `gamma' = 1 implies:

• A. Ideal solution behaviour
• B. Maximum positive deviation
• C. Maximum negative deviation
• D. Azeotropic behaviour

Hint:

Ideal solution behaviour also implies that the enthalpy of mixing (\( \Delta H_{\text{mix}} \)) and volume change of mixing (\( \Delta V_{\text{mix}} \)) are both equal to zero.
This is a standard theoretical benchmark in solution chemistry.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the physical meaning of an activity coefficient (\( \gamma \)) equal to 1.
Activity coefficients are used in solution thermodynamics to account for non-ideal behaviour in liquid phases.

Step 2: Key Formula or Approach:
The activity of component \( i \) in a liquid solution is given by:
\[ a_i = \gamma_i \cdot x_i \]
where \( x_i \) is the mole fraction and \( \gamma_i \) is the activity coefficient.
For an ideal solution, the activity of a component is exactly equal to its mole fraction:
\[ a_i = x_i \]

Step 3: Detailed Explanation:

• Ideal Solution Behaviour: When the activity coefficient \( \gamma_i = 1 \) for all components across the entire composition range, the liquid mixture is described as an ideal solution.
Under this condition, the mixture obeys Raoult's Law:
\[ p_i = x_i \cdot P^*_i \]
This occurs when the intermolecular forces between unlike molecules (A-B) are identical to the intermolecular forces between like molecules (A-A and B-B).

• Deviations from Ideality:
If \( \gamma_i \gt 1 \), the solution exhibits a positive deviation from ideality (molecules repel each other more than they attract, increasing vapour pressure).
If \( \gamma_i \lt 1 \), the solution exhibits a negative deviation from ideality (molecules attract each other strongly, decreasing vapour pressure).

• Azeotropic Behaviour: This occurs when the vapour composition equals the liquid composition (\( y_i = x_i \)) at a specific point, which is caused by strong deviations from ideality (\( \gamma_i \neq 1 \)).


Step 4: Final Answer:
An activity coefficient \( \gamma = 1 \) implies that the system is displaying ideal solution behaviour.