Acid-Base Equilibria 1. Application of equilibrium concepts. 2. Not much else new in the way of theory is presented. 3. Specific focus on aqueous (H O is 2 solvent) systems. 4. Assume we are at equilibrium unless otherwise specified. 1
I. Acids & Bases A. General ideas: 1. Some H nuclei are quite stable in their bonding arrangements. Ex.: H nuclei in methane (CH ) have almost no tendency 4 to leave their bonding e! pair and go elsewhere. CH + H O þ no reaction 4 2 Therefor, the H atoms in CH 4 are not acidic! 2
B. Brønsted-Lowry Theory 1. An acid is a substance that can donate a H +. 2. A base is a substance that can accept a H +. Requires non-bonding e! pair!!! 3. A general expression of an acid-base rxn.: HA + B W BH + + A! acid base acid base 3
4. Specific expression: HCl + H 2 O W H 3 O + + Cl! acid base acid base C. Conjugates: 1. Conjugate base is what is leftover after the acid has donated its H +. 2. Conjugate acid is what is leftover after the base has accepted its H +. 4
HNO 3 + NH 3 W NH 4 + + NO 3! You fill in the blanks above. Problems: a) Rxn. of CH COOH (acetic acid) with water: 3 b) Rxn. of NH (weak base) with water: 3 5
D. Acid & Base Strength (context dependent!!!) 1. 1st, a qualitative approach. Then, quantitative. 2. Think about H 2 O functioning as a base: HA (aq) + H 2 O (l) W H 3 O + (aq) + A! (aq) acid base acid base The 2 bases are competing for the H + ion. Will the H + ion spend most of its time associated with the weaker of the 2 bases or the stronger? 6
A base does what? Does a strong base do this better than a weak base? 3. A relative strength picture: When you dissolve 1 mole of HCl in 1 L of water you will get a higher [H O + ] than when you do the same with 3 CH COOH. Which is a stronger acid? 3 Closure question: If HA is a strong acid, is its conjugate base strong or weak? 7
A brief aside: When a H + ion is released in water, it does not exist as a free H + ion. It binds to water: 1. H 3 O + is called the hydronium ion. 2. H 5 O 2 +, H 7 O 3 +, etc., likely exist as well. 8
4. A quantitative approach (by far the best): K a a) Can you write an equilibrium constant expression for: HCl + H 2 O W H 3 O + + Cl! K eq = K a = [H 2 O] is so large, it is essentially constant. Therefore we can simplify K eq into K a. By definition: K eq [H 2 O] = K a 9
b) Can you write a K a expression for: CH 3 COOH + H 2 O W H 3 O + + CH 3 COO! K a = c) The K a value for HCl ~ 1 x 10 6, the value for CH 3 COOH = 1.8 x 10!5 Which is a stronger acid? HCl or CH 3 COOH Logic!? Counting page? 10
E. The Dissociation of Water 1. Water is interesting. It can be both an acid and a base: H 2 O + H 2 O W H 3 O + + OH! Lewis dot? acid base acid base On your own, write this reaction using Lewis structures instead of molecular formulas: 11
2. Can we write a K eq expression for the above? K eq = 3. Can we simplify this? Go to steps #4-6 first, then come back here. K w = 4. In pure water @ 25 C, [H 3 O + ] = 1.0 x 10!7 M. 12
5. In pure water at 25 C, [OH! ] =? 6. What is [H 2 O] =? 7. Now solve for K w : K w = [H 3 O + ] [OH! ] Problem: If 10!9, what is [OH! ]? [H 3 O + ] for a solution = 3.84 x 13
II. The ph Scale (power of Hydrogen) There are many reasons for using the ph, as opposed to [H 3 O + ]. Convenience in expressing the numbers is probably most important to you. A. By definition: ph =!log [H 3 O + ], so: [H 3 O + ] = 10!pH 1. Let s start with an example of ph. Calculate the ph of a 1.0 x 10!2 M solution of HBr (a strong acid). 14
2. Make sure you can also find [H 3 O + ] & [OH! ] if given ph. Calculate [H 3 O + ] & [OH! ] for a gastric juice sample with a ph = 2.09. B. Note that as [H 3 O + ] increases, ph decreases. Logic: ph = negative log of [H 3 O + ]. 15
C. What about rounding? Essentially the x / rule plus one.!log (1.0 x 10!9 ) = 9.00!log (3.28 x 10!5 ) = 4.484 126 156.!log (3.28 x 10!9 ) =?.?????????. Can you see the pattern yet? D. Why do we care about ph? (Apollo 13 clip?) 16
III. Reactions Between Acids and Bases A. Neutralization 1. HA (aq) + NaOH (aq) W H 2 O (l) + NaA (aq) acid base water salt 2. I like the following approach better: HA (aq) + OH! (aq) W H 2 O (l) + A! (aq) What would we call Na, in 1 above? 17
3. ph indicators have acid & conjugate base form. The forms are different colors: HInd + OH! W H O + Ind! (aq) (aq) 2 (l) (aq) acid base acid base Color 1 Color 2 4. ph paper has indicator linked to the paper 5. In a titration, the indicator tells you when you have just overshot (reached endpoint). Refer to Titration lab exercise for more details. 18
What volume of a 0.2000 m [NaOH] solution would be required to neutralize 40.00 ml of a 0.1207 M solution of HCl? 19
IV. Acid-Base Buffers A. Buffers function to limit change in ph when an acid or base is added to a system. Do you think the blood has buffers? B. A buffer is prepared by combining a weak acid with its conjugate base (in what ratio?) For acetic acid/acetate, CH 3 COOH combined with CH 3 COONa. 20
What happens to CH 3 COONa when it dissolves it in water? H 2 O CH 3 COONa ö C. Acid part of the buffer neutralizes added base, while the base neutralizes added acid. Added strong base CH 3 COOH + OH! ö Added strong acid CH 3 COO! + H 3 O + ö 21
D. The Henderson-Hasselbalch Transformation 1. Can we rearrange the K a expression to get an equation with ph =? [H O + ] [A! ] Start with K = 3 a Finish with ph = pk a + [HA] log([a! ]'[HA]) If ph =!log [H 3 O + ], what do you think pk a is? pk a = 22
2. Why is the Henderson-Hasselbalch Transformation useful? (Buffer issues.) a) Do you think it would be nice to have both HA and A! present in a buffer? yes or no? b) Why? 23
c) What happens to ph in the Henderson- Hasselbalch when [HA] = [A! ]? i) Substitute [HA] = [A! ] and simplify. What do you obtain? ii) What does the result in 2 c) i) mean in terms of most appropriate choice of weak acid to buffer at a given ph? 24
3. Medical Perspective: Which of dissociation of H 2 CO 3 is physiologically relevant? CO 2(aq) + H 2 O (l) W H 2 CO 3(aq) H 2 CO 3(aq) + H 2 O (l) W HCO 3!(aq) + H 3 O + (aq) pk a1 = 6.38 HCO 3! (aq) + H 2 O (l) W CO 3 2!(aq) + H 3 O + (aq) pk a2 =10.32 25
ph 4. Let s look at the Normal Values section of: http://www.nlm.nih.gov/medlineplus/ency/article/003855.htm P a CO 2 P a O 2 SaO 2 HCO 3! a) Can you relate the terms in ph'paco 2 'PaO 2 'SaO 2 ' HCO 3! to variables in the Henderson-Hasselbalch transformation? Which values relate directly to acid-base chemistry? ph = pk a + log([a! ]'[HA]) 26
b) How do you know if HCO 3! is acting like an acid or a base? See pk a values above and think of CO 2 eventually leaving the body. 27
Counting H 3 O + formed for strong acid & weak acid. To get more comfortable with quantitative aspects of acid strength, we are going to try counting the number of acid molecules that would dissociate to form H 3 O +. While we would normally use concentration values (and counting molecules is not quite proper), most students understand the concept better by thinking about numbers of molecules/ions, as opposed to concentrations of these. We will assume we are placing 1 x 10 6 molecules of HCl into water and allowing the dissociation reaction to proceed until equilibrium is reached. (Could we apply K a if we had not reached equilibrium?). (Reaction quotient tells you if you have reached equilibrium.) 28
For HCl, K a = HCl + H 2 O W H 3 O + + Cl! Start After 1 reacts After 10 react @ equilibrium 29
Now do the same for acetic acid. K a = CH 3 COOH + H 2 O W H 3 O + + CH 3 COO! Start After 1 reacts After 10 react @ equilibrium 30
Change in acid-conjugate base ratios as a function of ph ph 2 3 4 5 6 7 8 9 10 H 2 O H 3O + 10 5 10 4 1,000 100 10 1 OH - 1 1 1 1 1 1 1 10 1 100 1 1000 Aspirin pka: 3 HA 10 1 A - 1 1 1 10 1 100 1 1 1000 10 4 1 10 5 H 2 CO 3 pka: 6.37 HA A - R O C O pka = 3.0 + H 2 O R O C O - + H 3 O + H Aspirin, HA form Aspirin, A - form H O C O O Carbonic acid, HA form H pka = 6.37 + H 2 O H O O C O - Carbonic acid, A - form + H 3 O + 31
5. Can you fill in the blanks for carbonic acid (H 2 CO 3 ), above? Two approaches: a) Fill in ratios for ph values 2.37, 3.37, 4.37, etc., remember log term in H-H equation. b) Use the H-H equation to calculate the ratio of H 3 O + 'OH! for ph values 2, 3, 4, etc. 6. Does this approach give different way to look at acid strength? How dos this relate to loss of functions/in CO 2 poisoning (re. Apollo 13)? 32
Change in protein activity as a function of ph ph 2 3 4 5 6 7 8 9 10 HProtein 1000 Protein - 1 100 1 10 1 1 1 1 10 1 100 1 1000 pka = 6.0 HProtein + H 2 O + H 3 O + inactive Protein - active Most proteins have acid/base groups. Assume here that a protein has no activity in its HProtein form & is 100% active in the Protein - form. Would the activity be higher at ph 7.4 or 6.0??? What would a graph of activity vs. ph look like? 33
ph Activity Relationship 100 80 activity (%) 60 40 20 2 4 6 8 10 ph 34