Coaching Centre
Predicting the products of reactions
Evidence of chemical change, and how to recognise and predict the products of synthesis, decomposition and combustion reactions, with balanced equations and states, the thermal decomposition investigation, and the chemistry of First Nations methods that make toxic plant foods safe.
Throw a spoonful of green copper carbonate into a hot test tube and it turns black, while the gas coming off clouds a tube of limewater. Hold a lit splint to a jet of methane and you get a blue flame and droplets of water. Neither result is luck. Once you can name the type of reaction, you can usually write the products before you have seen them, and that is the main skill of this module.
Evidence that a reaction has happened
In Year 10 you met chemical reactions as rearrangements of atoms into new substances (see Year 10 Science, Chapter 3). At Stage 6 you need to be able to back that claim with observations. A chemical change makes at least one new substance, and the usual signs are:
- a colour change that isn't just dilution (blue copper(II) ions fading as zinc is added);
- a gas given off: bubbles, fizzing or a smell;
- a precipitate: an insoluble solid appearing when two solutions are mixed;
- a temperature change without heating or cooling from outside;
- light or a flame;
- a change that is hard to reverse, such as a new smell or the disappearance of a solid.
None of these is proof on its own. Dissolving ammonium nitrate gets very cold and boiling water gives off a gas, but both are physical changes. Chemists confirm a new substance with a test: hydrogen pops with a lit splint, oxygen relights a glowing splint, carbon dioxide turns limewater milky, and water turns anhydrous copper(II) sulfate from white to blue.
Classifying reactions
The syllabus names seven types of reaction. Three are in this lesson; precipitation and the two acid reactions are in lesson 3B; single displacement is in 3C.
| Type | General pattern | Example |
|---|---|---|
| Synthesis | A + B → AB | 2Mg(s) + O₂(g) → 2MgO(s) |
| Decomposition | AB → A + B | CaCO₃(s) → CaO(s) + CO₂(g) |
| Combustion | fuel + O₂ → oxides | C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g) |
| Precipitation | two solutions → insoluble solid | AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) |
| Acid–base | acid + base → salt + water | HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) |
| Acid–carbonate | acid + carbonate → salt + water + CO₂ | 2HNO₃(aq) + CaCO₃(s) → Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g) |
| Single displacement | A + BC → AC + B | Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) |
Some reactions fit more than one heading. Magnesium burning in air is a synthesis and a combustion. The marks come from the equation and the reasoning, so say which features you used to classify it.
Synthesis reactions
In a synthesis (or combination) reaction, two or more reactants join to form a single product. Most are elements combining: a metal with a non-metal gives an ionic compound, and two non-metals give a covalent compound. Use your Module 1 skills to write the product's formula from ion charges or valencies.
Aluminium powder is heated in a stream of chlorine gas. Predict the product and write a balanced equation.
Aluminium forms Al³⁺ and chlorine forms Cl⁻, so the product is AlCl₃, a white solid.
Skeleton: Al(s) + Cl₂(g) → AlCl₃(s). Chlorine comes in pairs, so make the Cl even: 2AlCl₃ needs 3Cl₂ and 2Al.
$$2\text{Al(s)} + 3\text{Cl}_2\text{(g)} \rightarrow 2\text{AlCl}_3\text{(s)}$$ Check: 2 Al and 6 Cl on each side.
Not every synthesis starts with elements. Metal oxides combine with water to make hydroxides (CaO + H₂O → Ca(OH)₂, the reaction that heats up when builders slake lime), and non-metal oxides combine with water to make acids (SO₃ + H₂O → H₂SO₄).
Decomposition reactions
A decomposition breaks one compound into two or more simpler substances. It needs energy, so it is usually driven by heat (thermal decomposition), electricity (electrolysis) or light (photolysis). The common Year 11 patterns are:
- metal carbonate → metal oxide + carbon dioxide: CuCO₃(s) → CuO(s) + CO₂(g)
- metal hydroxide → metal oxide + water: Cu(OH)₂(s) → CuO(s) + H₂O(g)
- electrolysis of water: 2H₂O(l) → 2H₂(g) + O₂(g)
- photolysis of silver halides (black-and-white photography): 2AgBr(s) → 2Ag(s) + Br₂(l)
- hydrogen peroxide, catalysed by MnO₂: 2H₂O₂(aq) → 2H₂O(l) + O₂(g)
Group 1 carbonates (except lithium's) don't decompose at Bunsen temperatures. The more reactive the metal, the more stable its carbonate, which links to the activity series in lesson 3C.
Combustion reactions
Combustion is a fast, exothermic reaction with oxygen. For fuels containing carbon, hydrogen and possibly oxygen, the products depend on the supply of oxygen:
- Complete combustion (plenty of O₂): CO₂ and H₂O. Blue flame.
- Incomplete combustion (limited O₂): CO and/or C (soot), plus H₂O. Yellow, sooty flame. Carbon monoxide binds to haemoglobin far more strongly than oxygen, which is why heaters must never run in closed rooms.
Write balanced equations for the complete combustion of octane, C₈H₁₈, and for its incomplete combustion to carbon monoxide.
Balance C, then H, then O last: C₈H₁₈ + O₂ → 8CO₂ + 9H₂O. Oxygen atoms on the right: 16 + 9 = 25, so 12½ O₂.
Double to clear the half: $$2\text{C}_8\text{H}_{18}\text{(l)} + 25\text{O}_2\text{(g)} \rightarrow 16\text{CO}_2\text{(g)} + 18\text{H}_2\text{O(g)}$$
Incomplete: C₈H₁₈ + O₂ → 8CO + 9H₂O needs 8 + 9 = 17 O atoms, 8½ O₂, so $$2\text{C}_8\text{H}_{18}\text{(l)} + 17\text{O}_2\text{(g)} \rightarrow 16\text{CO(g)} + 18\text{H}_2\text{O(g)}$$
Metals burn too: magnesium gives MgO with a white flame and iron wool gives Fe₃O₄ with orange sparks. These are combustion and synthesis at the same time.
- Card 1 is iron and sulfur, heated. Pick a type, then check the equation: Fe(s) + S(s) → FeS(s). Which feature tells you it is a synthesis?
- Go to card 2, copper(II) carbonate. After you choose, the picture shows the green solid turning black. Which test identifies the gas?
- Card 14 is hydrogen peroxide with MnO₂. Why is MnO₂ not written as a reactant in 2H₂O₂(aq) → 2H₂O(l) + O₂(g)?
- Work through all 16 cards and aim for 16 / 16. Note which ones you got wrong: they are usually the acid reactions or displacement, which the next two lessons cover.
Making toxic plants safe: First Nations food chemistry
Aboriginal and Torres Strait Islander Peoples have processed plants that are poisonous when raw for many thousands of years. The best-studied example is the seeds of cycads (Macrozamia and Cycas species), which contain a toxic compound called cycasin along with related compounds such as macrozamin. Eaten raw, they cause severe illness. Archaeological evidence from a rock shelter in south-west Western Australia suggests cycad seeds were being processed there around 13 000 years ago.
Each step has a chemical explanation:
- Crushing or slicing increases the surface area, so more of the toxin is exposed to water. This is the same idea that speeds up reactions in lesson 3G.
- Leaching in running water for days or weeks works because cycasin is a glycoside: a toxic molecule attached to a sugar unit. The sugar part has many –OH groups that hydrogen bond with water, so the compound is water soluble. Running water keeps the concentration in the surrounding water low, so the toxin keeps dissolving out.
- Fermenting (leaving the kernels to age, sometimes for months) lets microbes and enzymes break the toxin down in decomposition reactions.
- Roasting or baking supplies energy for thermal decomposition of the remaining toxin and makes the starch digestible.
Other foods treated in similar ways include the black bean or Moreton Bay chestnut (Castanospermum australe), which was sliced, leached and roasted, and some yams.
In Year 12 (Chemistry lesson 5H) you'll come back to these methods and explain the leaching step with solubility equilibrium.
Test yourself
Which equation represents a decomposition reaction?
One reactant breaks into two simpler products. The others are synthesis, combustion and single displacement.
What is the coefficient of O₂ when the equation for the complete combustion of butane, C₄H₁₀, is balanced with the smallest whole numbers?
2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. 6.5 is right per mole of butane but not a whole number.
A white solid is heated. A colourless gas is given off that turns limewater milky, and a white solid remains. The original solid is most likely
Zinc carbonate decomposes to ZnO and CO₂. Sodium carbonate does not decompose at Bunsen temperatures.
Cycad seeds were leached in running water for several days. Which property of cycasin does this method rely on?
Leaching dissolves the toxin out. Running water keeps carrying the dissolved toxin away.
A student heats 3.70 g of copper(II) carbonate (M = 123.56 g mol⁻¹) to constant mass. What mass of copper(II) oxide (M = 79.55 g mol⁻¹) should remain?
n = 3.70 ÷ 123.56 = 0.0299 mol; m(CuO) = 0.0299 × 79.55 = 2.38 g.
Label the thermal decomposition set-up.
Classify each reaction.
A metal carbonate decomposes on heating to a metal oxide and , which turns milky. Breaking a compound down with electricity is called . When a hydrocarbon burns in limited oxygen it can form carbon and soot. Leaching removes toxins that are .
Aboriginal and Torres Strait Islander Peoples made cycad seeds safe to eat by slicing or crushing them, leaving them in running water for several days, and then roasting them. Explain the chemistry behind each of these three steps.
Slicing or crushing the seeds increases the surface area in contact with water, so the toxin can dissolve out of the seed more quickly and more completely.
The toxin, cycasin, is a glycoside: it has a sugar part with many –OH groups that form hydrogen bonds with water, so it is water soluble. Leaving the pieces in running water dissolves it out (leaching). Running water constantly replaces the water around the seeds, keeping the concentration of dissolved toxin low, so dissolving continues until very little toxin remains.
Roasting supplies energy that thermally decomposes much of the remaining toxin into harmless products, and it cooks the starch so it can be digested.