Monoprotic Acid-Base Equilibria Monoprotic Weak Acids Monoprotic Weak Bases Fraction of Dissociation-Association Salts of Weak Acids Buffers

Weak Acids Weak Acid  an acid that is not completely dissociated in aqueous solution Weak Acid Equilibrium HA H + + A - weak acid conjugate base K a = Acid Dissociation Constant [H + ][A - ] [HA] (Available in Appendix G)

Weak Bases Weak Base  a base in which the hydrolysis reaction does not go to completion in aqueous solution Weak Base Equilibrium B + H 2 O BH + + OH - weak base conjugate acid K b = Base Hydrolysis Constant [BH + ][OH - ] [B]

Conjugate Relationships For conjugate acid-base pairs K a  K b =K w pK a +pK b =14 (at 25°C) This is a very useful relation to determine K b when you can look up K a

Conjugate Relationships The conjugate base of a weak acid is a weak base Stronger weak acid  Weaker conjugate base Weaker weak acid  Stronger conjugate base The conjugate acid of a weak base is a weak acid Stronger weak base  Weaker conjugate acid Weaker weak base  Stronger conjugate acid

Monoprotic Weak Acid Problems Method Write pertinent equations for all equilibria Write expression for mass balance Write expression for the charge balance

Monoprotic Weak Acid Problems Equilibria HA H + + A - K a = [H + ][A - ] [HA] H 2 O H + + OH - K W = [H + ][OH - ] Mass Balance F = formal concentration  the total amount of a compound dissolved in 1 L of solution F = [HA] + [A - ]  [HA] = F - [A - ]

Monoprotic Weak Acid Problems Charge Balance [H + ] = [A - ] + [OH - ] Approximation (a very good one) Assume that the weak acid reaction products will dominate over the water dissociation reaction in terms of product concentrations so [A - ] >> [OH - ] because the weak acid reaction dominates over the water dissociation reaction Charge balance equation becomes [H + ]  [A - ]

Monoprotic Weak Acid Problems Using our equilibrium expression K a = [H + ][A - ] = [H + ][H + ] = [H + ][H + ] [HA] [HA] F - [A - ] so let x = [H + ] and K a = x 2 F - x

Monoprotic Weak Acid Example Consider placing o-hydroxybenzoic acid in aqueous solution at a formal concentration of F = F K a = 1.07 x What is the [H + ] ? K a = x 2  x 2 + K a x - K a F = 0 F - x so using the quadratic equation [H + ] = = 6.80 x M and this is ~ [A - ]

Monoprotic Weak Acid Example What is the [OH - ] ? [OH - ] = = = 1.47 x M [H + ] 6.80 x Note our approximation [A - ] >> [OH - ] is justified! Another useful approximation is to neglect x in the denominator iff x < 0.01 F K a = x 2  x 2 F - x F Try this with the o-hydroxybenzoic acid problem- Does this meet our criterion? < (0.01)(0.050) NO

Monoprotic Weak Base Problems Weak base - similar to the weak acid approach except we solve for [OH - ] Equilibria B + H 2 O BH + + OH - K b = [BH + ][OH - ] [B] H 2 O H + + OH - K W = [H + ][OH - ]

Monoprotic Weak Base Problems Mass Balance F = [BH + ] + [B]  [B] = F - [BH + ] Charge Balance [BH + ] + [H + ] = [OH - ] In the weak base case we want to solve for [OH - ]

Monoprotic Weak Base Problems Approximation (a very good one) Assume that the weak base reaction products will dominate over the water dissociation reaction in terms of product concentrations so [BH + ] >> [H + ] because the weak base reaction dominates over the water dissociation reaction Charge balance equation becomes [BH + ]  [OH - ]

Monoprotic Weak Base Problems Using our equilibrium expression K b = [BH + ][OH - ] = [OH - ][OH - ] = [OH] 2 [B] [B] F - [BH + ] so let x = [OH - ] and K b = x 2 F - x

Monoprotic Weak Base Problems [OH - ] = Again a useful approximation is to neglect x in the denominator iff x < 0.01 F K b = x 2  x 2 F - x F

Fraction of Dissociation (for an acid) Fraction of Dissociation  the fraction of acid that is present in the form A -  = [A - ] = x [A - ] + [HA] x + (F -x)  = x F

Fraction of Association (for a base) Fraction of Association  the fraction of base that has reacted with water  = [BH + ] = x [BH + ] + [B] x + (F -x)  = x F

Salt of Weak Acids Sodium acetate dissolved in water Salts completely dissociate in solution Na + CH 3 COO - Na + + CH 3 COO - In solution CH 3 COO - + H 2 0 CH 3 COOH + OH - K a =1.75 x A - + H 2 0 HA + OH -

Salt of Weak Acids What if we dissolve F of Na + CH 3 OO - in solution? How would we find the pH? If you dissolve the salt of a weak acid the result is a weak base equilibrium problem. Equilibria CH 3 COO - + H 2 0 CH 3 COOH + OH - K b = [HA][OH - ] = K W = 1.01 x = 5.77 x [A - ] K a 1.75 x H 2 O H + + OH - K W = [H + ][OH - ]

Salt of Weak Acids Mass Balance F = [HA] + [A - ]  [A - ] = F - [HA] Charge Balance [H + ] = [OH - ] + [A - ] Weak base case  we want to solve for [OH - ] K b = [HA][OH - ] = K W = 1.01 x = 5.77 x [A - ] K a 1.75 x [HA][OH - ] = x 2 = 5.77 x [A - ] F - x Solve for x = [OH - ]

Buffer Solutions Buffer  a solution that resists changes in pH when acid or base is added or when the solution is diluted Typical Buffer A mixture of an acid and base (salt of the acid) conjugate pair which resists changes in pH The weak acid and conjugate base should have similar concentrations (10:1) Buffers break down at extremes of pH or dilution Buffers are most effective near the pK a of the acid

Buffer Solutions How do mixtures of weak acids and their conjugate bases act as buffers? Consider a weak acid of F =0.10 M and K a =1.0 x HA + H 2 O H 3 O + + A - K a = [H + ][A - ] = 1.0 x x x x [HA] Recall K a = x 2 F - x Solve for x x = 3.1 x M  = x/F = (3.1 x )/0.1 = 0.31  3.1 % dissociated Addition of additional A - makes HA dissociate less!

Buffer Solutions Consider a weak base of F =0.10 M and K b =1.0 x A - + H 2 O HA + OH - K b = [HA][OH - ] = 1.0 x x x x [A - ] Recall K b = x 2 F - x Solve for x x = 3.2 x M  = x/F = (3.2 x )/0.1 = 3.2 x  % associated Addition of additional HA makes A - associate less!

Buffer Solutions The concentration of HA and A - in solution are similar to the amounts added to solution A buffer acts through the reaction of added base with excess HA and the reaction of added acid with excess A - This is how it resists changes in pH As more A- is formed from the former reaction or as more HA is formed from the latter reaction the pH changes very little until we have a greater ratio than 1:10 or 10:1 between the acid and its salt

Henderson-Hasselbach Equation HA + H 2 O H 3 O + + A - K a = [H + ][A - ] [HA] [H + ] = K a [HA] [A - ] -log [H + ] = -log K a - log [HA] + log [A - ] pH = pK a + log [A - ] Use formal concentrations for [HA] all species pH = pK a + log [B] Use the K a for the acid form [BH + ]

Henderson-Hasselbalch Equation If [HA] = [A - ] then pH = pK a Effect of the [A-]/[HA] ratio on pH (Table 10-1) In any mixture of 5 different acids and bases in solution there will only be a single pH and the 5 Henderson- Hasselbalch Equations must be simultaneously met

Buffers Doing Their Job We add the acid form to a solution as the chloride salt (BH + Cl - ) Consider the tris buffer system tris(hydroxymethyl)aminomethane This salt completely dissociates when placed in solution BH + Cl - BH + + Cl - to form the weak acid

Buffers Doing Their Job What is the pH of a solution prepared by dissolving g of tris (F.W. = ) to 4.67 g of tris hydrochloride (F.W. = ) in 1.00 L of H 2 O? Our equilibrium looks like BH + B + H + [BH + ] = [4.67 g(1 mol/ g)]/ 1L = M [B] = [12.43 g(1 mol/ g)]/ 1L = M pH = pK a + log [B] = log = 8.61 [BH + ] NOTICE : Volume does not matter! You can use the number of moles of acid and conjugate base directly

Buffers Doing Their Job The reactions written above lie completely to the right (i.e. we do not need an arrow written to the left) and only cease when one of the reactants are completely exhausted Weak Base + Strong Acid react completely B + H + BH + K = [BH + ] = 1 [B] [H + ] K a Weak Acid + Strong Base react completely HA + OH - A - + H 2 O K = [A - ] = 1 [HA] [OH - ] K b

Buffers Doing Their Job One of the two reactants is the limiting reagent and the system reacts until that limiting reagent is completely consumed Here the mol of H + from HCl = (0.012 L)(1.00 M) = mol Recall we have mol of B so HCl is the limiting reagent What is the pH resulting from addition of 12.0 mL of 1.00 M HCl to the tris buffer in the previous problem? Addition of strong acid reduces the amount of tris base B + H + BH + weak base strong acid

Buffers Doing Their Job pH = pK a + log moles B = log = 8.41 moles BH NOTICE : Again volume does not matter! Now we can use Henderson-Hasselbalch B + H + BH + Initial moles Final moles

Buffer Solutions A buffer is the most effective at a pH = pK a How do we prepare a buffer in the lab? Calculate how many grams (mol) of weak acid to add to make final volume (V) and dilute with H 2 O to approximately 0.75V Monitor the pH Add concentrated base dropwise to adjust pH to desired value Pour solution into a volumetric flask and rinse initial container with H 2 O several times into volumetric flask Dilute with H 2 O to desired volume and mix well

Buffer Capacity Buffer Capacity  a measure of how well a solution resists changes in pH when adding strong acid or strong base Buffer Capacity   = (d C b /d pH) = - (d C a /d pH) where C b and C a are the number of moles required to change the pH by 1 unit F solution of H a with a pK a = 5.00 The top curve indicates the concentration of strong base necessary to yield the indicated pH The bottom curve is the derivative of the top curve (the buffer capacity)

Buffer Solution Facts When choosing a buffer you should choose the system with a pKa that is closest to the desired pH Buffers are “good buffers” over a range of pH’s corresponding to pK a - 1 to pK a + 1 Outside of this range there is not enough A - to react with strong acid or HA to react with strong base Realize that just as with all thermodynamic equilibrium constants the “true” Henderson- Hasselbalch Equation should contain activities rather than concentrations This leads to a dependence of the pKa and solution pH on temperature and the ionic strength of the solution

Buffer Solutions Assignment In addition to doing all of the assigned problems, look over Table 10-2 on pp Make sure that for each buffer listed you can: identify the acid and base forms of the buffer write the equilibrium expression that would be used in conjunction with the Henderson-Hasselbalch Equation You do not need to memorize the structures of individual buffer systems However, you should familiarize yourself with the buffers listed as indicated above