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Raoult’s Law
Ideal Solutions
Non- Ideal Solutions
Positive Deviation from Raoult’s Law
Negative Deviation from Raoult’s Law
Azeotrope
Maximum Boiling Azeotrope
Minimum Boiling Azeotrope
In 1986, a French Chemist named Francois Marte Raoult proposed a quantitative relation between partial pressure and mole fraction of volatile liquids. The law states that mole fraction of the solute component is directly proportional to its partial pressure.
Image 1: Types of Solutions
On the basis of Raoult’s Law, liquid-liquid solutions are classified into two types of solutions, they are:
Non-ideal Solutions
The solutions which obey Raoult’s Law at every range of concentration and at all temperatures are called Ideal Solutions. We can obtain ideal solutions by mixing two ideal components that is, solute and a solvent having similar molecular size and structure. For Example, consider two liquids A and B, and mix them. The formed solution will experience several intermolecular forces of attractions inside it, which will be:
A – A intermolecular forces of attraction
B – B intermolecular forces of attraction
A – B intermolecular forces of attraction
The solution is said to be an ideal solution, only when the intermolecular forces of attraction between A – A, B – B and A – B are nearly equal.
Image 2: Molecular structure is same in case of ideal solutions
Ideal Solutions generally have characteristics as follows:
They follow Raoult’s Law, which means partial pressure of components A and B in a solution will be P_{A} = P_{A}^{0} x_{A} and P_{B} = P_{B}^{0} x_{B }where P_{A}^{0} and P_{B}^{0} are respective vapour pressure in pure form and x_{A} and x_{B} are respective mole fractions of components A and B
The enthalpy of mixing of two components should be zero, that is, Δ_{mix} H = 0. This signifies that no heat is released or absorbed during mixing of two pure components to form ideal solution
The volume of mixing of two components should be zero that is, Δ_{mix} V = 0. This means that total volume of solution is equal to the sum of the volume of solute and solution. Adding further, it also signifies that there is no occurrence of contraction or expansion of volume while mixing of two components
The solute-solute interaction and solvent-solvent interaction is nearly equal to solute-solvent interaction
Note: Perfectly ideal solutions are rare in nature, only some solutions show some ideal behavior.
Examples of Ideal Solutions
n-hexane and n-heptane
Bromoethane and Chloroethane
Benzene and Toluene
CCl_{4} and SiCl_{4}
Chlorobenzene and Bromobenzene
Ethyl Bromide and Ethyl Iodide
n-Butyl Chloride and n-Butyl Bromide
The solutions which don’t obey Raoult’s law at every range of concentration and at all temperatures are called Non-Ideal Solutions. Non-ideal solutions deviate from ideal solutions and are also known as Non-Ideal Solutions.
Image 3: Graph between vapour pressuring and mole fraction
Image 4: The enthalpy of mixing is non-zero in non-ideal solution as some heat is released or absorbed in the process.
Non-ideal solutions depict characteristics as follows:
The solute-solute and solvent-solvent interaction is different from that of solute-solvent interaction
The enthalpy of mixing that is, Δ_{mix} H ≠ 0, which means that heat might have released if enthalpy of mixing is negative (Δ_{mix} H < 0) or the heat might have observed if enthalpy of mixing is positive (Δ_{mix} H > 0)
The volume of mixing that is, Δ_{mix} V ≠ 0, which depicts that there will be some expansion or contraction in dissolution of liquids
Non-ideal solutions are of two types:
Non-ideal solutions showing positive deviation from Raoult’s Law
Non-ideal solutions showing negative deviation from Raoult’s Law
Positive Deviation from Raoult’s Law occurs when the vapour pressure of component is greater than what is expected in Raoult’s Law. For Example, consider two components A and B to form non-ideal solutions. Let the vapour pressure, pure vapour pressure and mole fraction of component A be P_{A} , P_{A}^{0} and x_{A} respectively and that of component B be P_{B} , P_{B}^{0} and x_{B }respectively. These liquids will show positive deviation when Raoult’s Law when:
P_{A} > P_{A}^{0} x_{A} and P_{B} > P^{0}_{B} x_{B}, as the total vapour pressure (P_{A}^{0} x_{A} + P^{0}_{B} x_{B}) is greater than what it should be according to Raoult’s Law.
The solute-solvent forces of attraction is weaker than solute-solute and solvent-solvent interaction that is, A – B < A – A or B – B
The enthalpy of mixing is positive that is, Δ_{mix} H > 0 because the heat absorbed to form new molecular interaction is less than the heat released on breaking of original molecular interaction
The volume of mixing is positive that is, Δ_{mix} V > 0 as the volume expands on dissolution of components A and B
Examples: Following are examples of solutions showing positive deviation from Raoult’s Law
Acetone and Carbon disulphide
Acetone and Benzene
Carbon Tetrachloride and Toluene or Chloroform
Methyl Alcohol and Water
Acetone and Ethanol
Ethanol and Water
Negative Deviation occurs when the total vapour pressure is less than what it should be according to Raoult’s Law. Considering the same A and B components to form a non-ideal solution, it will show negative deviation from Raoult’s Law only when:
P_{A} < P_{A}^{0} x_{A} and P_{B} < P^{0}_{B} x_{B} as the total vapour pressure (P_{A}^{0} x_{A} + P^{0}_{B} x_{B}) is less than what it should be with respect to Raoult’s Law
The solute-solvent interaction is stronger than solute-solute and solvent-solvent interaction that is, A – B > A – A or B – B
The enthalpy of mixing is negative that is, Δ_{mix} H < 0 because more heat is released when new molecular interactions are formed
The volume of mixing is negative that is, Δ_{mix} V < 0 as the volume decreases on dissolution of components A and B
Examples: Following are examples of solutions showing negative deviation from Raoult’s Law
Chloroform and Benzene
Chloroform and Diether
Acetone and Aniline
Nitric Acid ( HNO_{3}) and water
Acetic Acid and pyridine
Hydrochloric Acid ( HCl) and water
Image 5: Graph between vapour pressure and mole fraction
Azeotropes are defined as a mixture of two liquids which has a constant composition in liquid and vapour phase at all temperatures. Azeotropes can’t be separated by fractional distillation, as the composition of vapour phase remains same after boiling. Because of uniform composition azeotropes are also known as Constant Boiling Mixtures.
There are two types of Azeotropes:
Maximum Boiling Azeotrope is formed when we mix two non-ideal solutions at some specific composition, showing large negative deviation from Raoult’s law.
Examples:
Nitric Acid (HNO_{3}) (68%) and water (32%) form maximum boiling azeotrope at boiling temperature of 393.5 K
Hydrochloric Acid (HCl) (20.24%) and water form maximum boiling azeotrope at boiling temperature of 373 K
Minimum Boiling Azeotrope is formed when we mix two non-ideal solutions at some specific composition, which shows large positive deviation from Raoult’s Law.
Example:
Watch this Video for more reference
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Ideal and Non-Ideal Solutions
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