Chem 106 Thursday, March 10, 2011 Chapter 17 Acids and Bases K a and acid strength Acid + base reactions: Four types (s +s, s + w, w + s, and w + w) Determining K from concentrations and ph ph of aqueous solutions of weak acids or bases, or salts Polyprotic acids 3/10/2011 1
OWL 17 OWL 18 3/10/2011 2
Which solution has a lower ph?... 1. 0.050 M sulfuric acid 2. 0.050 M nitric acid 28 5 0.050 M sulfur... 0.050 M nitric... 3/10/2011 3
Which solution has the lower ph? 1. 0.050 M sulfuric acid 2. 0.050 M nitric acid H 2 SO 4 has two ionizable H s, therefore [H 3 O + ] = 0.10 M, and ph = -log(0.10) = 1.00 HNO 3 has only one ionizable H, therefore [H 3 O + ] = 0.050 M, and ph = -log(0.050) = 1.30 3/10/2011 4
17.4 K a vs. Acid/Base strength 3/10/2011 5
Put in same order as table 17.3 Acid K a Conjugate base K b 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. That is, F- more readily GIVES UP a H + ion. So, F - is correspondingly is less able to ACCEPT a proton. 3/10/2011 6
Relative Acid/Base Strength from K for Overall Reaction 3/10/2011 7
How is K c for a H + transfer reaction related to the K a s of the two acids? When an acid and base react, the conjugate base of the acid and the conjugate acid of the base are formed. HF + NH 3 F - + NH 4 + K c > 1 pk a = 3.2 pk a = 9.2 The proton transfer equilibrium tends to form the conjugate base of the stronger acid. If the stronger acid is on the left, K C >1 If the stronger acid is on the right, K C < 1 If the two acids are of equal strength (equal K a s) K C = 1 3/10/2011 8
HF + NH 3 F - + NH 4 + You can easily prove for yourself that F - + NH 4 + HF + NH 3 K c < 1 If the stronger acid is on the left, K C >1 If the stronger acid is on the right, K C < 1 If the two acids are of equal strength (equal Ka s) K C = 1 3/10/2011 9
(that is, favors the products) HC 9 H 7 O 4 > C 6 H 5 COOH HC 9 H 7 O 4 > HClO 2 C 6 H 5 COOH > HClO 1 3 HC 9 H 7 O 4 > C 6 H 5 COOH C 6 H 5 COOH > HClO HC 9 H 7 O 4 > C 6 H 5 COOH > HClO 3/10/2011 10
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You can use the same reasoning to compare BASE strengths. I.e., the acid-base equilibrium tends away from the stronger base. F - + NH + 4 HF + NH 3 K b = 1x10-11 K b = 2x10-5 3/10/2011 12
CONJUGATE BASE 1 3 Conjugate ACID BASE ACID 2 4 3/10/2011 13
CONJUGATE BASE 1 3 Conjugate ACID BASE ACID 2 4 Lets look these up what are the structures and K a s Benzoic acid 6.5 x 10-5 Tris acid form 8.7 x 10-9 3/10/2011 14
K < 1 for the following reaction. Which are the stronger acid, weaker base? HOAc + Br- OAc- + HBr 17 1. HBr, OAc- 2. HOAc, Br- 9 6 3. HBr, Br- 2 4. HOAc, OAc- HBr, OAc- HOAc, Br- HBr, Br- HOAc, OAc- 3/10/2011 15
K < 1 for the following reaction. Which are the stronger acid and weaker base? HOAc + Br- OAc- + HBr 1. HBr, OAc- 2. HOAc, Br- 3. HBr, Br- 4. HOAc, OAc- 3/10/2011 16
17.6 Types of Acid-Base Reactions 3/10/2011 17
Types of Acid/Base Reactions 1. Strong Acid + Strong Base ----> Water + salt.. ph = Neutral Example: HCl + NaOH ----> H 2 O + NaCl Net Ionic Equation: H 3 O + + OH- ----> 2 H 2 O -------------------------------------------------------------------------------- 2. Weak Acid + Strong Base ----> Water + weak base... ph = Basic Example: HF + NaOH ----> H 2 O + NaF Net Ionic Equation: HF + OH - ----> H 2 O + F - (Conjugate base of a weak acid = Basic.) -------------------------------------------------------------------------------- 3. Strong Acid + Weak Base ----> Water + Weak Acid... ph = Acidic Example: HCl + F - ----> HF Net Ionic Equation: H 3 O + + F - ----> HF + H 2 O (Conjugate acid of a weak base = Acidic.) -------------------------------------------------------------------------------- 4. Weak Acid + Weak Base ----> Weak Acid + Weak Base.. Net Ionic Equ: HCN + F - ----> HF + CN - ( ph depends on K a s ) ------------------------------------------------------------------------------- 3/10/2011 18
Strong acid Strong base A HClO 4 + H 2 O -> H 3 O + + ClO 4 - NaOH -> OH - + Na + D H 3 O + + OH - --> 2 H 2 O 3/10/2011 19
Weak acid Strong base C HNO 2 (aq) + OH - (aq) NO 2- (aq) + H 2 O(l) Stronger acid K a = 1 x 10-4 Weaker acid K a = 1 x 10-14 Strong bases react completely 100% with any acid. Equal moles of acid base make essentially sodium nitrite (NaNO 2 ). So the real question is: what does the NO 2 - ion do? React slightly with water (here) to make OH -. 3/10/2011 20
Strong acid Weak base A HCl + H 2 O H 3 O + + Cl - H 3 O + (aq) + OAc - (aq) H 2 O(l) + HOAc(aq) Stronger acid Weak base Weaker acid Strong acids react completely 100% with any base. Equal moles of HCl and acetate make a solution of the weak acid (acetic acid). This solution is acidic. 3/10/2011 21
Determining K from concentrations and ph 3/10/2011 22
HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) Strong acids react completely with any base, even water. So [H 3 O + ] = M HA Weak acids react partially with water as determined by the dissociation constant K a. In this case, [H 3 O + ] < M HA 3/10/2011 23
A 0.015 M solution of hydrogen cyanate HOCN has a ph = 2.67. What is K a for HOCN? [H 3 O + ] = 10-2.67 = 0.00214 M HOCN + H 2 O H 3 O+ + OCN - I 0.015 0 0 C -0.00214 M +0.00214 M +0.00214 M E 0.01286 M 0.00214 M 0.00214 M = 3.56 x 10-4 3/10/2011 24
Predicting the ph of a weak acid or weak base 3/10/2011 25
What is the ph of a 0.0030 M solution of potassium cyanide (K b = 2.5 x 10-5 )? For dilute acids or bases, you may have to use the quadratic equation or successive approximations. For more concentrated solutions, one approximation is generally sufficient. 3/10/2011 26
This is the last part of the problem, which I did not finish in class (we got to [OH-] = 2.611 x 10-4 M. Use the quadratic equation or successive approximations if the concentration of the weak acid < 100 x K a. One approximation is sufficient if the concentration of the weak acid > 100 x K a. 3/10/2011 27
Polyprotic acids Strong (H 2 SO 4 ): K a s large Weak (H 3 PO 4 ): K a1 > K a2 > K a3 H 3 PO 4 H 2 PO 4 - + H 3 O + H 2 PO 4 - HPO 4 2- + H 3 O + HPO 4 2- PO 4 3- + H 3 O + K a1 = 7 x 10-3 K a2 = 6 x 10-8 K a3 = 4 x 10-13 3/10/2011 28