Strong and Weak Alkalis

Introduction

Sodium hydroxide solution and household ammonia are both alkalis: both turn red litmus blue and both neutralise acids. Yet at the same concentration, sodium hydroxide has a noticeably higher pH and conducts electricity much better. Just as acids can be strong or weak, so can alkalis. This page explains the difference using the two most important examples, sodium hydroxide and aqueous ammonia.

Core explanation

Strong alkalis. A strong alkali is completely ionised in aqueous solution, so all of its hydroxide is present as free OH⁻ ions. Sodium hydroxide is an ionic solid; when it dissolves, the lattice breaks up fully:

NaOH(s) → Na⁺(aq) + OH⁻(aq)

Other strong alkalis include potassium hydroxide, KOH, and the soluble hydroxides of Group 1 metals. Calcium hydroxide and barium hydroxide are also fully ionised in solution, although calcium hydroxide is only slightly soluble, so limewater never contains much OH⁻.

Weak alkalis. Aqueous ammonia is the standard weak alkali. Ammonia molecules do not contain hydroxide. Instead, a few of them accept a hydrogen ion from water:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

This is a reversible reaction that reaches equilibrium with most of the ammonia still present as NH₃ molecules. In a 0.1 mol/dm³ solution only about 1 in 75 molecules has reacted at any moment. Old bottles labelled "ammonium hydroxide" are misleading: there is very little NH₄⁺ and OH⁻ in them and no NH₄OH molecules to speak of.

Comparing 0.1 mol/dm³ solutions:

Property Sodium hydroxide Aqueous ammonia --- --- --- Extent of ionisation complete about 1% [OH⁻] 0.1 mol/dm³ about 0.0013 mol/dm³ pH 13 about 11 Conductivity high low

Same neutralising power. Although ammonia produces fewer OH⁻ ions at any moment, equal amounts of the two alkalis neutralise the same amount of acid. As acid removes OH⁻ ions, more ammonia reacts with water to replace them, until all of it has been used.

Why use a weak alkali? Because its OH⁻ concentration is low, aqueous ammonia is less aggressive towards skin and surfaces than sodium hydroxide of the same concentration. Ammonia also has a distinctive smell because some NH₃ molecules escape from solution, which sodium hydroxide solution does not do. Both still require care: ammonia vapour irritates the eyes and lungs, and concentrated sodium hydroxide is highly corrosive.

Step-by-step reasoning

To classify an alkali as strong or weak:

1. Write the equation for how it forms OH⁻ ions. 2. If it is a soluble metal hydroxide that fully separates into ions, it is strong (use →). 3. If it forms OH⁻ by partial reaction with water, it is weak (use ⇌). 4. Check with data: at equal concentration, the strong alkali has the higher pH and conductivity.

Visual explanation

Draw ten formula units of NaOH in one beaker, all shown as separate Na⁺ and OH⁻ ions. In a second beaker, draw ten NH₃ molecules, with just one converted into an NH₄⁺ ion and an OH⁻ ion, and double arrows showing that pair turning back into NH₃ and H₂O.

Real-world analogy

Imagine two teams asked to hand out leaflets. In the sodium hydroxide team, every member hands out their leaflet at once. In the ammonia team, only one or two members are handing out leaflets at any moment, though the others will step in as leaflets are taken. Both teams can deliver the same total, but not at the same instant.

Real-world example

Many glass and window cleaners contain dilute aqueous ammonia. It is alkaline enough to lift greasy films, and because it is weak and volatile it evaporates without leaving streaks. Stronger drain cleaners use sodium hydroxide instead, because they need a high OH⁻ concentration to break down fats and hair.

Why?

Why is ammonia only partly converted to ions? The NH₄⁺ ion readily gives its hydrogen ion back to OH⁻, reforming NH₃ and H₂O. The reverse reaction is strongly favoured, so equilibrium lies well to the left, with most ammonia remaining as molecules.

Common misconception

"Ammonia solution contains lots of NH₄OH." Ammonia dissolves mainly as NH₃ molecules surrounded by water. Only a small fraction forms NH₄⁺ and OH⁻ ions, and NH₄OH is not a stable molecule that can be isolated.

Worked example

Question: Solutions of alkali A and alkali B are both 0.01 mol/dm³. A has pH 12.0 and B has pH 10.6. Identify the weak alkali and explain.

Reasoning: A fully ionised alkali at 0.01 mol/dm³ gives [OH⁻] = 10⁻² mol/dm³, so [H⁺] = 10⁻¹² and pH = 12. B has a lower pH, so fewer OH⁻ ions.

Answer: B is the weak alkali; it is only partially ionised, so its OH⁻ concentration and pH are lower than those of A.

Quick check

1. Write the equation showing how ammonia produces hydroxide ions in water. Answer: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Exam focus

Name sodium hydroxide (or potassium hydroxide) as a strong alkali and aqueous ammonia as a weak alkali. Use ⇌ for ammonia. Explain differences in pH and conductivity using the number of OH⁻ ions, and remember that equal amounts neutralise the same amount of acid.

Advanced insight

In Brønsted–Lowry terms, ammonia is a weak base because it accepts protons only partially. Its conjugate acid, NH₄⁺, is a weak acid. This is why ammonium salts such as ammonium chloride give slightly acidic solutions: some NH₄⁺ ions donate protons to water, forming H₃O⁺.

Summary

Strong alkalis, such as NaOH and KOH, are completely ionised in water and release all their hydroxide as OH⁻ ions. Aqueous ammonia is a weak alkali: only a small fraction of NH₃ molecules react with water to form NH₄⁺ and OH⁻, in a reversible reaction. At equal concentration a strong alkali has a higher pH and conductivity, but equal amounts of either neutralise the same amount of acid.

Practice questions

1. Define a strong alkali and give an example. Answer: An alkali that is completely ionised in water; for example, sodium hydroxide. 2. Why does 0.1 mol/dm³ ammonia solution have a lower pH than 0.1 mol/dm³ sodium hydroxide? Answer: Ammonia is only partially converted to NH₄⁺ and OH⁻, so its OH⁻ concentration is much lower. 3. Explain why aqueous ammonia smells, but sodium hydroxide solution does not. Answer: Most ammonia stays as NH₃ molecules, which are volatile and escape from solution; NaOH exists as non-volatile ions. 4. Equal volumes of 0.1 mol/dm³ NaOH and 0.1 mol/dm³ NH₃ are each titrated with the same hydrochloric acid. Compare the volumes of acid needed. Answer: The volumes are equal, because both contain the same amount of base and ammonia reacts completely as its OH⁻ is removed.