Quick Notes

Quick Notes

Chemistry Chapter 6: Acid, Base and Salt
6.3 Strength of Acids and Alkalis
6.3 Strength of Acids and Alkalis
CHEMISTRY • Form 4 • Chapter 6: Acid, Base and Salt

6.3Strength of Acids and Alkalis

The strength of an acid or alkali is not determined by its concentration, but by its degree of dissociation or ionisation when dissolved in water. Acids produce hydrogen ions, \(\text{H}^+\), while alkalis produce hydroxide ions, \(\text{OH}^-\). Depending on whether these substances ionise completely or only partially in aqueous solution, they are classified as strong or weak acids and alkalis.

Learning Objectives

  • Define strong acid, weak acid, strong alkali, and weak alkali.
  • Explain the strength of an acid and alkali based on its degree of dissociation or ionisation in water.

Comparison of Complete vs. Partial Ionisation of Acids and Alkalis

Strong electrolytes fully break down into free ions, whereas weak electrolytes mostly remain as neutral molecules with very few ions.

Strong Acid · \(\text{HCl}\)
\(\text{H}^+\) \(\text{Cl}^-\) \(\text{H}^+\) \(\text{Cl}^-\) \(\text{H}^+\) \(\text{Cl}^-\)

Complete ionisation: only free ions remain.

Weak Acid · \(\text{CH}_3\text{COOH}\)
\(\text{CH}_3\text{COOH}\) \(\text{CH}_3\text{COOH}\) \(\text{CH}_3\text{COOH}\) \(\text{H}^+\) \(\text{CH}_3\text{COO}^-\) \(\text{CH}_3\text{COOH}\)

Partial ionisation: mostly intact molecules, few ions.

Strong Alkali · \(\text{NaOH}\)
\(\text{Na}^+\) \(\text{OH}^-\) \(\text{Na}^+\) \(\text{OH}^-\) \(\text{Na}^+\) \(\text{OH}^-\)

Complete dissociation: only free ions remain.

Weak Alkali · \(\text{NH}_3\)
\(\text{NH}_3\) \(\text{NH}_3\) \(\text{NH}_3\) \(\text{NH}_4^+\) \(\text{OH}^-\) \(\text{NH}_3\)

Partial ionisation: mostly intact molecules, few ions.

Brief Explanation

Strength depends on the degree of dissociation or ionisation in water.

Degree of dissociation determines whether an acid or alkali is strong or weak. It is not the same as concentration.

1. Strong Acids

A strong acid ionises completely in water to produce a high concentration of hydrogen ions, \(\text{H}^+\).

Example: Hydrochloric acid, \(\text{HCl}\).

All \(\text{HCl}\) molecules ionise completely into \(\text{H}^+\) and \(\text{Cl}^-\). No intact \(\text{HCl}\) molecules remain.

\[\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}\]

2. Weak Acids

A weak acid ionises partially in water to produce a low concentration of hydrogen ions, \(\text{H}^+\).

Example: Ethanoic acid, \(\text{CH}_3\text{COOH}\).

Only about \(1.54\%\) of the molecules ionise. Most remain as intact \(\text{CH}_3\text{COOH}\) molecules.

\[\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)}\]

3. Strong Alkalis

A strong alkali ionises completely in water to produce a high concentration of hydroxide ions, \(\text{OH}^-\).

Example: Sodium hydroxide, \(\text{NaOH}\).

Only \(\text{Na}^+\) and \(\text{OH}^-\) ions exist in the solution; no undissociated \(\text{NaOH}\) units remain.

\[\text{NaOH(aq)} \rightarrow \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}\]

4. Weak Alkalis

A weak alkali ionises partially in water to produce a low concentration of hydroxide ions, \(\text{OH}^-\).

Example: Aqueous ammonia solution, \(\text{NH}_3\).

Only about \(1.3\%\) of ammonia molecules ionise. Most remain as unreacted \(\text{NH}_3\) molecules.

\[\text{NH}_3\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)}\]

Hydrogen ions, \(\text{H}^+\), produced by acid molecules combine with water molecules, \(\text{H}_2\text{O}\), through a dative bond to form hydroxonium ions, \(\text{H}_3\text{O}^+\).

Try to Answer First

Answer mentally, then check the explanation.

1

Give the definition of a strong alkali and write its dissociation equation using sodium hydroxide, \(\text{NaOH}\), as an example.

Answer: A strong alkali is an alkali that ionises completely in water to produce a high concentration of hydroxide ions, \(\text{OH}^-\). The equation is \(\text{NaOH(aq)} \rightarrow \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}\).
2

Why does an ammonia solution, \(\text{NH}_3\), have a lower pH value than a potassium hydroxide solution, \(\text{KOH}\), of the same concentration?

Answer: Potassium hydroxide is a strong alkali that ionises completely in water to produce a high concentration of hydroxide ions, \(\text{OH}^-\). Ammonia is a weak alkali that ionises partially in water to produce a low concentration of hydroxide ions, \(\text{OH}^-\). A lower concentration of \(\text{OH}^-\) results in a lower pH value.
3

What does the reversible arrow (\(\rightleftharpoons\)) in the ionisation equation of ethanoic acid indicate?

Answer: It indicates that the ionisation is partial and reversible; ethanoic acid molecules dissociate to form \(\text{H}^+\) and \(\text{CH}_3\text{COO}^-\) ions, and these ions can simultaneously recombine to reform ethanoic acid molecules.

Common Mistakes

Avoid these errors when writing equations and explaining strength.

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Writing a single forward arrow (\(\rightarrow\)) instead of a reversible arrow (\(\rightleftharpoons\)) for the dissociation of weak acids (\(\text{CH}_3\text{COOH}\)) and weak alkalis (\(\text{NH}_3\)).
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Stating that strong acids or alkalis have a "high concentration of acid/alkali", rather than stating they produce a "high concentration of hydrogen/hydroxide ions".
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Forgetting that intact molecules still exist in solutions of weak acids and weak alkalis.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

"Strong acid" means the same thing as "concentrated acid".

Correct Fact

"Strong" refers to complete ionisation in water regardless of amount, whereas "concentrated" refers to a large amount of solute dissolved per unit volume of solution.

Misconception

Diprotic acids are always stronger than monoprotic acids.

Correct Fact

Basicity (monoprotic, diprotic) refers to the number of ionisable hydrogen atoms per molecule, while strength refers strictly to the degree of dissociation in water. For example, monoprotic \(\text{HCl}\) is a strong acid, whereas diprotic oxalic acid (\(\text{H}_2\text{C}_2\text{O}_4\)) is a weak acid.

Summary

Key facts to remember.

  • Strength is determined by the degree of dissociation (ionisation) in water.
  • Strong Acid: Ionises completely in water \(\rightarrow\) high \([\text{H}^+]\) (e.g., \(\text{HCl}\)).
  • Weak Acid: Ionises partially in water \(\rightarrow\) low \([\text{H}^+]\) (e.g., \(\text{CH}_3\text{COOH}\), degree of dissociation \(\approx 1.54\%\)).
  • Strong Alkali: Ionises completely in water \(\rightarrow\) high \([\text{OH}^-]\) (e.g., \(\text{NaOH}\)).
  • Weak Alkali: Ionises partially in water \(\rightarrow\) low \([\text{OH}^-]\) (e.g., \(\text{NH}_3\), degree of dissociation \(\approx 1.3\%\)).
  • Hydroxonium ions (\(\text{H}_3\text{O}^+\)) are formed by a dative bond between \(\text{H}^+\) and a water molecule.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following statements correctly defines a weak acid?

2

Why does hydrochloric acid, \(\text{HCl}\), have a higher concentration of hydrogen ions than ethanoic acid, \(\text{CH}_3\text{COOH}\), of the same molarity?

B. Fill in the Blanks

3

The strength of an acid or alkali depends on its degree of ________ in water.

4

A hydrogen ion, \(\text{H}^+\), combines with a water molecule through a dative bond to form a ________ ion.

C. Match the Substance with Its Classification

Drag each substance on desktop. On mobile, tap a substance and then tap the matching box.

Substances

Hydrochloric acid, \(\text{HCl}\) Ethanoic acid, \(\text{CH}_3\text{COOH}\) Sodium hydroxide, \(\text{NaOH}\) Ammonia solution, \(\text{NH}_3\)

Classifications

1Ionises completely in water to produce a high concentration of \(\text{H}^+\)
2Ionises partially in water to produce a low concentration of \(\text{H}^+\)
3Dissociates completely in water to produce a high concentration of \(\text{OH}^-\)
4Ionises partially in water to produce a low concentration of \(\text{OH}^-\)
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Strong acid Weak acid Strong alkali Weak alkali Degree of dissociation Hydroxonium ion