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Strengths of Acids and Bases

Strengths of Acids and Bases. Complete and Partial Dissociation. Strong and Weak Acids and Bases. Remember, strong acids and bases totally dissociate in solution. HNO 3 ( aq ) + H 2 O( l ) → NO 3 - ( aq ) + H 3 O + ( aq ) Weak acids and bases form equilibrium solutions.

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Strengths of Acids and Bases

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  1. Strengths of Acids and Bases Complete and Partial Dissociation

  2. Strong and WeakAcids and Bases • Remember, strong acids and bases totally dissociate in solution. • HNO3(aq) + H2O(l) → NO3-(aq) + H3O+(aq) • Weak acids and bases form equilibrium solutions. • HNO2(aq) + H2O(l) ⇄ NO2-(aq) + H3O+(aq)

  3. [NO2-][H3O+] [products] [reactants] [H2O][HNO2] Strong and WeakAcids and Bases • We can define an equilibrium constant, K, that shows the relationship between the reactants and products. • K = • For the reaction • HNO2(aq) + H2O(l) ⇄ NO2-(aq) + H3O+(aq) • K =

  4. [NO2-][H3O+] [NO2-][H3O+] [H2O][HNO2] [HNO2] Strong and WeakAcids and Bases • K = • We can use this equilibrium constant to define a term specifically for acids: acid dissociation constant, Ka. • Ka = K×[H2O] • Ka =

  5. [NO2-][H3O+] [A-][H3O+] [HNO2] [HA] Strong and WeakAcids and Bases • Ka = • We can generalize this equation to apply it to any acid. • For the reaction: HA + H2O ⇄ A- + H3O+ • where HA is the acid and A- is the conjugatebase • Ka =

  6. [A-][H3O+] [HA] Strong and WeakAcids and Bases • Ka = • For a strong acid, Ka is very high. • The concentrations of the products are higher than the concentrations of the reactants • For a weak acid, Ka is very low. • The concentrations of the products are lower than the concentration of the reactants

  7. Strong and WeakAcids and Bases

  8. Strong and WeakAcids and Bases • The larger the value of Ka, the more ionized the acid. • The more ionized the acid, the stronger the acid. • For example, benzoic acid (Ka = 6.3×10-5) is • weaker than methanoic acid (Ka = 1.8×10-4) • stronger than ethanoic acid (Ka = 1.8×10-5)

  9. [BH+][OH-] [B] Strong and WeakAcids and Bases • We can use the same set of steps to look at the dissociation constant of a base, Kb. • For the reaction: B + H2O ⇄ BH+ + OH- • where B is the base and BH+ is the conjugate acid • Kb =

  10. [BH+][OH-] [B] Strong and WeakAcids and Bases • Kb = • For a strong base, Kb is very high. • The concentrations of the products are higher than the concentrations of the reactants • For a weak base, Kb is very low. • The concentrations of the products are lower than the concentration of the reactants

  11. [A-][H3O+] [BH+][OH-] [HA] [B] Calculating Dissociation Constants • Ka and Kb can be calculated from experiment. • To find Ka or Kb • substitute measured quantities of all substances present at equilibrium int0 the expression for Ka or Kb • Ka = Kb =

  12. [CH3COO-][H+] (0.00134)(0.00134) [CH3COOH] 0.0987 Calculating Dissociation Constants Example 1: A 0.1000 M solution of ethanoic acid is only partially ionized. From measurements of pH of the solution, [H+] is determined to be 1.34×10-3 M. What is the acid dissociation constant? [H+]eq = 1.34×10-3 M CH3COOH ⇄ H+ + CH3COO- [CH3COOH]i = 0.1000 M Ka = = = 1.82×10-5

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