Explain ammonia as a Brønsted–Lowry base
Model ammonia accepting a proton from water to form ammonium and hydroxide 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
In Brønsted–Lowry theory, what does a base accept?
What you will learn
- Define ammonia as a proton acceptor
- Write the reversible reaction with water
- Identify the conjugate acid and base
Ammonia accepts a proton
A Brønsted–Lowry base accepts a proton, H⁺. The lone pair on nitrogen in NH₃ can bond to an incoming proton.
In water, NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. Ammonia is the base and NH₄⁺ is its conjugate acid; water donates the proton and OH⁻ is its conjugate base.
The equilibrium is incomplete
Ammonia is a weak base in water, so the reaction is reversible and only a fraction of dissolved NH₃ accepts a proton at equilibrium.
A basic solution contains OH⁻, but NH₃ does not release OH⁻ by dissociation as an ionic hydroxide would. The hydroxide is formed when water donates a proton.
Pause and explain
Which species donates a proton to NH₃ in water?
Worked example
Use Brønsted–Lowry theory to explain why aqueous ammonia is basic.
Show the worked solution
- NH₃ has a lone pair and accepts H⁺ from water.
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
- OH⁻ forms, making the solution basic; the equilibrium lies mostly toward NH₃ and water.
Answer Ammonia acts as a Brønsted–Lowry base by accepting H⁺ from water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The hydroxide ions make the solution basic.
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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 equilibrium equation for ammonia reacting with water.
Give me a hint
Track one proton moving from water to ammonia.
Compare my reasoning
- NH₃ accepts H⁺ to form NH₄⁺.
- H₂O loses H⁺ to form OH⁻.
- Use a reversible arrow for the weak-base equilibrium.
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Look for these in your work
- Atoms conserved
- Charges conserved
- Reversible arrow used
Identify the Brønsted–Lowry acid and base in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Give me a hint
An acid donates H⁺; a base accepts H⁺.
Compare my reasoning
- NH₃ gains a proton.
- H₂O loses a proton.
- Therefore NH₃ is the base and H₂O is the acid.
NH₃ is the base and H₂O is the acid; NH₄⁺ and OH⁻ are their respective conjugate partners.
Look for these in your work
- Transfer direction correct
- Acid named
- Conjugate pairs correct
A learner writes NH₃ ⇌ NH₄⁺ + OH⁻. Diagnose and correct the equation.
Give me a hint
Find the missing proton donor and check atom balance.
Compare my reasoning
- The written equation omits the water molecule that donates H⁺.
- Include H₂O on the left to supply H and O.
- Write NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
The balanced aqueous equilibrium is NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Look for these in your work
- Missing reactant identified
- Proton source explained
- Balanced equation given
Common mistakes
- Calling NH₃ an OH⁻ ion
- Writing NH₃ → NH₄⁺ without a proton donor
- Calling ammonia a strong base
What can you explain now?
In Brønsted–Lowry theory, what does a base accept?
Compare with the explanation
A proton
A proton is H⁺.
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Make it stick
Why does ammonia produce OH⁻ in water without containing hydroxide in its formula?
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