Liquid Solution : Ideal & Non-Ideal Solution

 

Ideal and Non-Ideal solution

(1)     Ideal solution : An ideal solution may be defined as the solution which obeys Raoult’s law over the entire range of concentration and temperature and during the formation of which no change in enthalpy and no change in volume takes place. So for ideal solutions the conditions are,

(i)      It should obey Raoult’s law, i.e., {{P}_{A}}=P_{A}^{0}{{X}_{A}}  and  {{P}_{B}}=P_{B}^{0}{{X}_{B}} .

(ii)     ΔHmixing = 0        

(iii)    ΔVmixing = 0

The solutions in which solvent-solvent and solute-solute interactions are almost of the same type as solvent-solute interactions, behave nearly as ideal solutions.

This type of solutions are possible if molecules of solute and solvent are almost of same size and have identical polarity. For example, solutions of following pairs almost behave as ideal solutions,

n-Heptane and n-hexane.; Chlorobenzene and bromobenzene.

Benzene and toluene; Ethyl bromide and ethyl iodide.

Ethylene bromide and ethylene chloride; Carbon tetrachloride and silicon tetrachloride.

For such solutions the vapour pressure of the solution is always intermediate between the vapour pressures of pure components A and B, i.e., P_{A}^{0} and  P_{B}^{0} .

 

(2)     Non-Ideal solution : The solutions which do not obey Raoult’s law and are accompanied by change in enthalpy and change in volume during their formation are called non-ideal solutions. So, for non-ideal solutions the conditions are :

(i)      It does not obey Raoult’s law. \[{{P}_{A}}\ne P_{A}^{0}{{X}_{A}};\,\,{{P}_{B}}\ne P_{B}^{0}{{X}_{B}}

(ii)     ΔHmixing ≠ 0

(iii)    ΔVmixing ≠ 0

Either component of non-ideal binary solution do not follow Raoult’s law. The non-ideal solutions are further divided into two types :

(a)       Solutions showing positive deviations

(b)       Solutions showing negative deviations

(a)       Solutions showing positive deviation : In this type of deviations, the partial vapour pressure of each component (say A and B) of solution is greater than the vapour pressure as expected according to Raoult’s law. This type of deviations are shown by the solutions in which solvent-solvent and solute-solute interactions are stronger than solvent-solute interactions.

For the non-ideal solutions exhibiting positive deviation. 

{{P}_{A}}>P_{A}^{0}\,{{X}_{A}},{{P}_{B}}>P_{B}^{0}\,{{X}_{B}} 

ΔHmixing = + ve; 

ΔVmixing = ve

e.g. of solutions showing positive deviations

(CH3)2CO + CS2 ; (CH3)2CO + C2H5OH

C6H6 + (CH3)2CO ; CCl4 + C6H6

CCl4 + CHCl3 ; CCl4 + C6H5CH3

H2O + CH3OH ; H2O + C2H5OH

CH3CHO + CS2 ; CHCl3 + C2H5OH

 

(b)       Solutions showing negative derivation : In this type of deviations the partial vapour pressure of each component of solution is less than the vapour pressure as expected according to Raoult’s law. This type of deviations are shown by the solutions in which solvent-solvent and solute-solute interactions are weaker than solvent-solute interactions.

For non-ideal solution showing negative deviation. 

\[{{P}_{A}}<P_{A}^{0}{{X}_{A}},\,\,\,{{P}_{B}}<P_{B}^{0}{{X}_{B}} 

ΔHmixing = − ve; 

ΔVmixing = − ve

e.g. of solutions showing negative deviations

CH3COOH + (CH3)2CO ; CHCl3 + C6H6

CHCl3 + (C2H5)2O ; H2O + HCl

H2O + HNO3 ; (CH3)2CO + C6H5NH2

 

Differences between ideal and non-ideal solutions

Ideal solutions Solutions with positive deviations Solutions with negative deviations
A……B interactions are similar to A……A and B……B interactions A……B interactions are smaller than A……A and B……B interactions A……B interactions are greater than A……A and B……B interactions
 \[{{P}_{A}}=P_{A}^{0}{{X}_{A}};\,\,{{P}_{B}}=P_{B}^{0}{{X}_{B}}  \[{{P}_{A}}>P_{A}^{0}{{X}_{A}};\,\,{{P}_{B}}>P_{B}^{0}{{X}_{B}}  \[{{P}_{A}}<P_{A}^{0}{{X}_{A}};\,\,{{P}_{B}}<P_{B}^{0}{{X}_{B}}
ΔHsol. = 0 ΔHsol. > 0 ΔHsol < O
ΔVmix = 0 ΔVmix > 0 ΔVmix < 0
Do not form azeotrope form minimum boiling point azeotrope form maximum boiling azeotrope

 

Azeotropic mixture

Azeotropes are defined as the mixtures of liquids which boil at constant temperature like a pure liquid and possess same composition of components in liquid as well as in vapour phase. Azeotropes are also called constant boiling mixtures because whole of the azeotropes changes into vapour state at constant temperature and their components cannot be separated by fractional distillation. Azeotropes are of two types as described below :

(1)     Minimum boiling azeotrope : For the solutions with positive deviation there is an intermediate composition for which the vapour pressure of the solution is maximum and hence, boiling point is minimum. At this composition the solution distills at constant temperature without change in composition. This type of solutions are called minimum boiling azeotrope. e.g;

 

Components Mass % of B Boiling points (K)

A

B

A

B

 Azeotrope
H2O C2H5OH 95.57 373 351.3 351.1
H2O C2H5CH2OH 71.69 373 370 350.72
CHCl3 C2H5OH 67 334 351.3 332.3
(CH3)2CO CS2 6.8 329.25 320 312.2

 

(2)     Maximum boiling azeotrope : For the solutions with negative deviations there is an intermediate composition for which the vapour pressure of the solution is minimum and hence, boiling point is maximum. At this composition the solution distill`s at constant temperature without the change in composition. This type of solutions are called maximum boiling azeotrope. e.g

 

Components Mass % of B Boiling points (K)

A

B

A

B

 Azeotrope
H2O

HCl

 20.3 373 188 383
H2O HNO3 58.0 373 359 393.5
H2O HClO4 71.6 373 383 476