Theory of distillation : Theory of distillation Ideal solutions : Ideal solutions Ideal solution is defined as the one which there is no change in the properties of the components other than dilution, when they are mixed to form a solution E.g. methanol & water which have similar properties
Benzene & toluene
Ethyl bromide & ethyl chloride Raoult’s law : Raoult’s law Raoult’s law states that the partial vapour pressure of each volatile component is equal to the vapour pressure of the pure constituent multiplied by its mole fraction in the solution at a given temp. pA=p0AXA
pB=p0BXB Dalton’s law : Dalton’s law Dalton’s law states that the total pressure exerted by a mixture of ideal gases may be considered as sum of the partial vapour pressure exerted by each gas, if alone were present & occupied the volume. P = pA + pB
P =p0AXA +p0BXB
pB=p0BXB Slide 5: v.p. of mixture v.p. of A v.p. of B 0% A 100% B 100% A 0% B v.p. of mixture v.p. of A v.p. of B 0% A 100% B 100% A 0% B v.p. of mixture v.p. of A v.p. of B 0% A 100% B 100% A 0% B v.p. composition P =p0AXA +p0BXB P >p0AXA +p0BXB P<p0AXA +p0BXB Applications : Applications According to an ideal solution, the component having relatively greater vapour pressure will be distilled first. This principle is used in simple distillation Real solutions : Real solutions Most systems show varying degree of deviation fro raoult’s law, depending on the nature of the liquids and the temp. These solutions are known as real solutions. E.g. carbon tetrachloride & cyclohexane
Chloroform & acetone Positive deviation : Positive deviation When vapour pressure is greater than the sum of the partial pressures of the individual components, such systems are said to exhibit positive deviation from raoult’s law.
P >p0AXA +p0BXB B’coz of differ in polarity, length of hydrocarbon chain & degree of association. Negative deviation : Negative deviation When vapour pressure is lower than the sum of the partial pressures of the individual components, such systems are said to exhibit negative deviation from raoult’s law.
P <p0AXA +p0BXB B’coz of hydrogen bonding, salt formation and hydration between components of solution Applications : Applications The area include fractional distillation of intermediates and drugs, purification of alcohol and other organic solvents. Volatility : Volatility The volatility of any substance in a solution may be defined as the equilibrium partial pressure of the substance in the vapour phase divided by the mole fraction of the substance in the solution Volatility of component A,
vA= = Partial vapour pressure of A Mole fraction of A in solution pA XA Relative Volatility : Relative Volatility Consider a mixture of liquid containing two substance A & B, in such cases, the volatility of the one component is expressed in terms of the second. Relative Volatility,
a = = Volatility of component A Volatility of component B pAXB XApB Relative Volatility : Relative Volatility According to dalton’s law, the partial vapour pressure of A & B may be expressed as:
pB=YBP Relative Volatility,
aAB = = YAPXB XAYBP YAXB XAYB