1 Acid-Base Reactions Chapter 17

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

4 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. HNO 3 (aq) + H 2 O(l)  H 3 O + (aq) + NO 3 - (aq) STRONG A base 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. Every A-B reaction has two acids and two bases. Equilibrium always lies toward the weaker pair. Here K is very large.

5 Acid-Base Reactions We know from experiment that HNO 3 is a strong acid. 1.It is a stronger acid than H 3 O + 2.H 2 O is a stronger base than NO K for this reaction is large

6 K and Acid-Base Reactions Acetic acid is only 0.42% ionized. It is a WEAK ACID HOAc + H 2 O   H 3 O + + OAc - WEAK A base acid STRONG B [H 3 O + ] is small, this means 1.H 3 O + is a stronger acid than HOAc 2.OAc - is a stronger base than H 2 O 3.K for this reaction is small

7 Acid/Base Reactions Strong acid + Strong base H + + Cl - + Na + + OH -  H 2 O + Na + + Cl - Net ionic equation H + (aq) + OH - (aq)  H 2 O(l) K = 1/K w = 1 x Mixing equal molar quantities of a strong acid and strong base produces a neutral solution.

8 Acid/Base Reactions Weak acid + Strong base CH 3 CO 2 H + OH -  H 2 O + CH 3 CO 2 - This is the reverse of the reaction of CH 3 CO 2 - (conjugate base) with H 2 O. OH - stronger base than CH 3 CO 2 - K = 1/K b = 5.6 x 10 4 Mixing equal molar quantities of a weak acid and strong base produces the acid’s conjugate base. The solution is basic.

9 Acid/Base Reactions Strong acid + Weak base H 3 O + + NH 3  H 2 O + NH 4 + This is the reverse of the reaction of NH 4 + (conjugate acid of NH 3 ) with H 2 O. H 3 O + stronger acid than NH 4 + K = 1/K a = 5.6 x 10 4 Mixing equal molar quantities of a strong acid and weak base produces the bases’s conjugate acid. The solution is acid.

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

11 Summary

12 ? MX + H 2 O ----> acidic or basic ? Consider NH 4 Cl NH 4 Cl(aq)  NH 4 + (aq) + Cl - (aq) (a)Reaction of Cl - with H 2 O Cl - + H 2 O ---->HCl + OH - baseacidacidbase Cl - ion is a VERY weak base because its conjugate acid is strong. Therefore, Cl - = neutral solution Salts

13 (b) Reaction of NH 4 + with H 2 O NH H 2 O  NH 3 + H 3 O + acid base baseacid NH 4 + ion is a moderate acid because its conjugate base is weak. Therefore, NH 4 + = acidic solution See TABLE 17.4 for a summary of acid-base properties of ions.Salts

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15 Calculate the pH of a 0.10 M Na 2 CO 3. A) Na + + H 2 O  neutral B) CO H 2 O  HCO OH - base acid acid base K b = 2.1 x Salts

16 Calculate the pH of a 0.10 M solution of Na 2 CO 3. K b = 2.1 x Step 1. Set up ICE table [CO 3 2- ][HCO 3 - ][OH - ] I C -x +x +x E x x x Salts

17 Salts Assume x ≈ 0.10, because 100K b < C o x = [HCO 3 - ] = [OH - ] = M Step 2. Step 2.Solve the equilibrium expression

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

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