Ammonia as a Base

Introduction

Most of the bases you have met so far are metal oxides or metal hydroxides, and every alkali seemed to owe its behaviour to hydroxide ions. Ammonia, NH₃, breaks that pattern. It contains no metal and no oxygen, yet it neutralises acids, turns universal indicator blue and is used to make millions of tonnes of salts every year. On this page you will see why ammonia is a base, how it forms ammonium salts, and why it can even do so as a gas with no water present.

Core explanation

A base is a hydrogen ion acceptor. Earlier you learned that acids are sources of H⁺ ions and that neutralisation is the reaction H⁺ + OH⁻ → H₂O. A broader and more useful idea is that a base is any substance that accepts H⁺ ions . The hydroxide ion is a base because it accepts H⁺ to make water. Oxide ions accept H⁺ too. Ammonia is a base for the same reason: it can accept an H⁺ ion.

How ammonia accepts H⁺. The nitrogen atom in NH₃ has five outer electrons. Three are shared in bonds with hydrogen atoms; the remaining two form a lone pair . This lone pair can form a new bond to a hydrogen ion, which has no electrons of its own. The result is the ammonium ion :

NH₃ + H⁺ → NH₄⁺

The ammonium ion is tetrahedral, with four equivalent N–H bonds, and carries a single positive charge.

Ammonia with acids. Because the key step is simply NH₃ accepting H⁺, ammonia reacts with any acid to form an ammonium salt, and no water is produced:

- NH₃ + HCl → NH₄Cl (ammonium chloride) - NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate) - 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ (ammonium sulfate)

Compare this with a metal hydroxide: NaOH + HCl → NaCl + H₂O. With ammonia, the "acid + base → salt + water" pattern becomes simply "acid + ammonia → ammonium salt".

Without water at all. Ammonia and hydrogen chloride are both gases. When they meet, they react directly to form a white smoke of tiny ammonium chloride crystals:

NH₃(g) + HCl(g) → NH₄Cl(s)

No solution and no water are involved. This proves that a base does not need hydroxide ions to neutralise an acid.

Ammonia in water. Ammonia is very soluble in water. A small fraction of the dissolved molecules take H⁺ from water molecules:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

The hydroxide ions produced make the solution alkaline, typically around pH 11 for a laboratory bench solution. The reaction is reversible and only a small proportion of ammonia molecules react at any moment, so aqueous ammonia is a weak alkali .

Step-by-step reasoning

To predict the product of ammonia with an acid:

1. Identify the negative ion of the acid (Cl⁻, NO₃⁻, SO₄²⁻). 2. Replace the acid's hydrogen with NH₄⁺, which carries a 1+ charge. 3. Balance charges: one NH₄⁺ for each 1− charge, two for SO₄²⁻. 4. Balance the equation; there is no water on the product side.

Visual explanation

Imagine a model of NH₃ as a short tripod: nitrogen on top, three hydrogens as legs, and a lobe of electron density (the lone pair) pointing upwards. An H⁺ ion docks onto that lobe, turning the tripod into a symmetrical four-armed tetrahedron, NH₄⁺. In the titration simulation, the same idea explains why ammonia solution raises pH as it is added.

Real-world analogy

A lone pair is like an empty hook on a coat stand. The hook does not need to hold a coat to exist, but as soon as a coat (H⁺) is offered, it can be hung there. Ammonia is a base because it carries this free hook.

Real-world example

Ammonium sulfate and ammonium nitrate are among the most widely used nitrogen fertilisers. Factories make them by reacting ammonia, produced by the Haber process, with sulfuric or nitric acid. Farmers spread them to supply soluble nitrogen that crops take up to build proteins.

Why?

Why does the ammonia–hydrogen chloride reaction give a white smoke? The product, ammonium chloride, is an ionic solid. As the gases meet, ions form and immediately cluster into countless microscopic crystals, which scatter light and appear as a white cloud.

Common misconception

"All bases must contain OH⁻ ions." Ammonia has no hydroxide ions in its formula. It only produces OH⁻ indirectly, by taking H⁺ from water. What makes something a base is its ability to accept H⁺, not the presence of hydroxide in its formula.

Worked example

Question: Write a balanced equation for ammonia reacting with phosphoric acid, H₃PO₄, to form ammonium phosphate.

Reasoning: The phosphate ion is PO₄³⁻. Three NH₄⁺ ions (each 1+) are needed to balance the 3− charge, so the salt is (NH₄)₃PO₄. Three NH₃ molecules supply the three NH₄⁺ ions, each taking one of the acid's three hydrogens.

Answer: 3NH₃ + H₃PO₄ → (NH₄)₃PO₄

Quick check

1. What feature of the ammonia molecule allows it to accept a hydrogen ion? Answer: The lone pair of electrons on the nitrogen atom, which forms a bond to the H⁺ ion.

Exam focus

Know the equation NH₃ + H⁺ → NH₄⁺ and be able to write equations for ammonia with hydrochloric, nitric and sulfuric acids. Remember that no water forms. Describe aqueous ammonia as a weak alkali and explain this using the reversible reaction with water.

Advanced insight

In the Brønsted–Lowry model, an acid is a proton donor and a base is a proton acceptor. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, water acts as an acid by donating a proton, while in its reactions with acids, water can act as a base. NH₄⁺ and NH₃ form a conjugate pair, and the ammonium ion is itself a weak acid, which is why solutions of ammonium salts are slightly acidic.

Summary

Ammonia is a base because the lone pair on its nitrogen atom accepts H⁺ to form the ammonium ion, NH₄⁺. It reacts with acids to form ammonium salts without producing water, even as a gas. In water it forms a few OH⁻ ions reversibly, making it a weak alkali. Ammonium salts are important fertilisers.

Practice questions

1. Write the equation for ammonia gas reacting with hydrogen chloride gas, including state symbols. Answer: NH₃(g) + HCl(g) → NH₄Cl(s) 2. Give the formula of the salt formed when ammonia reacts with sulfuric acid, and explain the formula. Answer: (NH₄)₂SO₄, because two NH₄⁺ ions, each 1+, are needed to balance the 2− charge on the sulfate ion. 3. Explain why aqueous ammonia is described as a weak alkali. Answer: Only a small fraction of the ammonia molecules react reversibly with water to form NH₄⁺ and OH⁻, so the concentration of hydroxide ions is low. 4. State one way in which the reaction of ammonia with an acid differs from the reaction of sodium hydroxide with an acid. Answer: Ammonia forms an ammonium salt only, whereas sodium hydroxide forms a salt and water.