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The solubility of a partially soluble salt is decreased when a common ion is added. Consider the equilibrium established when acetic acid, HC2H3O2, is added to water. At equilibrium H+ and C2H3O2- are constantly moving into and out of solution, but the concentrations of ions is constant and equal. If a common ion is added, e.g. C2H3O2- from NaC2H3O2 (which is a strong electrolyte) then [C2H3O2-] increases and the system is no longer at equilibrium. So, [H+] must decrease. | Additional Aspects of Aqueous Equilibria Chapter 17 David P. White University of North Carolina, Wilmington Copyright 1999, PRENTICE HALL 1 1 1 1 The Common Ion Effect The solubility of a partially soluble salt is decreased when a common ion is added. Consider the equilibrium established when acetic acid, HC2H3O2, is added to water. At equilibrium H+ and C2H3O2- are constantly moving into and out of solution, but the concentrations of ions is constant and equal. If a common ion is added, e.g. C2H3O2- from NaC2H3O2 (which is a strong electrolyte) then [C2H3O2-] increases and the system is no longer at equilibrium. So, [H+] must decrease. Copyright 1999, PRENTICE HALL Buffered Solutions Composition and Action of Buffered Solutions A buffer consists of a mixture of a weak acid (HX) and its conjugate base (X-): The Ka expression is A buffer resists a change in pH when a small amount of OH- or H+ is added. Copyright 1999, PRENTICE HALL Buffered Solutions Composition and Action | Additional Aspects of Aqueous Equilibria Chapter 17 David P. White University of North Carolina, Wilmington Copyright 1999, PRENTICE HALL 1 1 1 1 The Common Ion Effect The solubility of a partially soluble salt is decreased when a common ion is added. Consider the equilibrium established when acetic acid, HC2H3O2, is added to water. At equilibrium H+ and C2H3O2- are constantly moving into and out of solution, but the concentrations of ions is constant and equal. If a common ion is added, e.g. C2H3O2- from NaC2H3O2 (which is a strong electrolyte) then [C2H3O2-] increases and the system is no longer at equilibrium. So, [H+] must decrease. Copyright 1999, PRENTICE HALL Buffered Solutions Composition and Action of Buffered Solutions A buffer consists of a mixture of a weak acid (HX) and its conjugate base (X-): The Ka expression is A buffer resists a change in pH when a small amount of OH- or H+ is added. Copyright 1999, PRENTICE HALL Buffered Solutions Composition and Action of Buffered Solutions When OH- is added to the buffer, the OH- reacts with HX to produce X- and water. But, the [HX]/[X-] ratio remains more or less constant, so the pH is not significantly changed. When H+ is added to the buffer, X- is consumed to produce HX. Once again, the [HX]/[X-] ratio is more or less constant, so the pH does not change significantly. Copyright 1999, PRENTICE HALL Buffered Solutions Composition and Action of Buffered Solutions Copyright 1999, PRENTICE HALL Buffered Solutions Buffer Capacity and pH Buffer capacity is the amount of acid or base neutralized by the buffer before there is a significant change in pH. Buffer capacity depends on the composition of the buffer. The greater the amounts of conjugate acid-base pair, the greater the buffer capacity. The pH of the buffer depends on Ka. Copyright 1999, PRENTICE HALL Buffered Solutions Buffer Capacity and pH If Ka is small (i.e., if the equilibrium concentration of undissociated acid is close to the .
