Chem 105 Tuesday March 8, 2011 Chapter 17. Acids and Bases 1) Define Brønsted Acid and Brønsted Base 2) Proton (H + ) transfer reactions: conjugate acid-base pairs 3) Water and other amphiprotic substances 4) Water autoionization and K w 5) ph and poh 6) Weak acids and K a 7) Weak bases and K b 8) Strong acids and bases 3/8/2011 1
Acids and bases are crucial to the function of many chemical and biochemical systems. Most biochemical reactions are catalyzed by H + transfer reactions occurring within the enzyme active site. H + ion is very chemically reactive - so H + never actually occurs by itself. It is always attached to something else such as water to make H 3 O +. Naked H + (the proton) can be seen only in a mass spectrometer in a high vacuum system. 3/8/2011 2
Define: Brønsted acid = H + donor in a H + -transfer reaction HCl(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) In the forward direction, HCl acts as a Brønsted acid (donates H + ion to water). In the reverse direction, H 3 O + acts as a Brønsted acid (donates H + ion to Cl - chloride ion). 3/8/2011 3
HCl(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) Define: Brønsted base = H + acceptor in a H + -transfer reaction In the forward direction, H 2 O acts as a Brønsted base (accepts H + ion from HCl). In the reverse direction, Cl - chloride ion acts as a Brønsted base (accepts H + ion from H 3 O + ). 3/8/2011 4
conjugate base = acid with H + removed conjugate acid = base plus a H + Acid Base Conjugate base Conjugate acid HCl(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) a conjugate acid/base pair a conjugate acid/base pair 3/8/2011 5
3/8/2011 6 Table 17-2, p. 765
Write a proton transfer reaction between ammonia (NH 3 ) and water, with ammonia acting as a base. Identify the conjugate acid/base pairs. Base Acid NH 3 + H 2 O Conjugate acid NH 4 + + OH - Conjugate base Notice that water molecules (in different situations) can act as a Bronsted acid or base. Therefore water is an amphiprotic substance. 3/8/2011 7
H 2 O can act as a Brønsted Acid (H + donor), or a Brønsted Base (H + acceptor). NH 3 (aq) + H 2 O(l) NH 4+ (aq) + HO - (aq) Ammonia dissolved in water: water as a Bronsted acid HNO 3 (aq) + H 2 O(l) NO 3- (aq) + H 3 O + (aq) Nitric acid dissolved in water: water as a Bronsted base 3/8/2011 8
N H H H H O H Showing the anticipated direction of electron flow in a chemical reactions that involves breaking or making a bond. A curved arrow goes from the electron source to the electron sink. N H H H O H H N H H H H O H N H H H O H H In organic chemistry, we call this is curved arrow notation or electron pushing. 3/8/2011 9
Acid-base chemistry originates from the properties of pure water. Water itself contains about 1 x 10-7 M concentrations of H + and OH - ions. This is due to an auto-ionization or spontaneous splitting reaction. Determined by measuring the (very small) electrical conductance of pure water, and comparing this to the conductance of dilute salt solutions. Conductance -> Adding 1 x 10-7 M NaCl (0.1 μm) to purified water about doubles the conductivity of the water. Therefore, the water itself must have about 1 x 10-7 M ions, which would have to arise from autoionization (H + and OH - ). 0 0 0.1 1 [NaCl] μm 3/8/2011 10
AUTOIONIZATION = pure water reacting with itself to a small extent. H 2 O(l) + H 2 O(l) H 3 O + (aq) + HO - (aq) [H 3 O + ] = [HO - ] = 1.00 x 10-7 M (experimentally determined) To preserve electric neutrality, these must be equal concentrations. The equilibrium constant for autoionization is K w, also called the ION PRODUCT of water. products [H 3 O ][OH K w 3 reactants 1 ] [H O ][OH [H 2 O] (l) = 1 because it is K w = (1.0 x 10-7 ) 2 = 1.00 x 10-14 the bulk solvent phase. [H 2 O] is essentially 3/8/2011 constant at 55 M. 11 ]
This equation governs the behavior of acids and bases in water. Since the product [H + ] [OH - ] is constant, if one increases, the other must decrease. Let s say you add 0.0010 mol of NaOH to 1.0 L of pure water. Now what happens? (qualitative, then quantitative) 2 H 2 O(l) H 3 O + (aq) + OH - (aq) Instantaneous concentrations after adding NaOH, but before the equilibrium shifts. Q =[H 3 O + ][OH - ] = 10-7 x 10-3 = 10-10 K w = 10-14 Q = 10,000 x K w Q = K, Q > K, Q < K, Le Chatelier predicts equilibrium shifts to the LEFT. 3/8/2011 12
Set up an ICE table How does added OH - affect the auto-ionization? 2 H 2 O(liq) H 3 O + (aq) + OH - (aq) initial 0 0 after adding OH- 0 0.0010 change +x +x equilib x 0.0010 + x K w = [H 3 O + ][OH - ]= (x) (0.0010 + x) Because x << 0.0010 M, we estimate that [OH - ] 0.0010 M [H 3 O + ] = x = K w / 0.0010 = 1.0 x 10-11 M 3/8/2011 13
A common way to express acidity (and basicity) is with ph Define: ph = - log [H 3 O + ] (= - log 10 [H 3 O + ]) In a neutral solution, [H 3 O + ] = 1.00 x 10-7 at 25 o C ph = -log (1.00 x 10-7 ) = - (-7.000) = 7.000 3/8/2011 14
So, what is the ph of the 0.0010 M NaOH solution? [H 3 O + ] = 1.0 x 10-11 M ph = - log (1.0 x 10-11 ) = 11.00 General conclusion Basic (or alkaline) solution ph > 7 Neutral ph = 7 Acidic solution ph < 7 3/8/2011 15
The practical limits of ph are about -1 to 15 11 10-11 10-3 -1 10 3/8/2011 16
The ph of Coke is 3.12. Because ph = - log [H 3 O + ] then log [H 3 O + ] = - ph Take antilog and get [H 3 O + ] = 10 -ph [H 3 O + ] = 10-3.12 = 7.6 x 10-4 M Due to citric acid and dissolved CO 2. H O 2 C O H C O 2 H C O 2 H O O H H O O H O O H O 3/8/2011 17
In general Other p Scales px = -log 10 X and so poh = - log [OH - ] K w = [H 3 O + ] [OH - ] = 1.00 x 10-14 at 25 o C Take the -log of both sides -log (10-14 ) = - log [H 3 O + ] + (-log [OH - ]) pk w = 14 = ph + poh 3/8/2011 18
[H + ]=10 -ph [OH - ]=10 -poh 3/8/2011 19
Which solution is more alkaline? 27 1. poh = 3 2. poh = 4 13 poh = 3 poh = 4 3/8/2011 20
Which solution is more alkaline? 1. poh = 3 2. poh = 4 poh = -log[oh-] [OH-] = 10 -poh [OH-] = 1 x 10-3 M 3/8/2011 21
Equilibria Involving Weak Acids and Bases Aspirin is a good example of a weak acid. A weak acid (or base) = one that ionizes to a small extent in water (< 5%). 3/8/2011 22
Acid Conjugate Base acetic, CH 3 CO 2 H CH 3 CO 2-, acetate ammonium, NH + 4 NH 3, ammonia Hydrogen carbonate, HCO - 3 CO 2-3, carbonate 3/8/2011 23
Acid dissociation constant K a acetic acid, CH 3 CO 2 H (HOAc) HOAc(aq) + H 2 O(l) H 3 O + (aq) + OAc - (aq) Acid K a = [ H 3 O + ] [ O A c - ] [ H O A c ] = 1. 8 x 1 0-5 Conj. Base (acetate ion) [H 2 O] (l) = 1 because it is the bulk solvent phase. [H 2 O] is essentially constant at 55 M. 3/8/2011 24
There are many weak acids, and their K a s range from about 10-1 to 10-14. The larger the value of K a, the greater the tendency of the acid to react with water forming H 3 O +. Stronger weak acids produce more H + ions when dissolved in water, compared to weaker weak acids. 3/8/2011 25
HClO 3/8/2011 26
Acids K a Conjugate Bases Increase acid strength 3/8/2011 27
Log form of K a pk a = -log K a pka 7.46 3.74 3.34 HClO 3/8/2011 28
Which is the strongest acid? 1. Hydrogen sulfide H 2 S K a 1.0 x 10-7 2. Propanoic acid CH 3 CH 2 CO 2 H K a 1.3 x 10-5 3. Hypochlorous acid HClO K a 3.5 x 10-8 28 10 0 Hydrogen sulfi... Propanoic acid... Hypochlorous a... 3/8/2011 29
Which one is the strongest acid? 1. Hydrogen sulfide H 2 S K a 1.0 x 10-7 2. Propanoic acid CH 3 CH 2 CO 2 H K a 1.3 x 10-5 3. Hypochlorous acid HClO K a 3.5 x 10-8 pka 0.00000010 7.00 0.000013 4.88 0.0000000357.46 This part of the log (the characteristic ) carries the only the power of ten. This part of the log (the mantissa ) carries the significant figures (2 here). Acid with higher K a, or lower pk a, is the strongest acid. 3/8/2011 30
HF + H 2 O(l) H 3 O + (aq) + F - Acid Conj. base HOAc + H 2 O(l) H 3 O + (aq) + OAc - Weaker Acid Conj. base Look at two weak acids HF and acetic acid: HF is a stronger acid, as I try to represent with the different sized forward and reverse arrows. Now look at the two bases F - and OAc -. The reverse arrow for the bottom reaction is larger relative the forward arrow, compared to the reverse arrow in the top reaction. Thus the base OAc- is a stronger base than F-. 3/8/2011 31
Hydrolysis Reaction = Reaction of Base with Water, e.g., F - from NaF A weak base has K b < 1 Leads to small [OH - ] and a ph of 12 -> 7 3/8/2011 32
Acids K a Conjugate Bases K b Increasing acid strength Increasing base strength 3/8/2011 33
Put in same order as table 17.3 Acid Ka Conjugate base Kb HF 7.20 x 10-4 F- 1.39 x 10-11 Aspirin 3.00 x 10-4 Asp- 3.33 x 10-11 HCN 4.00 x 10-10 CN- 2.50 x 10-5 Think of it this way: HF is a stronger acid because F - ion more readily GIVES UP a H + ion. So, F - is correspondingly- less able to ACCEPT a proton 3/8/2011 34
Strong Acids and Bases 3/8/2011 35
Strong acids ionize completely in water: HCl(aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) Weak acids ionize partially in water: HF(aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) Strong acids you should know HCl, HBr, HI HNO 3 HClO 4 H 2 SO 4 3/8/2011 36
Strong bases ionize completely in water: NaOH(aq) Na + (aq) + HO - (aq) Weak bases ionize partially in water: F - (aq) + H 2 O (l) HF(aq) + HO - (aq) Strong bases you should know Group I Group II LiOH NaOH KOH Ca(OH) 2 (s) slightly soluble Sr(OH) 2 Ba(OH) 2 3/8/2011 37
2.29e-5 poh [ OH - = 4. 340 ] = 10 = 10 - poh - 4. 340-5 = 4. 571 x 10 Carrying an extra sig fig M But recall that Ba(OH) 2 (aq) Ba 2+ (aq) + 2 OH - (aq) So you only need ½ the concentration of Ba(OH) 2, i.e. 2.285 x 10-5 M 3/8/2011 38
What concentration of aqueous rubidium hydroxide is needed to make a solution with poh = 2.5? 1. 1.15 M 2. 0.115 M 26 3. 0.0158 M 4. 0.00316 M 5. 0.000158 M 3 4 6 1 1.15 M 0.115 M 0.0158 M 0.00316 M 0.000158 M 3/8/2011 39
What concentration of aqueous rubidium hydroxide is needed to make a solution with poh = 2.500? 1. 1.15 M 2. 0.115 M 3. 0.0158 M 4. 0.00316 M 5. 0.000158 M (on board) 3/8/2011 40