1. Raoult’s Law

  1. Raoult’s Law states that the partial vapor pressure of a component in a solution is directly proportional to its mole fraction in the solution.
  2. The formula is: Pₐ = Xₐ Pₐ⁰, where:
    • Pₐ = partial vapor pressure of component A
    • Xₐ = mole fraction of component A
    • Pₐ⁰ = vapor pressure of pure component A
  3. It applies to solutions where the solute does not volatilize and there is no chemical reaction between components.

2. Ideal Solutions

  1. Ideal solutions obey Raoult’s law at all concentrations and temperatures.
  2. The interactions between solute-solvent are similar to solute-solute and solvent-solvent interactions.
  3. Such solutions show:
    • No change in volume during mixing.
    • No change in enthalpy during mixing.
  4. Examples of ideal solutions include benzene and toluene, and hexane and heptane.

3. Non-Ideal Solutions

  1. Non-ideal solutions do not obey Raoult’s law across the concentration range.
  2. In these solutions, solute-solvent interactions differ significantly from solute-solute and solvent-solvent interactions.
  3. They show either:
    • Positive deviation from Raoult’s law: vapor pressure is higher than predicted.
    • Negative deviation from Raoult’s law: vapor pressure is lower than predicted.
  4. Examples of non-ideal solutions include:
    • Positive deviation: ethanol and acetone.
    • Negative deviation: chloroform and acetone.

4. Positive and Negative Deviations

  1. Positive deviation occurs when solute-solvent interactions are weaker, causing increased vapor pressure.
  2. Negative deviation occurs when solute-solvent interactions are stronger, reducing vapor pressure.
  3. Positive deviation examples: ethanol-water, acetone-carbon disulfide.
  4. Negative deviation examples: water-HCl, acetone-chloroform.

5. Key Points

  1. Raoult’s law is fundamental for understanding vapor pressure in solutions.
  2. In an ideal solution, enthalpy and volume changes during mixing are zero.
  3. Non-ideal solutions show deviations due to differences in interaction strengths.
  4. Positive and negative deviations affect the vapor pressure of solutions.
  5. Examples of ideal solutions include benzene and toluene; examples of non-ideal solutions include ethanol and water.
  6. The study of Raoult’s law helps in applications like distillation and solution preparation.

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