Lecture 3: Acids, bases and pH

(Campbell and Farrell: Chapter 2.3 – 2.4)

Acid – proton ‘donor’

Base – proton ‘acceptor’

HA H+ + A-

ACID

(protonated)

CONJUGATE

BASE

(deprotonated)

Write chemical equations for the dissociation of the following acids and identify the acid/ conjugate base pair for each:

1. HCO3

2. HCOOH (formic acid)

3. CH3COOH (acetic acid)

HA H+ + A-

Acid dissociation constant (Ka)

Ka = [H+][A-]

HA

• Measure of the strength of an acid

• Ka - strength

Which acid in the following list is the STRONGEST and which is the WEAKEST?

H20 H+ + OH-

ACID CONJUGATE

BASE

Dissociation of water

Also referred to as self-ionization of water

Dissociation of water

H2O H+ + OH-

Ion product of water (Kw)

Kw = [H+][OH-]

• Kw is always = 1 x 10-14 regardless of whether a solution is neutral, basic

or acidic

pH = -log10[H+]

• pH [H+] basic

• pH [H+] acidic

What is the pH of the following solutions?

1. 1 x 10-3 M HCl

2. 1 x 10-4 M NaOH

• pKa Ka weak

• pKa Ka strong

pKa = -log10Ka

Monoprotic acid – can donate one proton (‘mono’ = 1)

e.g. HCl

Diprotic acid – can donate two protons (‘di’ = 2)

e.g. oxalic acid HOOC-COOH

Triprotic acid – can donate three protons (‘tri’ = 3)

e.g. Phosphoric acid H3PO4

Polyprotic acid – can donate three or more protons

(‘poly’ = many)

Fig. 2-12, p. 50

Enzymes have pH ‘optima’, i.e. pH values where they work best

Sometimes we have to prepare solutions of specific pH

HOW?

We need an equation that relates the pH of a solution to the relative amounts of acid and base

components

Henderson-Hasselbalch equation

Henderson-Hasselbalch equation

• relates the pH and of a weak acid solution and the relative amounts of acid and conjugate base in the solution.

1) how to prepare a solution of a weak acid at a certain pH

2) what the pH is when an acid and conjugate base are mixed in a certain ratio

Titration curves

• Titration:Titration: an experiment in which measured amounts of acid (or base) are added to measured amounts of base (or acid)

Equivalent (eq)

• The quantity of base that reacts with 1 mol of [H+] or the amount of acid that yields 1 mol of [H+]

• ‘Equivalents of OH- added’ (the x-axis of titration curve) can be read as ‘amount (mol) of conjugate base formed per (mol) amount of acid’

• Equivalence point:Equivalence point: the point in an acid-base titration at which enough base has been added to exactly neutralize the acid (or vice versa)• The equivalence point is reached after the addition of

1 equavalent (eq) of base for every equivalent of acid

Titration ENDPOINT

• Inflection point:Inflection point: the point in an acid-base titration at which enough base has been added to neutralize half of the acid (or vice versa)• The inflection point is reached after the addition of 0.5 equivalent (eq) of base for every equivalent of acid.• Titration MIDPOINT

Relation between pH, pKa and ‘protonation’

If pH < pKa, then substance will be protonated (H+ on)

If pH > pKa, then substance will be deprotonated (H+ off)

deprotonatedprotonated

The Henderson-Hasselbalch equation and the titration curve

Calculate the relative amounts of acid and base present and the pH of the solution when 1 mol of acetic acid (pKa = 4.76) is titrated with:

1. 0.1 mol NaOH

2. 0.3 mol NaOH

3. 0.5 mol NaOH

4. 0.7 mol NaOH

5. 0.9 mol NaOH