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Q1.14:
What is meant by positive and negative deviations from Raoult's law and how is the sign of (\Delta_{mix}H) related to positive and negative deviations from Raoult's law?

Solution :

1. Theoretical Foundation: Raoult's Law

Raoult's Law states that for an ideal solution, the partial vapor pressure of each volatile component in the solution is directly proportional to its mole fraction in the solution. For a binary solution of components A and B: $P_A = P_A^0 x_A$ and $P_B = P_B^0 x_B$. Deviations occur when intermolecular forces between solute and solvent differ from those in pure components.

2. Positive Deviation from Raoult's Law

Positive deviation occurs when the vapor pressure of the solution is higher than that predicted by Raoult's Law. This happens when the intermolecular forces between component A and B (A-B interactions) are weaker than the forces present in pure components (A-A and B-B interactions).

  • Vapor Pressure: $P_{total} > P_A^0 x_A + P_B^0 x_B$
  • Enthalpy of Mixing ($\Delta_{mix}H$): Since the A-B interaction is weaker, energy is absorbed to overcome the stronger original attractions. Thus, $\Delta_{mix}H > 0$ (Endothermic process).
  • Volume of Mixing ($\Delta_{mix}V$): The molecules are pushed further apart, leading to expansion. Thus, $\Delta_{mix}V > 0$.

3. Negative Deviation from Raoult's Law

Negative deviation occurs when the vapor pressure of the solution is lower than that predicted by Raoult's Law. This happens when the intermolecular forces between component A and B (A-B interactions) are stronger than those in pure components (A-A and B-B interactions).

  • Vapor Pressure: $P_{total} < P_A^0 x_A + P_B^0 x_B$
  • Enthalpy of Mixing ($\Delta_{mix}H$): The formation of stronger A-B bonds releases energy. Thus, $\Delta_{mix}H < 0$ (Exothermic process).
  • Volume of Mixing ($\Delta_{mix}V$): The molecules are pulled closer together, leading to contraction. Thus, $\Delta_{mix}V < 0$.

4. Comparative Summary Table

Characteristic Positive Deviation Negative Deviation
Intermolecular Forces $A-B < A-A$ or $B-B$ $A-B > A-A$ or $B-B$
$\Delta_{mix}H$ Positive ($> 0$) Negative ($< 0$)
$\Delta_{mix}V$ Positive ($> 0$) Negative ($< 0$)

5. Graphical Representation

Vapor Pressure vs Mole Fraction x_A = 1, x_B = 0 x_A = 0, x_B = 1 Positive Negative

Final Solution: Positive deviation arises from weaker solute-solvent interactions resulting in an endothermic mixing process ($\Delta_{mix}H > 0$), while negative deviation arises from stronger solute-solvent interactions resulting in an exothermic mixing process ($\Delta_{mix}H < 0$).


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