Acids & neutralisation
Acids & neutralisation
Acids, alkalis and strength
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An acid releases H⁺(aq) in aqueous solution. An alkali is a water-soluble base that produces OH⁻(aq). Familiar examples are listed in the table.
Strength and concentration are different ideas. A strong acid ionises essentially completely in water; a weak acid ionises only partly. Concentration tells you how much acid is present per unit volume.
| Common acid | Formula | Common alkali | Formula |
|---|---|---|---|
| Hydrochloric acid | HCl | Sodium hydroxide | NaOH |
| Sulfuric acid | H2SO4 | Potassium hydroxide | KOH |
| Nitric acid | HNO3 | Ammonia | NH3 |
| Ethanoic acid | CH3COOH | — | — |
Diluting a strong acid makes it less concentrated, not weak. Use the ionisation equations below to see the difference between essentially complete and partial ionisation.
| Term | Meaning |
|---|---|
| Strong acid | Essentially completely dissociated in aqueous solution. |
| Weak acid | Partially dissociated in aqueous solution. |
| Concentration | Amount of solute per unit solution volume. |
Equal concentrations, different strengths
Equal volumes of hydrochloric acid and ethanoic acid have the same concentration in mol dm⁻³. Compare their initial hydrogen-ion concentrations and the amounts in moles of hydroxide ions needed for complete neutralisation. Explain both comparisons.
Worked answer
HCl is essentially completely dissociated; ethanoic acid is only partially dissociated, so HCl has a higher H+ concentration initially. Both acids are monoprotic, and the samples contain the same total amount of acid.
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Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
(a) Write the formulae of hydrochloric acid, nitric acid, ethanoic acid and ammonia. (4 marks)
(b) State the ion characteristic of acids and the ion produced by alkalis in aqueous solution. (2 marks)
(c) Explain, using an equation, how aqueous ammonia produces hydroxide ions. (2 marks)
Total: 8 marks
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(a) HCl [1], HNO3 [1], CH3COOH [1], NH3. [1]
(b) Acids: H+(aq) [1]; alkalis: OH−(aq). [1]
(c) NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq). [1] Ammonia accepts H+ from water, leaving OH−. [1]
(a) Explain the difference between a strong acid and a weak acid. (2 marks)
(b) Explain why diluting hydrochloric acid does not turn it into a weak acid. (1 mark)
(c) Equal volumes of equally concentrated HCl and CH3COOH are separately neutralised completely by NaOH. Compare the amounts in moles of NaOH needed and explain your answer. (2 marks)
Total: 5 marks
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(a) A strong acid dissociates essentially completely in water [1]; a weak acid dissociates only partially. [1]
(b) Dilution reduces amount per unit volume, while strength describes extent of dissociation. [1]
(c) The same amount of NaOH is needed. [1] Equal volume and concentration give equal total acid amounts, and both acids are monoprotic, so each mole neutralises one mole of OH−. [1]
Neutralisation and salts
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- Salt
- An ionic compound formed when the replaceable hydrogen ions of an acid are replaced by metal ions or ammonium ions (NH4+).
Neutralisation is the reaction of an acid with a base. When H⁺(aq) reacts with OH⁻(aq), water forms. The full equation depends on the base: acid plus aqueous ammonia forms an ammonium salt without a water product.
The other ions form the salt. Use the acid to identify the negative ion, then balance charges with the positive ion from the base.
Different bases give different additional products. Metal oxides and hydroxides form salt and water; carbonates also produce carbon dioxide. Use the table and balanced equations to choose the correct mole ratio.
| Reactants | Products |
|---|---|
| Acid + metal hydroxide | Salt + water |
| Acid + metal oxide | Salt + water |
| Acid + metal carbonate | Salt + water + carbon dioxide |
| Acid + ammonia | Ammonium salt |
| Acid | Salt family |
|---|---|
| Hydrochloric | Chloride |
| Sulfuric | Sulfate |
| Nitric | Nitrate |
| Ethanoic | Ethanoate |
Choose the correct salt and balance
Write balanced equations, including state symbols, for (a) magnesium oxide with hydrochloric acid, (b) sodium hydroxide with sulfuric acid, (c) calcium carbonate with nitric acid and (d) aqueous ammonia with hydrochloric acid.
Worked answer
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
Write balanced full equations, including state symbols, for the following aqueous-acid reactions.
(a) Sulfuric acid with potassium hydroxide solution. (2 marks)
(b) Nitric acid with magnesium oxide. (2 marks)
(c) Ethanoic acid with sodium hydroxide solution. (2 marks)
(d) Hydrochloric acid with calcium carbonate. (2 marks)
(e) Nitric acid with aqueous ammonia. (2 marks)
(f) Write the net ionic equation for H+ reacting with OH−. (1 mark)
Total: 11 marks
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(a) H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l). [2]
(b) 2HNO3(aq) + MgO(s) → Mg(NO3)2(aq) + H2O(l). [2]
(c) CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l). [2]
(d) 2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g). [2]
(e) HNO3(aq) + NH3(aq) → NH4NO3(aq). [2]
For (a)–(e), award one mark for correct formulae/products and one for balancing and states. (f) H+(aq) + OH−(aq) → H2O(l). [1]