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2.7Inorganic Chemistry

Acids, Bases and Salt Preparations — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

10 min read 14 parts Lesson 14 of 23 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Lesson summary

Acids, Bases and Making Soluble Salts — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Write equations for the reactions of acids with metals, bases and carbonates and describe how to prepare a pure dry sample of a soluble salt.A salt forms when the hydrogen of an acid is replaced by a m…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Crystallisation
  • Effervescence

Part 1 of 6Reactions of acids

A salt forms when the hydrogen of an acid is replaced by a metal or ammonium ion. The salt name has two parts: the metal comes from the base or metal and the second part from the acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate).

1
Acid + metal → salt + hydrogen: Zn + H2SO4 → ZnSO4 + H2 (only for metals above hydrogen, and not too reactive).
2
Acid + metal oxide (base) → salt + water: CuO + H2SO4 → CuSO4 + H2O.
3
Acid + metal hydroxide / alkali → salt + water: NaOH + HCl → NaCl + H2O.
4
Acid + metal carbonate → salt + water + carbon dioxide: CaCO3 + 2HCl → CaCl2 + H2O + CO2.
5
Acid + ammonia → ammonium salt: NH3 + HNO3 → NH4NO3, used as a fertiliser.

A base is a substance that neutralises an acid; an alkali is a base that dissolves in water. Carbonates fizz because CO2 is released, and the gas turns limewater milky.

Part 2 of 6Solubility rules

Deciding the preparation method depends on solubility: all sodium, potassium and ammonium salts are soluble; all nitrates are soluble; chlorides are soluble except silver and lead(II); sulfates are soluble except lead(II), barium and calcium; carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium (calcium hydroxide is sparingly soluble).

Part 3 of 6Preparing a soluble salt from an insoluble base or metal

Examplecopper(II) sulfate crystals from copper(II) oxide and sulfuric acid.

Warm about 50 cm3 of dilute sulfuric acid gently in a beaker (do not boil).
Excess
Add copper(II) oxide a little at a time, stirring, until it is in excess (black solid stops dissolving) so that all the acid is used up.
Filter off the excess oxide into an evaporating basin; the filtrate is blue copper(II) sulfate solution.
Heat to evaporate about half the water until the crystallisation point (a drop on a glass rod forms crystals when cooled),
  • then leave to cool slowly so crystals form.
Filter off crystals, wash with a little cold distilled water and dry between filter papers.

Using excess of a solid is key because the excess is easily filtered. The same method works for zinc oxide, magnesium carbonate or zinc metal with acids; effervescence stops when all the acid is used.

Part 4 of 6Preparing a soluble salt from an alkali: titration

For sodium, potassium or ammonium salts, both reactants are soluble, so no excess can be filtered off.

1
The exact volumes are found by titration with an indicator.
2
Then the experiment is repeated with the same volumes but without the indicator; the solution is evaporated to crystallise the salt.
3
Example: NaOH + HCl → NaCl + H2O gives sodium chloride.
4
Final crystals are dried in a warm oven or between filter papers.

Part 5 of 6Choosing the correct preparation method

Starting materialsMethodWhy
Insoluble oxide/carbonate + acidAdd excess solid, filter, crystalliseExcess removable by filtering
Reactive-but-safe metal (Zn, Mg) + acidAdd excess metal, filter, crystalliseHydrogen given off, visible end point
Soluble alkali + acidTitration, then evaporateNeither can be filtered off

Do not use sodium or potassium metal with acid because the reaction is dangerously violent, and do not use copper because it does not react. Always write the word equation, then the balanced symbol equation, and check the solubility of the salt before deciding on the method.

Part 6 of 6Worked examples

  1. Write a balanced equation for the reaction of zinc oxide with sulfuric acid. [2]2 marks

  2. Why is excess copper(II) oxide added when preparing copper sulfate? [2]2 marks

  3. Describe how to prepare crystals of sodium nitrate from sodium hydroxide solution and nitric acid. [4]4 marks

  4. Name the gas formed when magnesium carbonate reacts with acid and give the test. [2]2 marks

Key terms

TermMeaning
SaltCompound formed when H+ of an acid is replaced by a metal or ammonium ion
BaseSubstance that neutralises an acid
ExcessMore of a reactant than needed so the other is completely used up
FiltrateLiquid that passes through filter paper
CrystallisationFormation of crystals from a saturated solution
EffervescenceFizzing due to gas release

Avoid these mistakes

1
Using titration for an insoluble base — excess and filtration are used instead.
2
Boiling to dryness, which can decompose crystals; stop at the crystallisation point.
3
Forgetting that carbonates produce CO2 and water, not hydrogen.
4
Using a hydroxide that is insoluble as an alkali in titration.

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