Solutions

A solution is a homogeneous mixture of two (or more) substances, the composition of which may vary between certain limits. A solution consisting of two  components is called binary solution. The component which is present in large quantity is called solvent and the component which is small in quantity is called solute. If both components are in same physical state.

Types of solution : Depending upon the nature of solute and solvent, solutions are classified as follows :

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:

1. Nature of solute and solvent: Polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents.

2. 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

“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.

Vapour Pressure: 

• The pressure exerted by vapours over the liquid surface at equilibrium is called vapour pressure of the liquid.

• If solute is non volatile solid or liquid the vapour pressure of solution is equal to partial vapour pressure of solvent in the solution.

• If the solute is volatile solid or liquid, then vapour pressure will be equal to the sum of partial vapour pressure of solute and that of solvent.

Raoult’s Law:

“The partial vapour pressure of any component in the solution is directly proportional to its mole fraction”.

For a binary solution of two components A and B,

PA  = XA

PB = XB

Where

P0A  = vapour pressure of component A in pure state.

PA  = vapour pressure of component A in the solution.

P0B  = vapour pressure of component B in pure state.

PB  = vapour pressure of component B in the solution

Limitations of Raoult’s Law

• Raoult’s law is applicable only to very dilute solutions.

• It is applicable to solutions containing non-volatile solute only.

• It is not applicable to solutes which dissociate or associate in a particular solution

Raoult’s Law in Combination with Dalton’s Law of Partial Pressure:

PT = XA P0A + XB P0B = P0B  + (P0A -P0B) XA                   

Where

PT = Total Vapour Pressure of the Solution.

Ideal and Non-Ideal Solutions:

• Ideal Solution:

These solutions obey Raoult’s law at all compositions of solute in solvent at all temperature 

Two liquids A and B form and ideal solution when A –A  and B–B molecular attractions will be same and hence A–B molecular attraction will be almost same as A–A and B–B molecular attraction.

For Ideal Solution:

1. dHmixing = 0, i.e. no heat should be absorbed or evolved during mixing

2. dVmixing  = 0, i.e. no expansion or contraction on mixing

Examples , Ethyl chloride and ethyl bromide, n–hexane and n–heptane , CCl4 and SiCl4

• Non-Ideal Solution:

These solutions deviate from ideal behaviour and do not obey Raoult’s law over entire range of composition.  

For non ideal solutions,

1. dHmixing  ≠ 0

2. dHmixing ≠ 0

Here we may have two cases

A) Positive Deviation:

1. PA  > XA & PB > XB

2. dHmix > 0

3. dVmix  > 0

Example: Cyclohexane and Ethanol

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

2.Nitric acid (68%) / water, boils at 120.2 °C

3.Perchloric acid (71.6%) / water, boils at 203 °C

4.Hydrofluoric acid (35.6%) / water, boils at 111.35 °c

Elevation of Boiling Point by a Non-Volatile Solute :

Since the addition of a non-volatile solute lowers the vapour pressure of the solvent, the vapour pressure of a solution is always lower than that of the pure solvent, and hence it must be heated to a higher temperature to make its vapour pressure equal to atmospheric pressure.

Osmosis and Osmotic Pressure:

• Osmosis: The phenomenon of the passage of pure solvent from a region of lower concentration (of the solution) to a region of its higher concentration through a semi-permeable membrane.

• Osmotic Pressure: Excess pressure which must be applied to a solution in order to prevent flow of solvent into the solution through the semi-permeable membrane.

• PV = nRT

where  

•     P=    Osmotic pressure

• V   =    volume of solution

• n =    no. of moles of solute that is dissolved

• R   =    Gas constant

• T =    Absolute temperature

• Isotonic Solutions: A pair of solutions having same osomotic pressure is called isotonic solutions.

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