Distillation of Real Solutions PDF

Distillation of Real solution  Distillation of Binary Mixtures The relationship between vapor pressure (and hence boiling point) and composition of binary liquid phases is underlying principle in distillation. The higher vapor pressure of a liquid that is, the more volatile an...

Distillation of Real solution  Distillation of Binary Mixtures The relationship between vapor pressure (and hence boiling point) and composition of binary liquid phases is underlying principle in distillation. The higher vapor pressure of a liquid that is, the more volatile and has lower boiling point. A mixture of these substances having the composition a is distilled at the boiling point b. The composition of the vapor v1 in equilibrium with the liquid at this temperature is C; the composition of the distillate when it is condensed.  A- Positive deviation The distillation give azeotropic mixture which produce more volatile (high v.p)constituent plus mixture of constant composition and constant Tb (low). L V T P V L A 100% B 100% A 100% B 100%  Distillation diagram The distillation give low boiling point azeotrope or high V.P azeotrope For example (EtOH+H2O) and (MeOH+ C6H6)  B- Negative deviation The distillation give high boiling azotrope or low V.P. azeotrope for example (H2O+HOAC) and (CHCl3+ Me2CO)  Distillation of HCl+H2O azotropic mixture with 20.22% composition at 108.08 0C (High boiling) or (Low V.P.) azotrope. The composition of this mixture is constant and sufficiently accurate and reproducible so that the solution at 20.22% composition can be used a standard solution in analytical chemistry  C- Immiscible liquids When a mixture of two immiscible liquids heated, the distillation may be effected when the sum of the partial pressure ( ∑Pi) is exceeds to atmospheric pressure. ∑Pi = 1 atmosphere This principle is applied in steam distillation for many organic compound insoluble in water.  Steam distillation occurs below the normal boiling temperature of solute so that it is useful to obtaining volatile oils from plant tissues with out decomposed the oils. Example: Bromobenzene boils in steam distillation at 95 0C Tb Bromobenzene =156.2 0C Tb water = 100 0C  Colligative properties When a non volatile (solute) combined with a volatile (solvent), the solute reduces the escaping tendency (V.P.) of solvent on basis of Rault’s law. solute The Colligative properties can be summarized as the following: 1- lowering of V.P. (ΔP α C) 2- Osmotic pressure (π α C) 3- Depression of Tf (ΔTf α C) 4- Elevation of Tb (ΔTb α C) These 4 properties called Colligative properties depend chiefly on the number rather than the nature of the constituents (on the concentration)  Lowering of vapor pressure For a dilute solution, according to Rault,s law P1=X1 P01 X1=1-X2 ………. (1) solvent, (2) solute P1= (1-X2) P01 P= P0-P01X2 P01-P=P01X2 (P01-P)/P01 =X2 or ΔP/P01 = X2 = n2/(n1+n2) Where ΔP: lowering vapor pressure ΔP/P01 :relative V.P, lowering Now: In very dilute solution n1>>n2 so that n2/(n1+n2) ≈ n2/n1 And ΔP/P01 = X2 = n2/n1 =(W2/Mw2)/ (w1/Mw1) [(W2/Mw2)/(w1/ Mw1) *1000/1000)] =m*Mw1/1000 ΔP/P01 = X2= m*Mw1/1000 ΔP/P01 = 0.018 m ……… for aqueous solution P0- P = ∆P