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Equilibria of Weak Acids and Bases

Equilibria of Weak Acids and Bases. Strong/Weak Acids. Strong HBr, HI, HCl, HNO 3 , H 2 SO 4 , HClO 4 Readily transfer H + ion to water Completely dissociated Strong electrolyte Weak HC 2 H 3 O 2 , HCN, HNO 2 , HClO, HCO 3 - Does not readily transfer H+ ion Most remains un-ionized.

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Equilibria of Weak Acids and Bases

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  1. Equilibria of Weak Acids and Bases

  2. Strong/Weak Acids • Strong • HBr, HI, HCl, HNO3, H2SO4, HClO4 • Readily transfer H+ ion to water • Completely dissociated • Strong electrolyte • Weak • HC2H3O2, HCN, HNO2, HClO, HCO3- • Does not readily transfer H+ ion • Most remains un-ionized

  3. Strong/Weak Bases • Strong, CaO • Strong affinity for H+ ions • Some substance that readily yield OH- ions • Good electrolyte • Weak • Weak affinity for H+ ions • Some substance react only partially with water to form OH- ions

  4. Strength of Conjugate Acid-Base Pairs • Stronger the acid, the weaker the conjugate base • Stronger the base, the weaker the conjugate acid

  5. Acid Dissociation Constant • Quantifies the strength of an acid • Use HA to represent acid molecule • HA (aq) + H2O (l) H3O+ (aq) + A- (aq) • Keq = [H3O+][A-]/[HA][H2O] • Water is essentially a constant in dilute solutions, therefore • Keq [H2O] = [H3O+][A-]/[HA]

  6. Acid Dissociation Constant • Keq [H2O] = Ka = dissociation constant • For any acid, Ka = [H3O+][A-]/[HA] • The greater Ka, the stronger the acid • All weak acids Ka < 1 • For polyprotic acids, dissociation takes place in separate steps. Each dissociation has its own Ka.

  7. Polyprotic acid dissociation • Dissociation of carbonic acid • Step 1: H2CO3 (aq) + H2O (l) H3O+ (aq) + HCO3- (aq) • Ka1 = [H3O+][HCO3-]/[H2CO3] = 4.5 x 10-7 • Step 2: HCO3- (aq) + H2O (aq) H3O+ (aq) + CO32- • Ka2 = [H3O+][CO32-]/[HCO3-] = 5.6 x 10-11 • Ka is very small, therefore weak acid

  8. Base Dissociation Constant • Kb • B (aq) + H2O (l) HB+ (aq) + OH- (aq) • Kb = [HB+][OH-]/[B]

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