Acid–Base Reactions Practice Questions

Acid–base reactions turn up in almost every chemistry exam, from the first year of secondary school to university entrance tests. They look varied, but they reduce to a handful of patterns. This set of 25 questions works through all of them, starting with the basics and building up to calculations.

Try each question on paper before checking the answers at the bottom. If you get stuck, these guides will help:

• Acids vs bases for the core reactions
• Naming salts for product names
• Brønsted–Lowry acids and bases for proton-transfer questions

Section A: predict the products (word equations)

Complete each word equation.

1. hydrochloric acid + sodium hydroxide → ___ + ___
2. sulfuric acid + copper(II) oxide → ___ + ___
3. nitric acid + calcium carbonate → ___ + ___ + ___
4. hydrochloric acid + magnesium → ___ + ___
5. ethanoic acid + potassium hydroxide → ___ + ___
6. sulfuric acid + ammonia → ___

Section B: balanced symbol equations

Write a balanced equation, with state symbols, for each reaction.

7. Hydrochloric acid with potassium hydroxide.
8. Sulfuric acid with sodium hydroxide.
9. Nitric acid with magnesium oxide.
10. Hydrochloric acid with sodium carbonate.
11. Phosphoric acid with sodium hydroxide, fully neutralised.
12. Zinc with sulfuric acid.

Section C: ionic equations

13. Write the ionic equation for any neutralisation of a strong acid by a strong alkali.
14. Write the ionic equation for the reaction between an acid and a carbonate.
15. Why is the enthalpy change of neutralisation almost the same (about −57 kJ/mol) for HCl + NaOH and for HNO₃ + KOH?

Section D: identify acids, bases and conjugates

For each reaction, identify the Brønsted–Lowry acid and base on the left-hand side.

16. HNO₃ + H₂O → H₃O⁺ + NO₃⁻
17. NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
18. HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O
19. HCl(g) + NH₃(g) → NH₄Cl(s)

Section E: calculations

20. What mass of sodium chloride is formed when 4.00 g of sodium hydroxide reacts completely with hydrochloric acid? (Na 23.0, O 16.0, H 1.0, Cl 35.5)

21. What volume of 0.200 mol/dm³ sulfuric acid is needed to neutralise 25.0 cm³ of 0.300 mol/dm³ sodium hydroxide?

22. 5.00 g of calcium carbonate reacts with excess hydrochloric acid. What volume of carbon dioxide forms at room temperature and pressure (molar volume 24.0 dm³/mol)? (Ca 40.1, C 12.0, O 16.0)

23. 0.486 g of magnesium ribbon reacts with excess dilute hydrochloric acid. How many moles of hydrogen are produced, and what volume at room temperature and pressure? (Mg 24.3)

24. 20.0 cm³ of 0.500 mol/dm³ HCl is added to 1.00 g of calcium carbonate. Which reactant is in excess, and by how many moles?

25. A student adds 25.0 cm³ of 1.00 mol/dm³ HCl to 25.0 cm³ of 1.00 mol/dm³ NaOH in a polystyrene cup. The temperature rises by 6.8 °C. Calculate the enthalpy change of neutralisation. (Assume the solution has density 1.00 g/cm³ and specific heat capacity 4.18 J/g/°C.)

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Answers

1. sodium chloride + water

2. copper(II) sulfate + water

3. calcium nitrate + water + carbon dioxide

4. magnesium chloride + hydrogen

5. potassium ethanoate + water

6. ammonium sulfate (no water is formed, because ammonia is a base without hydroxide)

7. HCl(aq) + KOH(aq) → KCl(aq) + H₂O(l)

8. H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

9. 2HNO₃(aq) + MgO(s) → Mg(NO₃)₂(aq) + H₂O(l)

10. 2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

11. H₃PO₄(aq) + 3NaOH(aq) → Na₃PO₄(aq) + 3H₂O(l)

12. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

13. H⁺(aq) + OH⁻(aq) → H₂O(l)

14. 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g). (If the carbonate is an insoluble solid, such as CaCO₃, it’s written as a solid: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g).)

15. Both are strong acids and strong alkalis, so they’re completely ionised. The only reaction that actually happens is H⁺ + OH⁻ → H₂O; the other ions are spectators. So the energy change is the same.

16. Acid: HNO₃. Base: H₂O.

17. Acid: H₂O. Base: NH₃.

18. Acid: HCO₃⁻. Base: OH⁻.

19. Acid: HCl. Base: NH₃. (No water is needed for a Brønsted–Lowry acid–base reaction.)

20.

• NaOH + HCl → NaCl + H₂O (ratio 1 : 1)
• Moles NaOH = 4.00 ÷ 40.0 = 0.100 mol
• Moles NaCl = 0.100 mol
• Mass NaCl = 0.100 × 58.5 = 5.85 g

21.

• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• Moles NaOH = 0.300 × 0.0250 = 7.50 × 10⁻³ mol
• Moles H₂SO₄ = 7.50 × 10⁻³ ÷ 2 = 3.75 × 10⁻³ mol
• Volume = 3.75 × 10⁻³ ÷ 0.200 = 0.01875 dm³ = 18.75 cm³ (18.8 cm³ to 3 s.f.)

22.

• CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
• Mr CaCO₃ = 40.1 + 12.0 + 48.0 = 100.1
• Moles CaCO₃ = 5.00 ÷ 100.1 = 0.0500 mol
• Moles CO₂ = 0.0500 mol
• Volume = 0.0500 × 24.0 = 1.20 dm³ (1200 cm³)

23.

• Mg + 2HCl → MgCl₂ + H₂
• Moles Mg = 0.486 ÷ 24.3 = 0.0200 mol
• Moles H₂ = 0.0200 mol
• Volume = 0.0200 × 24.0 = 0.480 dm³ (480 cm³)

24.

• Moles HCl = 0.500 × 0.0200 = 0.0100 mol
• Moles CaCO₃ = 1.00 ÷ 100.1 = 9.99 × 10⁻³ mol
• CaCO₃ needs 2 HCl each, so 9.99 × 10⁻³ mol CaCO₃ would need 0.0200 mol HCl. Only 0.0100 mol is available.
• HCl is limiting; calcium carbonate is in excess.
• CaCO₃ that reacts = 0.0100 ÷ 2 = 5.00 × 10⁻³ mol
• Excess CaCO₃ = 9.99 × 10⁻³ − 5.00 × 10⁻³ = 4.99 × 10⁻³ mol (about 0.50 g left over)

For more on this type of question, see limiting reagent.

25.

• Total volume = 50.0 cm³, so mass of solution ≈ 50.0 g
• Heat released q = m × c × ΔT = 50.0 × 4.18 × 6.8 = 1421 J = 1.42 kJ
• Moles of water formed = moles HCl = 1.00 × 0.0250 = 0.0250 mol
• ΔH = −1.42 ÷ 0.0250 = −56.8 kJ/mol (negative because the reaction is exothermic)

This is very close to the accepted value of about −57 kJ/mol. The small difference comes mainly from heat lost to the cup and the surroundings, which is why an insulated cup with a lid gives better results.

How to approach questions like these

For every acid–base question, the same four-step habit saves marks. First, identify the reaction type: acid with base, carbonate, metal or ammonia. Second, write the word equation and name the salt from the acid. Third, balance the symbol equation, paying special attention to diprotic acids like sulfuric acid and to Group 2 hydroxides. Finally, for calculations, convert everything to moles before using the ratio, and only convert back to grams or volumes at the end.

Scoring guide

• 20–25: excellent. You’re ready for exam questions on this topic.
• 14–19: good. Revisit the sections where you dropped marks.
• Below 14: work through the linked guides and try again in a few days.

Mistakes that come up again and again

1. Unbalanced sulfuric acid equations. H₂SO₄ needs two NaOH.
2. Forgetting CO₂ in carbonate reactions.
3. Writing water as a product of acid + ammonia. The product is just the ammonium salt.
4. Leaving out state symbols when the question asks for them.
5. Forgetting the negative sign on an exothermic enthalpy change.

Key takeaways

• Acid + base → salt + water; acid + carbonate → salt + water + CO₂; acid + metal → salt + hydrogen.
• The net ionic equation for strong acid + strong base neutralisation is H⁺ + OH⁻ → H₂O.
• In Brønsted–Lowry terms, identify which species loses a proton (acid) and which gains one (base).
• In calculations, always use the mole ratio from the balanced equation.
• Check pH-based answers with the pH calculator.