1. A2 Chemistry: F325 – Equilibria, Energetics and Elements
Acids, Bases and Buffers
Plymstock School
P.J.McCormack

2. Learning Outcomes. By the end of the lesson I will be able to....
29 November, 2018
Learning Outcomes.
By the end of the lesson I will be able to....
All..
State the meaning of the terms acid and base
Write an equation for an acid reacting with a base and label the conjugate acid and conjugate base.
Most..
Write an expression for Kw
Explain what the term ionic product of water is
Some..
Calculate the pH and hydrogen ion concentration
Low
High
Key Words: ionic product of water, conjugate acid, conjugate base
P.J.McCormack

3. 29 November, 2018
Key Terminology
Acid Proton (H+) donor Base Proton acceptor Buffer A substance that can resist change in pH (a weak acid and a salt of a weak acid) Ka The acid dissociation constant (shows the extent of the acid dissociation)
P.J.McCormack

4. 29 November, 2018
Acid-base Equilibria
The Brønsted-Lowry theory of acids and bases defines an acid as a proton donor and it may only function as such when in the presence of a proton acceptor – a base. HCl + NH3 → Cl- + NH4+ acid base conjugate base conjugate acid HCl and Cl- are a conjugate pair. NH3 and NH4+ are also a conjugate pair.
P.J.McCormack

5. Amphoteric Nature of Water
29 November, 2018
Amphoteric Nature of Water
HCl can also donate a proton to water:
HCl + H2O  H3O+ + Cl-
In this case the water is acting as a base and the H3O+ is its conjugate acid. (H2O and H3O+ are a conjugate pair).
H3O+ is called the oxonium ion.
Water can also act as an acid:
NH3 + H2O  OH- + NH4+
The OH- is the conjugate base of the acid water.
Water is an amphiprotic solvent because it may either donate or accept protons.
P.J.McCormack

6. H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
29 November, 2018
Ionisation of Water
H2O(l) H+(aq) + OH-(aq) or
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
The equilibrium expression can be written:
Kc = [H+] [OH-]
[H2O]
P.J.McCormack

7. 29 November, 2018
Ionic Product of Water
In this the concentration of the water is constant so the expression can be modified to:
Kw = [H+] [OH-]
Where Kw is called the ionic product of water.
At 298 Kw = 1 x mol2 dm-6
P.J.McCormack

8. 29 November, 2018
Ionic Product of Water
Since, for every molecule of H2O that splits up, one H+ and one OH- is formed, therefore
[OH-] = [H+]
or 1 x = [H+]2
[H+] = 1 x 10-7 mol dm-3 and
[OH-] = 1 x 10-7 mol dm-3
P.J.McCormack

9. Tell me two things... (for each)
that you have done well today
that you know now that you didn’t know at the start of the lesson
that you could have done better next lesson
that you would like to know more about 2

10. pH = -log10[H+] The pH Scale
29 November, 2018
The pH Scale
The acidity of a solution depends on the concentration of H+ ions and is measured on the pH scale.
The definition of pH is:
pH = -log10[H+]
P.J.McCormack

11. 29 November, 2018
The pH Scale pH
[H+]
/mol dm-3 1 8
1 x 10-8
1 x 10-1 9
1 x 10-9 2
1 x 10-2 10
1 x 10-10 3
1 x 10-3 11
1 x 10-11 4
1 x 10-4 12
1 x 10-12 5
1 x 10-5 13
1 x 10-13 6
1 x 10-6 14
1 x 10-14 7
1 x 10-7
A difference of one pH number means a tenfold difference in acidity. So pH 2 is ten times the acidity of pH 3.
P.J.McCormack

12. Calculating pH pH = -log10[H+] pH = -log10[0.01] = 2.0
29 November, 2018
Calculating pH
pH = -log10[H+]
If a strong acid has a concentration of mol dm-3.
The pH is calculated
pH = -log10[0.01]
= 2.0
P.J.McCormack

13. 29 November, 2018
Calculating pH
Calculate the pH of an acid solution in which [H+(aq)] = 8.10 x 10-6 mol dm-3.
Answer = 5.09
Answer = 5.09
P.J.McCormack

14. 29 November, 2018
Calculate [H+] from pH
[H+(aq)] = 10-pH A strong acid has a pH of 3.4. What is the value of [H+(aq)]? [H+] = = 3.98 x 10-4 mol dm-3
P.J.McCormack

15. 29 November, 2018
Weak and Strong Acids
A strong acid is an acid that fully dissociates HCl → H+ + Cl- A weak acid is one that does not fully dissociates CH3COOH H+ + CH3COO-
P.J.McCormack