1. Title: Lesson 4 Strong and Weak Acids and Bases
Learning Objectives:
Understand the difference between strong and weak acids and bases
Complete an experiment to explore the difference in properties of strong/weak acids and bases

2. Refresh
100 cm3 of a NaOH solution of pH 12 is mixed with 900 cm3 of water. What is the pH of the resulting solution? 1 3 11 13

3. Strong acids example:
Good proton donor. Dissociation reactions go to completion, their conjugate bases are not readily able to accept a proton.
Weak acids example:
Poor proton donors. Equilibria of dissociation reaction lies to the left, their conjugate bases are readily able to accept a proton.
Conjugate base Cl- shows virtually no basic properties
Conjugate base CH3COO- is a stronger base than Cl-
Acid dissociation favours the production of weaker conjugate bases.

4. Bases follow a similar pattern…
Conjugate acid Na+ shows virtually no acidic properties
Strong base example:
Good proton acceptor. Dissociation reaction goes to completion, their conjugate acids do not readily donate protons.
Weak base example:
Poor proton acceptor. Equilibria of dissociation reaction lies to the left, their conjugate acids readily donate protons.
Conjugate acid NH4+ shows virtually no acidic properties
Base dissociation favours the production of weaker conjugate acids.

5. Strong Bases (Grp 1 hydroxides & barium hydroxide)
Strong Acids
(only six; know 1st three for IB)
Strong Bases (Grp 1 hydroxides & barium hydroxide)
Weak Acids carboxylic and carbonic acids
Weak Bases
ammonia and amines
H2SO4,
sulfuric acid*
LiOH,
lithium hydroxide
CH3COOH, ethanoic acid and other organic acids
C2H5NH2, ethylamine and other amines
HNO3,
nitric acid
NaOH,
sodium hydroxide
H2CO3,
carbonic acid
Note CO2(aq) = H2CO3(aq)
NH3,
ammonia
Note NH3(aq) = NH4OH(aq)
HCl,
hydrochloric acid
KOH,
potassium hydroxide
H3PO4,
phosphoric acid
HI,
hydroiodic acid
Ba(OH)2,
barium hydroxide
HBr,
hydrobromic acid
HClO4,
perchloric acid
Try to remember the main strong acids and bases… Then you can probably deduce any others you come across will be weak…

6. Strong, Concentrated and Corrosive
In everyday English, strong and concentrated are often used interchangeably. In chemistry, they have distinct meanings:
strong: completely dissociated into ions
concentrated: high number of moles of solute per liter (dm3) of solution
corrosive: chemically reactive
Similarly, weak and dilute also have very different chemical meanings:
weak: only slightly dissociated into ions
dilute: a low number of moles of solute per liter (dm3) of solution
It is possible for an acid or base to be strong but in a dilute solution, or weak and present in a concentrated solution.

7. Strong, Concentrated and Corrosive
Similarly, weak and dilute also have very different chemical meanings:
weak: only slightly dissociated into ions
dilute: a low number of moles of solute per liter (dm3) of solution

8. Strong and Weak Acids Strong Acids:
HA(aq)  H+(aq) + A-(aq) … The acid fully dissociates into ions
For example: HCl(aq)  H+(aq) + Cl-(aq)
Includes: hydrochloric, sulfuric, phosphoric, nitric
Strong acids have weak conjugate bases
Weak Acids:
HA(aq)  H+(aq) + A-(aq) … The acid only partially dissociates into ions
For example: HF(aq)  H+(aq) + F-(aq)
Includes: hydrofluoric, ethanoic, carbonic
Weak acids have strong conjugate bases

9. Strong and Weak Bases Strong Bases:
BOH(aq)  B+(aq) + OH-(aq) …..the base fully dissociates into ions
For example: NaOH(aq)  Na+(aq) + OH-(aq)
Includes: group (I) hydroxides, barium hydroxide
Strong bases have weak conjugate acids
Weak Bases:
BOH(aq)  B+(aq) + OH-(aq) …..the base only partially dissociates into ions
For example: NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
Includes: ammonia, amines
Weak bases have strong conjugate acids

10. Simulation

11. Distinguishing between strong and weak acids and bases
The equilibrium has a profound effect on the properties of the acid/base
Compared with strong acids of the same concentration, weak acids will:
Have lower electrical conductivity (less mobile ions in from the same moldm-3)
React more slowly (reactions depend on the concentration of H+ ions)
pH is higher (lower concentration of H+ ions)
Change pH more slowly when diluted
However, they neutralise the same volume of alkali
Weak bases follow a similar pattern

13. Solutions

14. Experiment
Complete the experiment here, investigating strong/weak acids and bases
When you give explanations, focus on reasons in terms of the equilibrium rather than just ‘because it is strong/weak’

15. Key Points Strong acids/bases dissociate fully into ions
Weak acids/bases only partially dissociate, forming an equilibrium
The strong/weak character has a significant effect on the chemical properties