Weak Acids and Bases

“WEAK” Acids and bases that ionize less (much less) than 100% The amount of H 3 O + or OH - in solution is MUCH smaller than the initial concentration of weak acid or base. Relative strength of an acid or base is measured by an equilibrium constant. –K a : acid strength –K b : base strength

For any weak acid (HA) HA (aq) + H 2 O(l) H 3 O + (aq) + A - (aq) K a = [H 3 O + ][A - ] HA Acid strength increases with size of K a

Weak Acid (Acetic Acid) CH 3 COOH + H 2 O CH 3 COO - + H 3 O + acetic acid hydronium Consider the acid strengths of the acetic acid and hydronium ion K a acetic acid = 1.8x10 -5 K a H 3 O + = 1.0 The “stronger” acid is ______ so it will ACT as an acid and lose that proton; more ____ will be present than _____ The equilibrium lies on the WEAKER side, so this is a _______ favored reaction

For a general weak base (B) B (aq) + H 2 O (l) BH + (aq) + OH - (aq) K b = [BH + ][OH - ] [B] Base strength increases with size of K b

Weak Base (NH 3 ) NH 3 + H 2 O NH OH - ammonium hydroxide Consider the base strengths of NH 3 and OH - K b NH 3 = 5.6x K b OH - = 1.0 The “stronger” base is ______ so it will ACT as a base to gain the proton; more ____ will be present than _____ The equilibrium lies on the WEAKER side, so this is a _______ favored reaction

Relating an acid or base to its conjugate CH 3 COOH + H 2 O CH 3 COO - + H 3 O + acetic acidacetate (conjugate) K a K b = K w 25°C) What is the K b for acetate if the K a of acetic acid is 1.8 x ?

Comparing Acids to Acids Its important to be able to compare the strengths of all of the different acids to conjugate acids and bases and to conjugate bases. By convention acid and conjugate base strengths are used. Table 702 looks intimidating, but its not!!!!!!

Table 17.3 (pp. 702) Acid Name AcidKaBaseKbBase Name ____________ acid strength ____________(conjugate) base strength Open your book up to this page!

Using the table Who is a stronger acid, H 2 SO 4 or H 2 SO 3 ? Is benzoic acid, C 6 H 5 CO 2 H, stronger or weaker than acetic acid? Which has the stronger conjugate base, acetic acid or boric acid? Which is the stronger base, ammonia or the acetate ion? Which has the stronger conjugate acid, ammonia or the acetate ion? MORE Ka’s in appendix H MORE K b ’s in appendix I

pKa Another way to compare acid strengths using a logarithmic scale pK a = - logK a pK a value decreases as acid strength increases Problems What is the pKa of benzoic acid (K a = 6.3 x10 -5 ) Is an acid with a pKa of 2.87 stronger or weaker?

Now on to calculating pH using Ka Calculate the pH of a M benzoic acid solution? ( K a = 6.3 x )

Calculating pH of Weak Base What is the pH of a M NH 3 solution? (K b = 1.8 x )

Using pH to determine K A 0.10 M solution of lactic acid has a pH of What is the K a for lactic acid?

SALTS Refer to handout in curriculum guide