ACID - BASE CHEMISTRY What is an acid? What is a base? hydrogen ion donor What is an acid? What is a base? How does HNO3, a strong acid, behave in water? How does HNO2, a weak acid, behave in water? hydrogen ion acceptor Strong electrolyte: HNO3 ---> H+ + NO3- 100% Weak electrolyte: HNO2 = H+ + NO2- partial

How does NH3 behave in water? Write a reaction for the general behavior of any weak acid. Write a reaction for the general behavior of any weak base. Weak electrolyte: NH3 + H2O = NH4+ + OH- HA + H2O = H3O+ + A- or HA = H+ + A- B + H2O = BH+ + OH-

How about an equilibrium constant for weak acids? HA + H2O = H3O+ + A- or HA = H+ + A- (H3O+) (A-) (HA) (H2O) (H+) (A-) (HA) K = Ka = acid dissociation constant For dilute solutions (<1M): (H2O) = 56 M and is a constant

How about an equilibrium constant for weak bases? B + H2O = BH+ + OH- (BH+) (OH-) (B) Kb = base dissociation constant

The Bronsted-Lowry Concept Conjugate pairs HCl Cl- CH3COOH CH3COO- NH4+ NH3 HNO3 NO3- How does a conjugate pair differ?

Behavior of water at 25oC: H2O = H+ + OH- Kw = (H+)(OH-) = 1.0 x 10-14 This reaction is endothermic. Why? What happens to Kw if temperature increases?

pH pH = -log (H+) (H+) = 10-pH What is pH of 0.0271M HCl? If pH = 3.48, what is (H+)?

What is the pH of pure water at 25oC? H2O = H+ + OH- Kw = 1.0 x 10-14 Neutrality

Other useful relationships pX = -log (X) (X) = 10-pX _______ = -log (OH-) ______ = 10-pKa pKw = _______ Kw = (H+)(OH-) convert to log form

How are pH, pOH, (H+), and (OH-) related at 25oC? acidic (H+) > (OH-) neutral (H+) = (OH-) basic (H+) < (OH-)

pH acid rain (NOx, SOx) pH of 4.2 - 4.4 in Washington DC area 0-14 scale for the chemists 2 3 4 5 6 7 8 9 10 11 12 acidic (H+) > (OH-) neutral @ 25oC (H+) = (OH-) distilled water basic or alkaline (H+) < (OH-) normal rain (CO2) pH = 5.3 – 5.7 fish populations drop off pH < 6 and to zero pH < 5 natural waters pH = 6.5 - 8.5

Can you have a negative pH? What is the pH of 6.0M HCl? What is the pH of 10M NaOH? What is the pH of 10-8 M HCl?

How about the pH of a weak acid solution? To the CHM 102 webpage…. Good pH tutorial: http://www.chem.ubc.ca/courseware/pH/index.html

practice… 1. Calculate pH of 0.0016 M NaOH. 2. What is (H+) of solution with pOH = 2.61.

3. What is the pH of neutrality at 60oC where Kw = 9.62 x 10-14? 4. What is the pOH of neutrality at 60oC? 5. How many times more concentrated is a pH of 4 compare to a pH of 7?

answers… 5. 1000 x 4. pOH = 6.50 3. pH = 6.50 2. (H+) = 4.1 x 10-12 M