Solutions
Homogeneous mixtures
The component present in larger amount and determines the physical state of the solution is called solvent
The component which is present in lesser amount
Composition and properties are uniform throughout the mixture
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Solubility
Solubility: Maximum amount of substance that can be dissolved in a specified amount of solvent at a specified temperature is called its solubility.
Saturated solution: A solution in which no more solute can be dissolved at a given temperature is said to be a saturated solution.
Unsaturated solution: A solution in which the solute can be dissolved at a given temperature is said to be a non-saturated solution or unsaturated solution
Factors affecting solubility of a solid in a liquid:
- Nature of solute and solvent: Polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents.
- Effect of temperature:
a) If the dissolution process is endothermic (∆sol H > 0), the solubility increases with rise in temperature.
b) If dissolution process is exothermic (∆sol H < 0), the solubility decreases with rise in temperature.
Factors affecting solubility of a gas in a liquid:
Effect of temperature: As dissolution is an exothermic process, then according to Le Chatelier’s principle, the solubility should decrease with increase of temperature.
Effect of pressure:
Henry’s law Pgas = KH x Xgas
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“The partial pressure of the gas (p) is directly proportional to the mole fraction of the gas (x) in the solution”
KH is the Henry’s law constant and is different for different gases at a particular temperature.
Higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid.
Limitation of Henry’s Law
(i) This law is applicable to the gaseous solutes which show only ideal behaviour at high temperature and low pressure.
(ii) This law is applicable to solute which does not undergo association or dissociation when the solute is dissolved in solvent.
(iii) This law is applicable to gaseous solute which does not form any product by the chemical reaction, when gaseous solute is dissolved in liquid solvent.
Raoult’s law
It states that for a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in
If Y1 and Y2 are the mole fractions of the components 1 and 2 respectively in the vapour phase then, P1 = Y1 Ptotal and P2 = Y2 Ptotal
Ideal Solutions
- The Solutions which obey Raoult’s Law at all Compositions or Concentrations
- The enthalpy of mixing of pure components to form the solution is zero. ∆Hmix = 0
- The volume change during mixing is also zero. ∆Hmix = 0
Examples:
- Toluene and Benzene
- Ethyl Iodide and Ethyl Bromide
- Bromobenzene and Chlorobenzene
- n-Heptane and n-Hexane
Non – Ideal Solutions
The solutions which do not obey Raoult’s law at all compositions or concentrations
Types of non-ideal solution
Non – Ideal Solution with Positive Deviation
Non – Ideal Solution with Negative Deviation
Azeotropes
The mixtures of liquids which boil at constant temperature like a pure liquid and possess same composition of components in liquid as well as vapour phase are called constant boiling mixtures or azeotropic mixtures.
Minimum Boiling Azeotropes
Show large positive deviations from Raoult’s law
The mixture will have a minimum boiling point which is less than the boiling point of two components.
95% ethanol + 5% water
97% chloroform + 3% water
Maximum Boiling Azeotropes
Show large negative deviations from Raoult’s law
The mixture will boil at a higher temperature than that of the pure components.
1.Sulfuric acid (98.3%) / water, boils at 338 °C
Colligative properties
These are the properties of dilute solutions of non-volatile solute in a volatile solvent which depend only on the number of solute particles but not on its nature.
The types of colligative properties
- Relative lowering of vapour pressure
- Elevation in boiling point
- Depression in freezing point
- Osmotic pressure
Relative lowering of vapour pressure
- When a non- volatile solute is dissolved in a solvent, vapour pressure of the solution is lower than that of the pure solvent which is known as lowering of vapour pressure.
- Relative lowering of vapour pressure is equal to the mole fraction of the solute in the solution.
P10= Vapour pressure of pure solvent
P1= Vapour pressure of pure solution
n2= number of moles of solute
n1= number of moles of solvent
Hence the equation can be written as
For a dilute solution, n1 >>> n2
Where w1, w2 are the masses of solvent and solute and M1 and M2 are the molecular masses of solvent and solute respectively
Elevation in boiling point
The boiling point of a solution containing a non-volatile solute is always higher than the boiling point of the pure solvent. This increase in boiling point is termed as elevation in boiling point.
The difference in the Boiling Points of Solution and that of Pure Solvent is known as Elevation in Boiling Point.
Where Tb0 and Tb are boiling points of pure solvent and solution respectively
It is observed that the elevation in boiling point is directly proportional to molal concentration of the solute.
