Predict ammonia displacement from ammonium salts
Use acid–base proton transfer to explain how an alkali releases ammonia from ammonium ions.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Explain the stated molecular, acid–base and atmospheric reaction pathways; balance equations and distinguish direct reactions from catalytic cycles. Written practice is self-checked
Before you begin
- Use covalent bond order, lone pairs, Brønsted–Lowry proton transfer, oxidation numbers and balanced equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which ion accepts a proton from NH₄⁺?
What you will learn
- Write the net ionic equation
- Use ammonium and hydroxide as an acid–base pair
- Balance a representative ammonium-salt equation
Hydroxide removes a proton from ammonium
The ammonium ion can donate H⁺, while hydroxide accepts it: NH₄⁺ + OH⁻ → NH₃ + H₂O. This is a Brønsted–Lowry acid–base reaction.
A strong alkali supplies OH⁻, shifting the proton-transfer reaction toward ammonia and water. The ammonia is displaced from the ammonium salt; spectator ions remain in solution.
Write complete equations with spectator ions
For example, NH₄Cl + NaOH → NH₃ + H₂O + NaCl. The net ionic equation is NH₄⁺ + OH⁻ → NH₃ + H₂O.
Check that nitrogen, hydrogen, oxygen, charge and spectator ions are conserved. This is a chemical equation for understanding, not a procedure for preparing or collecting ammonia.
Pause and explain
What is the net ionic equation?
Worked example
Write the equation for the reaction of ammonium chloride with sodium hydroxide and identify the acid–base roles.
Show the worked solution
- NH₄⁺ donates H⁺ and is the Brønsted–Lowry acid.
- OH⁻ accepts H⁺ and is the base, forming H₂O.
- The remaining Na⁺ and Cl⁻ form NaCl.
Answer NH₄Cl + NaOH → NH₃ + H₂O + NaCl. NH₄⁺ donates a proton to OH⁻; Na⁺ and Cl⁻ are spectator ions.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the net ionic equation for ammonium reacting with hydroxide.
Give me a hint
Transfer one proton from NH₄⁺ to OH⁻.
Compare my reasoning
- NH₄⁺ becomes NH₃ by losing H⁺.
- OH⁻ becomes H₂O by accepting H⁺.
- Check the total charge is −1 on both sides.
NH₄⁺ + OH⁻ → NH₃ + H₂O.
Look for these in your work
- Species correct
- Proton transfer shown
- Charge balanced
Complete: NH₄NO₃ + NaOH → ____ + ____ + ____.
Give me a hint
Use the net ionic equation, then retain spectator ions.
Compare my reasoning
- The ammonium ion forms NH₃.
- Hydroxide forms H₂O.
- Na⁺ and NO₃⁻ remain as NaNO₃.
NH₄NO₃ + NaOH → NH₃ + H₂O + NaNO₃.
Look for these in your work
- Products identified
- Equation balanced
- Spectator salt retained
A learner writes NH₄Cl + NaOH → NH₄OH + NaCl. Correct the chemistry.
Give me a hint
Treat ammonium as a proton donor and hydroxide as a proton acceptor.
Compare my reasoning
- The net reaction transfers H⁺ from NH₄⁺ to OH⁻.
- This forms NH₃ and H₂O, not a stable NH₄OH product in the equation.
- Add NaCl as the spectator salt.
NH₄Cl + NaOH → NH₃ + H₂O + NaCl.
Look for these in your work
- Acid–base roles correct
- Ammonia and water formed
- Balanced salt equation
Common mistakes
- Producing N₂ instead of NH₃
- Omitting water from the net ionic equation
- Treating Na⁺ as the proton acceptor
What can you explain now?
Which ion accepts a proton from NH₄⁺?
Compare with the explanation
OH⁻
Hydroxide accepts H⁺ to form water.
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Make it stick
Why does a strong alkali release NH₃ from an ammonium salt?
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