ABNORMAL COLLIGATIVE PROPERTIES The colligative properties of solutions depend on the number of solute particles present in solution.Various relations derived for colligative poperties hold good in dilute solutions only when there is no change in molecular state of solute. In case the total number of particles of the solute changes in solution, the colligative property shall also change accordingly. The number of particles in solution may change in two ways: (i) By dissociation: When the substance is an electrolyte, i.e., the substance undergoes ionization and number of particles increases in solution, the ions act as particles. The number of particles, thus, increases on ionization and the value of colligative property increases accordingly. (ii) By association: When the substance undergoes association, i.e., two or more molecules of the solute associate to form single giant particle, the number of particles decreases and consequently the value of colligative property decreases. Thus, Normal value of colligative property No. of particles of solute taken No. of particles of solute after ionization or Abnormal value of colligative property association Thus, the ratio, Abnormal colligative property/Normal colligative may have the value either more than 1 or less than 1. The ratio is termed van¶t Hoff factor which is represented by µi¶. Thus, i = Pobs./Pnormal = ( p)obs./( p)normal = ( Tb)obs./( Tb)normal = ( Tf)obs./( Tf)normal = Actual no. of particles in solution/No. of particles taken (i) Suppose one molecule of an electrolyte gives µn¶ ions on dissociation and µ ¶ is its degree of ionization. Obviously, Number of ions produced = n and Number of unionized molecules = 1 ± Total number of particles in solution = 1 ± +n 1 van¶t Hoff factor µi¶ = 1+(1/n±1) /1 < 1 if n is 2 or more = 1±i/1±1/n As Colligative property 1/mol. then nA (A)n (1 í ) /n Total number of particles in solution = 1 ± + /n = 1 + (1/n ± 1) Thus.= 1 + (n ± 1) Thus. mass will always be higher than normal mol. i = Normal mol. <1 2 . mass In case of association. mass Thus. mass/Observed mol. i = Normal mol. mass will always be less than normal mol. mass property/Normal colligative property = Normal mol. mass/Observed mol. mass. mass = 1 + (n í 1) >1 Observed mol. So observed colligative mass/Observed mol. van't Hoff factor µi¶ = 1+(n±1) /1 > if n is 2 or more and = i±1/n±1 (ii) Suppose µn¶ molecules associate to form one giant molecule and µ ¶ is the degree of association. mass = 1 + (1/n ± 1) Observed mol. K3[Fe(CN3)] FeCl3. KCi CH3COOH.S. ² AB ? A+ + Bí 1í AB2?A2++2Bí 1í 2 + A2B?2A +2Bí 1í 2 AB3 ? A3+ + 3Bí 1í 3 A3B 3A+ + B3í 1í 2 nA ? An 1í /n No. glucose NaCl. etc. 2 3. sucrose. 3 . of van¶t Hoff Abnormal particles factor µi¶ molecular in the mass solution from1 mole solute 1 1 mnormal 1+ mnormal/1+ 2.No Solute type Example Ionisation or association 1. Qaternary electrolyte AB3 or A3B type 4 4 1/n AlCl3. A2B type K2[PtCl6] 3 3 1+2 1+2 1+3 1+3 [1±(1± 1/n) ] mnormal/1+2 mnormal/1+2 mnormal/1+3 mnormal/1+3 mnormal/[1±(1± 1/n)] 4. Ternary electrolyte AB2 type or CaCl2. BaCl2 H2SO4. Non electrolyte Binary electrolyte AB type Urea. K3PO4 mnormal/[1+(n± [1+(ní1) ] 1)] 5. Association Anní1?A+ní1)+ní1)Bí of solute (ní1) Benzoic acid 1í forming dimer General in benzene electrolyte One mole of solute giving n ions in the solution n 6. Thus the no. Consider the following reaction for association nA --> (A)n 4 . i.m Osmotic pressure. Examples: Acetic acid in benzene. i = or i = or i = Using Van't Hoff factor. the colligative properties are modified as follows: Relative lowering of vapour pressure. chloro acetic acid in naphthalene Association of two molecules (dimerisation) of acetic acid in benzene.. abnormal results (for all colligative properties and molar masses) are obtained. Van't Hoff in 1886 introduced a factor 'i' called Van't Hoff factor. The deviation depends on extent of dissociation or association of solute. of molecules decreases and hence colligative properties decrease.. i. To calculate the extent of association or dissociation.Van't Hoff Factor and Abnormal Molecular Masses Van't Hoff factor 'i' is defined as ratio of the experimental value of colligative property to the calculated value (normal value) of colligative property. When the solute associates or dissociates in solution.e. T b = i Kbm Depression in freezing point. Elevation of boiling point.e. experimental value for colligative properties and molar mass deviates from calculated or normal value. = i CRT Association of Solute Many organic solutes when dissolved in non-aqueous (non polar) solvent associate to form a bigger molecule. Van't Hoff factor 'i' is defined as ratio of the experimental value of colligative property to the calculated value (normal value) of colligative property. T f = i Kf . 1 (1 ) 0 moles before association moles after association i= i= obviously. Examples: Dissociation of NaCl in aqueous solution. 5 .. NaCl (1 . i < 1 i. colligative properties decrease on association. In such case no. colligative properties increases.e.e.) Na+(aq) + Cl. i > 1 i. of particles increases and hence colligative properties increase. Dissociation of Solute Some solutes (organic acids. Consider the following dissociation reaction.(aq) moles after dissociation i= obviously. bases and salts) in aqueous solution undergo dissociation..