Chemistry · Acids bases and salts

Writing neutralisation equations

You know acid plus alkali gives salt and water, but the formulas will not balance.

A neutralisation equation starts from the reactants and builds the salt from its ions. Get the salt formula right, and balancing is short.

This lesson is part of SPM Chemistry acids, bases and salts. The next step is writing ionic equations with correct charges.

How do I build the salt formula?

Take the metal ion from the base and the negative ion from the acid. Swap the charge numbers to find the formula.

Nitric acid gives NO₃⁻ and calcium hydroxide gives Ca²⁺. Swapping gives Ca(NO₃)₂, with brackets because the radical appears twice.

Worked example 1: acid with an alkali

Sulfuric acid reacts with potassium hydroxide.

Potassium is K⁺ and sulfate is SO₄²⁻, so the salt is K₂SO₄.
Write the skeleton: H₂SO₄ + KOH → K₂SO₄ + H₂O.
Balance potassium: H₂SO₄ + 2KOH → K₂SO₄ + H₂O.
Balance hydrogen and oxygen: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.

Check: hydrogen is 2 + 2 = 4 on the left and 4 on the right. Oxygen is 4 + 2 = 6 on the left and 4 + 2 = 6 on the right.

Worked example 2: acid with a carbonate

Hydrochloric acid reacts with calcium carbonate. The salt is CaCl₂, and carbonate gives carbon dioxide and water.

Equation: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. Check: hydrogen 2, chlorine 2, calcium 1, carbon 1, oxygen 3 on each side.

For a metal oxide, such as magnesium oxide, the products are only salt and water: 2HCl + MgO → MgCl₂ + H₂O.

The mistake that loses marks

A student writes H₂SO₄ + KOH → KSO₄ + H₂O and then balances it by changing a subscript. The salt formula KSO₄ is wrong, because K⁺ and SO₄²⁻ need two potassium ions.

Never change subscripts to balance. Fix the salt formula from the charges, then use coefficients only.

Check yourself

Write a balanced equation for nitric acid reacting with calcium hydroxide.

Answer

Calcium is Ca²⁺ and nitrate is NO₃⁻, so the salt is Ca(NO₃)₂.

Skeleton: HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O. Two nitrates on the right need 2HNO₃. Then hydrogen is 2 + 2 = 4, so the water coefficient is 2.

2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O. Oxygen: 6 + 2 = 8 on the left and 6 + 2 = 8 on the right.

What to study next

Go to writing ionic equations with correct charges, or turn equations into quantities with calculating acid-base quantities.

The mole and stoichiometry steps tool checks the amounts in your equation. To have a teacher read your equations with you, see online one-to-one Chemistry tuition.

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Common questions

Why does the salt formula come first?

The coefficients in the equation depend on the salt formula. If the formula is wrong, balancing cannot fix it. Build the salt from the charges of its ions first, then balance the atoms.

Does every neutralisation make water?

Acids with alkalis, metal oxides, metal hydroxides and carbonates make water. A carbonate also makes carbon dioxide. Ammonia is the exception: an acid and NH₃ give only an ammonium salt.

Do I write state symbols?

Yes, whenever the question asks for them. Write (aq) for solutions, (s) for solids, (l) for water and (g) for gases.

How do I balance hydrogen and oxygen?

Balance the metal and the acid radical first, then hydrogen, then oxygen. Water is usually the last to be balanced, and its coefficient often sorts the hydrogen and oxygen counts at the same time.

Keep going

A teacher reading your working one-to-one can see whether the error is in the salt formula or in the balancing, and target that step with fresh equations.

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