Ionic equations and tests for ions and gasesIB MYP Chemistry: Revision notes
Section 1
Ionic equations and spectator ions
Many reactions happen in water between ions. An ionic equation shows only the ions (or substances) that actually change. Ions that are present but unchanged are spectator ions, and they are left out.
To write one: write the full equation with state symbols, split every soluble ionic substance (aq) into its ions, cross out the spectator ions that appear unchanged on both sides, then check that the atoms and the charges balance.
Worked example: silver nitrate + sodium chloride.
Full: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Split into ions: Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻
Na⁺ and NO₃⁻ are spectators, so the ionic equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Solids, liquids and gases are never split into ions. Only (aq) ionic substances are written as separate ions.
Section 2
Solubility rules and precipitation
A precipitate is an insoluble solid formed when two solutions are mixed. To predict one, use the solubility rules:
- All nitrates are soluble.
- All salts of Group 1 metals (sodium, potassium) and ammonium salts are soluble.
- Chlorides are soluble, except silver chloride and lead chloride.
- Sulfates are soluble, except barium sulfate and lead sulfate (calcium sulfate is only slightly soluble).
- Carbonates and hydroxides are insoluble, except those of Group 1 and ammonium.
Example: barium chloride + sodium sulfate gives insoluble barium sulfate, so Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Precipitation reactions are used to make insoluble salts and to remove ions from waste water.
Section 3
Ionic equation for neutralisation
Acids contain H⁺(aq) ions and alkalis contain OH⁻(aq) ions. In every neutralisation between a strong acid and an alkali, the H⁺ and OH⁻ ions react to make water.
H⁺(aq) + OH⁻(aq) → H₂O(l)
For hydrochloric acid and sodium hydroxide, the Na⁺ and Cl⁻ ions are spectators; they stay dissolved as sodium chloride solution. Because the ionic equation is the same each time, all these neutralisations release similar heat.
Section 4
Tests for negative ions (anions)
- Chloride, Cl⁻: add dilute nitric acid, then silver nitrate solution. A white precipitate of silver chloride forms.
- Sulfate, SO₄²⁻: add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate forms.
- Carbonate, CO₃²⁻: add dilute acid. It fizzes, and the gas turns limewater milky (carbon dioxide).
The acid is added first in the chloride and sulfate tests to react with any carbonate ions, which would otherwise also give a white precipitate and cause a false positive.
Always name the reagents in order: acid first, then silver nitrate (chloride) or barium chloride (sulfate).
Section 5
Tests for positive ions (cations)
Ammonium, NH₄⁺: warm with sodium hydroxide solution. Ammonia gas is given off and turns damp red litmus paper blue.
Metal ions: add sodium hydroxide solution. Many metal hydroxides are insoluble, so the colour of the precipitate identifies the ion:
- Copper(II), Cu²⁺: blue precipitate
- Iron(II), Fe²⁺: green precipitate
- Iron(III), Fe³⁺: red-brown precipitate
- Calcium, magnesium and aluminium (Ca²⁺, Mg²⁺, Al³⁺): white precipitate. The aluminium precipitate dissolves in excess sodium hydroxide; calcium and magnesium do not.
Example: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s).
Section 6
Tests for gases
- Hydrogen, H₂: a lit splint gives a squeaky pop.
- Oxygen, O₂: relights a glowing splint.
- Carbon dioxide, CO₂: turns limewater milky (cloudy).
- Chlorine, Cl₂: bleaches damp blue litmus paper (it turns red, then white). It is a poisonous, pungent gas, so tests must be done in a fume cupboard.
- Ammonia, NH₃: turns damp red litmus paper blue because it is alkaline.
Litmus paper must be damp for the ammonia and chlorine tests, because the gases need to dissolve to show their effect.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ionic equations and tests for ions and gases
- A technician at a water-treatment laboratory in Nairobi mixes a colourless solution of silver nitrate with a colourless solution of sodium chloride. A white solid forms at once, and the mixture is then filtered. The technician writes the reaction first as a full equation and then as an ionic equation.Explain why a white solid forms when the two solutions are mixed.2 marks
- A student in a school laboratory in Manchester is given three bottles of colourless solutions labelled sodium carbonate, sodium sulfate and ammonium chloride. A technician has mixed up the labels, so the student must identify each solution using chemical tests.Describe how the student could use dilute hydrochloric acid to show which bottle contains sodium carbonate.2 marks
- A metal-plating factory in Pune releases waste water into a drain. An environmental science club collects three samples, X, Y and Z, and adds a few drops of sodium hydroxide solution to each. Sample X forms a blue precipitate, sample Y forms a green precipitate and sample Z forms a red-brown precipitate. Each sample contains only one type of metal ion.Identify the metal ion present in each of samples X, Y and Z.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).