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Just When It’s Safe to Go Back Into the Water. The Acidic or Basic Quality of Salts. Acid Equilibrium Review. Weak acids form equilibria in aqueous solutions according to the following general pattern HX (aq) H +1 (aq) + X -1 (aq) The equilibrium constant follows the form
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Just When It’s Safe to Go Back Into the Water The Acidic or Basic Quality of Salts
Acid Equilibrium Review • Weak acids form equilibria in aqueous solutions according to the following general pattern • HX (aq) H+1 (aq) + X-1 (aq) • The equilibrium constant follows the form • Ka = [H+1][X-1]/[HX] • The reaction can be shifted according to LeChatelier’s Principle
Base Equilibrium Review • Weak bases form equilibria in water similar to the example of ammonia • NH3 + HOH NH4+1 + OH-1 • The equilibrium constant would be • Kb = [NH4+1][OH-1]/[NH3] • Again, LeChatelier’s Principle can be used to explain or predict shifts
And, Water Has Its Equilibrium Too • Water forms an equilibrium that can be affected by any substance dissolved into solution • The equilibrium involves the water molecule, hydrogen ion, and hydroxide ion • H2O H+1 + OH-1 • The equilibrium constant, Kw, takes the form • Kw = [H+1][OH-1] • Kw = 1 x 10-14
Conjugate Acids and Bases • An acid-base reaction produces new materials which may be classified as conjugate acids (or bases) • Weak acid (or base) equilibria often produce anions (or cations) with significant base (or acid) properties • The weaker the original acid (or base) the stronger its conjugate base (or acid)
Acetic Acid, for example • When acetic acid dissolves in water it forms hydrogen ion and acetate ion • CH3COOH (aq) CH3COO-1 + H+1 • The acetate ion has some basic properties (It is the conjugate base for acetic acid in the reaction.) • Therefore, it can be expected that solutions (which might carelessly be considered neutral) containing acetate ion might be basic
An Acetate Solution Example • Suppose a solution of sodium acetate, CH3COO-Na+, is made • The acetate and sodium ions separate • CH3COO-Na+ CH3COO- + Na+ • Each ion may have acidic or basic properties • To analyze this we see what would be produced if each ion reacted with water
These Are Called Hydrolysis Reactions • Sodium ion reacting with water • Na+1 + HOH NaOH + H+1 • The products are NaOH and H+1. The NaOH is strong and dissociates 100%. Thus, no difference between [H+1] and [OH-1] is created • Acetate ion reacting with water • CH3COO-1 + HOH CH3COOH + OH-1 • The products are CH3COOH and OH-1. Acetic acid is weak and doesn’t dissociate 100%. So, more [OH-1]>[H+1]. It’s a basic solution.
Generalizing for Hydrolysis • Check the reaction impact of the ions from a salt (MX) with water • M+1 + HOH MOH + H+1 • X-1 + HOH HX + OH-1 • If either the acid or base produced is weak the salt has acidic/basic properties • These reactions are equilibrium reactions and will have K values
Using a specific example of sodium acetate the hydrolysis reaction is The Ka value for acetic acid is The K formula for the hydroysis reaction would be Inverting the Ka gives Mulitplying this by Kw gives So, Kh = Kw/Ka CH3COO- CH3COOH + OH-1 Ka = [H+1][CH3COO-]/[CH3COOH] Kh = [CH3COOH][OH-]/[CH3COO-] 1/Ka = [CH3COOH]/[CH3COO-][H+] Kw/Ka = [CH3COOH][H+][OH-] [CH3COO-][H+] Kh = [CH3COOH][OH-]/[CH3COO-] K Values for Hydrolysis Reactions
Predict a pH for 0.25 M sodium acetate solution. The reaction for acetate ion is: CH3COO- CH3COOH + OH- Ka for acetic acid is 1.8x10-5 Kh = 1x10-14/1.8x10-5 = 5.56x10-10 Equilibrium “chart” CH3COO- CH3COOH + OH- S 0.25 0 0 D -x x x Eq 0.25-x x x 5.56x10-10 = x2/0.25 * x = 1.18 x 10-5 So, pOH = 4.93 and pH = 9.07 A Specific Example * The “x” term was dropped as x << 0.25
