3.5 ACIDS AND BASES: STRENGTH OF ACIDS AND BASES
3.5.4
Distinguish between strong and weak acids and bases in terms of the extent of dissociation and electrical conductivity.
3.5.5
State whether a given acid or base is strong or weak.
IDENTIFYING STONG ACIDS/BASES AND WEAK ACIDS/BASES
STRONG ACIDS
hydrochloric acid, HCl
nitric acid, HNO3
sulfuric acid, H2SO4
weak acids
carbonic acid, H2CO3
acetic acid (vinegar), CH3COOH
STRONG BASES
sodium hydroxide, NaOH
potassium hydroxide, KOH
weak bases
ammonia, NH3
Strong acids are strong acids even when diluted with lots of water.
Weak acids are always weak acids even when they are highly concentrated.
DEFINING “STONG” AND “WEAK” IN TERMS OF IONIZATION
STRONG
= ionizes completely in water
→ 100% dissociation of the molecular form
– the molecular form does not exist in the solution
HCl (aq) → H+(aq) + Cl–(aq)
[H+] = [HCl]
H2SO4 (aq) → H+(aq) + HSO4–(aq)
NaOH (aq) → Na+(aq) + OH–(aq)
[OH–] = [NaOH]
= only partially ionizes in water
 ions can reform the molecule
Some (most) of the substance is in its molecular form, some is in its ionic form
CH3COOH(aq)  CH3COO–(aq) + H+(aq)
[H+] << [CH3COOH]
NH3 (aq) + H2O(l)  NH4+(aq) + OH–(aq) [OH–] << [NH3]
WEAK
A strong acid and a weak acid could have the same pH if:
 the strong acid was diluted
and
 the weak acid was more concentrated
Solution type
strong concentrated base
strong, diluted base or weak concentrated base
very dilute acid, very dilute base
strong, diluted acid or weak concentrated acid
strong concentrated acid
Approximate pH
13
10
7
4
1
DISTINGUISHING BETWEEN STONG ACIDS/BASES AND WEAK ACIDS/BASES
pH
Equal concentrations of strong and weak acids produce different H+ ion concentrations.
1.0 mol dm-3 HCl → [H+] = 1.0 mol dm-3
pH 0
-3
+
-3
1.0 mol dm CH3COOH → [H ] = 0.00134 mol dm
pH 2.87
At the same concentration:
 strong acids have a lower pH than weak acids
 strong bases have a higher pH than weak bases.
Reaction Rate
depends on the concentration of the reactive chemicals (H+ or OH–)
At the same concentration, strong acids will react faster/more vigorously than weak acids.

Reactive Metals
Acids react with reactive metals to produce hydrogen gas, H2
Mg(s) + 2 HCl(aq) → MgCl2 (aq) + H2 (g)
Mg(s) + 2 CH3COOH(aq) → Mg(CH3COO)2 (aq) + H2 (g)
Both acids react, but the Mg disappears faster and the H2 bubbles faster with HCl

Carbonates (CO32–) Acids react with carbonates to produce carbon dioxide gas, CO2
o baking soda and vinegar makes bubbles
CaCO3 (s) + 2 HNO3 (aq) → Ca(NO3)2 (aq) + H2O (l) + CO2 (g)
NaHCO3(s) + CH3COOH(aq) → Na(CH3COO)2 (aq) + H2O (l) + CO2 (g)

Conductivity
Solutions with a higher concentration of ions conduct better.
Strong acids have higher conductivities than weak acids.
Strong bases have higher conductivities than weak bases.