6.3 Strength of Acids and Alkalis

©scfong

At the end of the lesson, you should be able to:
6.3.1 Define strong acid, weak acid, strong alkali and weak

alkali
6.3.2 Explain the strength of acid and alkali based on its

degree of dissociation in water

©scfong

The strength of an acid depends on the degree of dissociation
or ionisation of the acid in water.

Strong acids

A strong acid is an acid which dissociate completely in
water to produce a high concentration of hydrogen ions.

HCl H+ + Cl–

©scfong

Strong acids

H+ Cl- H+ All the molecules ionises to
produce Cl– ions and H+ ions
Cl-
(No molecule of hydrogen
H+ Cl- chloride, HCl exists in this solution)

Cl-

H+

Completely ionised
High concentration of hydrogen ions, H+
Lower pH value

©scfong

Weak acids

A weak acid is an acid which ionises/dissociate partially in
water to produce a lower concentration of hydrogen ions.
Ethanoic acid is a weak acid.

CH3COOH ⇌ H+ + CH3COO–

©scfong

Weak acids • The degree of dissociation of ethanoic

H+ CH3COO- acid molecules is 1.54%.
• In other words, from 100 molecules of
CH3COO- H+
ethanoic acid, CH3COOH, only one
CH3COO- CH3COO- H+ molecule of ethanoic acid, CH3COOH
ionises to hydrogen ions, H+ and
H+
ethanoate ions, CH3COO–.
• The rest still exist as molecules of ethanoic

acid, CH3COOH.

Partially ionised
Low concentration of hydrogen ions, H+
Higher pH value

©scfong

HCl H+ + Cl– CH3COOH ⇌ H+ + CH3COO–

©scfong

pH is a measure of the number of H+ ions in solution, with a lower pH
meaning more H+ ions.

Because strong acids dissociate fully in solution, they contain more H+
ions per molecule of acid, producing a lower pH.

Compared to weak acids of the same concentration, strong acids:
 have lower pH values
 are better conductors of electricity
 react more quickly.

high H+ low H+

©scfong

Strong alkalis

Alkalis can be classified as strong and weak in the same
way as acids. A strong alkali, such as sodium hydroxide,
fully dissociates in solution.

A strong alkali is an alkali which ionises completely in
water to produce a high concentration of hydroxide ions.

NaOH Na+ + OH–

©scfong

Strong alkalis

Na+ OH- Na+
OH-

Na+

OH- OH-
Na+

Completely ionised
High concentration of hydroxide ions, OH-
Higher pH value

©scfong

Weak alkalis

A weak alkali is an alkali which ionises partially in water to
produce a low concentration of hydroxide ions.
Ammonia is a weak alkali.

NH3 + H2O ⇌ NH4+ + OH–
Partially ionised
Low concentration of hydroxide ions, OH-
Lower pH value

©scfong

Strong alkalis Weak alkalis

NH3

Na+ OH- NH3 NH3

OH- Na+ H2O H2O
Na+ OH-
NH4+ OH-
OH- Na+ H2O NH3

NH3 + H2O ⇌ NH4+ + OH–

©scfong