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COMMON ION EFFECT

The document discusses the common ion effect, which is the reduction of salt dissociation due to the addition of a common ion, illustrated through examples such as silver chloride and barium iodate. It explains how solubility equilibria and the solubility product constant (Ksp) are affected by this phenomenon in various chemical contexts, including weak acids and bases. Additionally, it outlines practical applications of the common ion effect, such as in the purification of salts and the 'salting out' of soaps.

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Course Title: COMMON ION EFFECT Course Code: CHM-404 Semmester: 4th E
Common ion effect and industrial applications
The reduction of the degree of dissociation of a salt by the addition of a common-ion is called the common-ion effect. Example In a saturated solution of silver chloride, we have the equilibrium AgCl (s) Ag+ (aq) + Cl– (aq) When sodium chloride is added to the solution, the concentration of Cl– ions will increase. The equilibrium shown above will be shifted to the left to form more of solid Ag Cl. Thus the solubility of AgCl, a typical sparingly soluble salt, will decrease. Common ion effect
Factors effecting degree of dissociation  Nature of solute  Nature of solvent  Concentration  Temperature common ion effect
Example Dissociation of hydrogen sulphide in presence of hydrochloric acid H2S 2H+ + S2- By applying the law of mass action, we have Ka = [H+]2 [S2-]/ [H2S] To the above solution of H2S , if we add hydrochloric acid, then it ionizes completely as HCl H+ + Cl- common ion effect
Solubility equilibria and solubility product When an ionic solid substance dissolves in water, it dissociates to give separate cations and anions. For example, for a sparingly soluble salt, say Ag Cl, we can write the equilibrium equations as follows : AgCl(S) Ag+ + Cl- common ion effect
Solubility equilibria and solubility product According to law of mass action K= [Ag+] [Cl–] / [AgCl] The amount of Ag Cl in contact with saturated solution does not change with time and the factor [Ag Cl] remains the same. common ion effect
Solubility equilibria and solubility product As [AgCl] is constant so equilibrium expression is Ksp= [Ag+] [Cl–] where [Ag+] and [Cl– ] are expressed in mol/L The equilibrium constant in the new context is called the solubility product constant (or simply the solubility product) and is denoted by Ksp. common ion effect
Solubility of barium iodate in presence of barium nitrate Barium iodate, Ba(IO3)2, has a solubility product Ksp = [Ba2+][IO3 -]2 = 1.57 x 10-9 Its solubility in pure water is 7.32 x 10-4 M. common ion effect
Common ion effect with weak acids and bases common ion effect
Common ion effect with weak acids and bases common ion effect
Activity Calculate the solubility of silver chromate, Ag2CrO4, in a 0.100 M solution of AgNO3. (Ksp for Ag2CrO4 = 9.0 × 10– 12) Activity Calculate the Ksp for Bismuth sulphide (Bi2S3), which has a solubility of 1.0 × 10– 15 mol/L at 25°C. Activity The solubility of BaSO4 is 2.33 × 10– 4 g/ml at 20°C. Calculate the solubility product of BaSO4 assuming that the salt is completely ionised. common ion effect
Common ion effect on solubility Adding a common ion decreases solubility, as the reaction shifts toward the left to relieve the stress of the excess product. Adding a common ion to a dissociation reaction causes the equilibrium to shift left, toward the reactants, causing precipitation. Applications of common ion effect
Common ion effect on solubility Example AgCl(s) Ag+ + Cl– If S be the solubility of AgCl, we have Ksp = [S mol/l Ag+] [S mol/l Cl–] Suppose 0.25 mol/L excess of HCl is added to the solution. Then ion product (Q) will be Q = [S mol/l Ag+] [(S + 0.25) mol/l Cl– ] If Q > Ksp Precipitation If Q = Ksp Saturated solution If Q < Ksp No precipitation Applications of common ion effect
Salting out of soap RCOONa(aq.) RCOO- (aq.) + Na+ (aq.) NaCl(aq.) Na+ (aq.) + Cl- (aq.) Applications of common ion effect
Purification of salt Applications of common ion effect NaCl(aq.) Na+ (aq.) + Cl- (aq.) HCl(aq.) H+ (aq.) + Cl- (aq.)
Lime Softening CO2 + Ca(OH)2 CaCO3 + H2O Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + 2H2O Applications of common ion effect
 Harris, B. C. and W.H. Harris. 2010. Quantitative Chemical Analysis. 8th Edition. Freeman and Company New York.  Christian, G.D. 2006. Analytical Chemistry. Sixth edition, John Wiley and Sons, New York.  Skoog, D. A. and D.M. West 2005. Fundamentals of Analytical Chemistry. Hot Reinehart Inc., London.  Kealey, D and P.J.Haines, 2002. Analytical Chemistry, Bios Scientific Publishers Limited, Oxford, UK.  Sharma, B. K. 2004. Instrumental methods of chemical analysis, In; Introduction to Analytical chemistry: Goel Publishing House Meerut, 23th Edition.  Reilley, C. 1993. Laboratory Manual of Analytical Chemistry. Allyn & Bacon, London. Reference books
COMMON ION EFFECT