Raoult's Law: Positive and Negative Deviation
Raoult's law: The partial vapor pressure of each component of an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution.
In ideal solutions, the intermolecular forces between all molecules are uniform.
Also read Raoult's law for Volatile and Non-volatile Solutes
Positive Deviation from Raoult's Law
Condition:
Positive deviation from Raoult's law occurs when the intermolecular forces between the solute and solvent molecules are weaker than the intermolecular forces within the pure solute and pure solvent.
Consequences:
Vapor Pressure Increases: Due to weaker intermolecular forces, molecules escape the solution very easily and increases the vapor pressure than predicted by Raoult's law.
Positive Enthalpy of Mixing: Energy is required to overcome the weaker intermolecular forces. Therefore, enthalpy of mixing is positive, i.e. ΔHmix > 0
Positive Volume of Mixing: Due to weaker intermolecular forces, the total volume of the solution increases than the sum of the individual volumes of the components. Therefore, volume of mixing is positive, i.e. ΔVmix > 0.
Examples:
1. Mixtures of ethanol and water
2. Mixtures of acetone and carbon disulfide
3. Mixtures of acetone and Benzene
4. Mixtures of ethanol and Cyclohexane
5. Mixtures of carbon Tetrachloride and Toluene or Chloroform
Explanation:
Mixtures of ethanol and water: Ethanol molecules are held together by relatively weak hydrogen bonds while water molecules are held together by strong hydrogen bonds.
When ethanol and water are mixed, the hydrogen bonds between ethanol and water molecules are weaker than the hydrogen bonds in pure water. This weaker interaction allows both ethanol and water molecules to escape the liquid phase very easily and due to this, amount of vapour increases. So, vapor pressure increases than predicted by Raoult's law.
Due to weaker interaction, volume of solution after the mixing of these two liquids also increases. Therefore, ΔHmix and ΔVmix both are positive and the solution exhibits a minimum boiling point azeotrope.
Negative Deviation from Raoult's Law
Condition:
Negative deviation from Raoult's law occurs when the intermolecular forces between the solute and solvent molecules are stronger than the intermolecular forces within the pure solute and pure solvent.
Consequences:
Vapor Pressure Decreases: Due to stronger intermolecular forces, molecules are held together more tightly in the solution and decreases the vapor pressure than predicted by Raoult's law.
Negative Enthalpy of Mixing: Energy released when the stronger intermolecular forces occurs. Therefore, enthalpy of mixing is negative, i.e. ΔHmix < 0.
Negative Volume of Mixing: Due to stronger intermolecular forces, the total volume of the solution is less than the sum of the individual volumes of the components. Therefore, volume of mixing is positive, i.e. ΔVmix < 0.
Examples:
1. Mixtures of chloroform and acetone
2. Mixtures of water and hydrochloric acid
3. Mixtures of water and Nitric Acid
4. Mixtures of chloroform and diethyl ether
5. Mixtures of aniline and Phenol
Explanation:
Mixtures of chloroform and acetone: Chloroform molecules have relatively weak dipole-dipole interactions while acetone molecules also have dipole-dipole interactions.
When chloroform and acetone are mixed, a strong dipole-induced dipole interaction, specially hydrogen bonding, occurs between the hydrogen atom of chloroform and the oxygen atom of acetone. This interaction is stronger than the intermolecular forces in the pure liquids.
This stronger interaction reduces the tendency of the molecules to escape into the vapor phase and due to this, the vapor pressure decreases than predicted by Raoult's law.
Due to stronger interaction, volume of solution after the mixing of these two liquids also decreases. Therefore, ΔHmix and ΔVmix both are negative and the solution exhibits a maximum boiling point azeotrope.
Remember:
Positive Deviations: Weaker solute-solvent interactions.
Negative Deviations: Stronger solute-solvent interactions.
Also read Deviation of Vapour Pressure Curve from Ideal Behaviour
Which liquid pairs show negative deviation from Raoult’s Law ? Explain and draw the graph for it by taking an example of chloroform and acetone. NIOS Sr. Secondary October 2024
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