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Acid-Base Reactions

Acid-Base Reactions. Chapter 17. ACIDS CONJUGATE BASES. weak. STRONG. weak. STRONG. Acid-Base Reactions. Reactions always go from the stronger A-B pair (larger K) to the weaker A-B pair (smaller K). Acid-Base Reactions. A strong acid is 100% dissociated.

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Acid-Base Reactions

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  1. Acid-Base Reactions Chapter 17

  2. ACIDSCONJUGATE BASES weak STRONG weak STRONG Acid-Base Reactions Reactions always go from the stronger A-B pair (larger K) to the weaker A-B pair (smaller K).

  3. Acid-Base Reactions A strong acid is 100% dissociated. STRONG ACID = a good H+ donor—must have a WEAK CONJUGATE BASE Or a poor H+ acceptor. HNO3(aq) + H2O(l) fg H3O+(aq) + NO3-(aq) STRONG Abase acid weak B • Every A-B reaction has two acids and two bases. • Equilibrium always lies toward the weaker pair. • Here K is very large.

  4. Acid-Base Reactions We know from experiment that HNO3 is a strong acid. 1. It is a stronger acid than H3O+ 2. H2O is a stronger base than NO3- 3. K for this reaction is large

  5. K and Acid-Base Reactions Acetic acid is only 0.42% ionized. It is a WEAK ACID HOAc + H2O fg H3O+ + OAc- WEAK Abase acidSTRONG B [H3O+] is small, this means 1. H3O+ is a stronger acid than HOAc 2. OAc- is a stronger base than H2O 3. K for this reaction is small

  6. Acid/Base Reactions Strong acid + Strong base H+ + Cl- + Na+ + OH-fg H2O + Na+ + Cl- Net ionic equation H+(aq) + OH-(aq) fg H2O(l) K = 1/Kw = 1 x 1014 Mixing equal molar quantities of a strong acid and strong base produces a neutral solution.

  7. Acid/Base Reactions Mixing equal molar quantities of a weak acid and strong base produces the acid’s conjugate base. The solution is basic. Weak acid + Strong base CH3CO2H + OH- fg H2O + CH3CO2- This is the reverse of the reaction of CH3CO2- (conjugate base) with H2O. OH- stronger base than CH3CO2- K = 1/Kb = 5.6 x 104

  8. Acid/Base Reactions Mixing equal molar quantities of a strong acid and weak base produces the bases’s conjugate acid. The solution is acid. Strong acid + Weak base H3O+ + NH3fg H2O + NH4+ This is the reverse of the reaction of NH4+ (conjugate acid of NH3) with H2O. H3O+ stronger acid than NH4+ K = 1/Ka = 5.6 x 104

  9. Acid/Base Reactions Weak acid + Weak base pH of solution depends on relative strengths of cation and anion. • Product cation = CA of weak base. • Product anion = CB of weak acid.

  10. Summary

  11. Salts MX + H2O ----> acidic or basic? Consider NH4Cl NH4Cl(aq) D NH4+(aq) + Cl-(aq) (a) Reaction of Cl- with H2O Cl- + H2O ----> HCl + OH- baseacidacidbase Cl- ion is a VERY weak base because its conjugate acid is strong. Therefore, Cl- = neutral solution

  12. Salts (b) Reaction of NH4+ with H2O NH4+ + H2O D NH3 + H3O+ acidbasebaseacid NH4+ ion is a moderate acid because its conjugate base is weak. Therefore, NH4+ = acidic solution See TABLE 17.4 for a summary of acid-base properties of ions.

  13. Salts Calculate the pH of a 0.10 M Na2CO3. A) Na+ + H2O D neutral B) CO32- + H2O DHCO3- + OH- baseacidacidbase Kb = 2.1 x 10-4

  14. Salts Calculate the pH of a 0.10 M solution of Na2CO3. Kb = 2.1 x 10-4 Step 1. Set up ICE table [CO32-] [HCO3-] [OH-] I 0.10 0 0 C -x +x +x E 0.10 - x xx

  15. Salts Step 2. Solve the equilibrium expression Assume 0.10 - x ≈ 0.10, because 100•Kb < Co x = [HCO3-] = [OH-] = 0.0046 M

  16. Salts Step 3. Calculate the pH [OH-] = 0.0046 M pOH = - log [OH-] = 2.34 pH + pOH = 14, so pH = 11.6 the solution is ________.

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