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C. Chemical Reactions · Chemical Reactions

Predicting Products: Decomposition Reactions

C2 C3About 70 minutes
Learning goals
  • I can predict the products of decomposition reactions of binary compounds, carbonates, hydrogen carbonates, hydroxides, chlorates, oxyacids and hydrates.
  • I can explain why most decomposition reactions need energy (heat, light or electricity).
  • I can see a decomposition reaction as the reverse of a synthesis reaction.
  • I can write balanced equations, with states, for decomposition reactions in everyday and industrial settings.

Words to know

decomposition reaction
A single compound breaks down into two or more simpler substances.
electrolysis
Using electricity to decompose a compound, for example splitting water into hydrogen and oxygen.
thermal decomposition
Decomposition caused by heating.
hydrate
An ionic compound with water molecules built into its crystals, such as CuSO₄·5H₂O, copper(II) sulfate pentahydrate.
anhydrous
Without water; the form of a salt left after the water of hydration is driven off.
catalyst
A substance that speeds up a reaction without being used up, such as manganese(IV) oxide or the enzyme catalase for hydrogen peroxide.

Taking things apart

When you bake a cake, baking soda in the batter breaks down in the heat and releases carbon dioxide gas, which makes the cake rise. A car's airbag inflates in a few hundredths of a second because a solid in it suddenly breaks down into nitrogen gas. Cement plants heat crushed limestone to over 1000 °C to drive out carbon dioxide. All of these are decomposition reactions: one compound breaking into simpler substances.

Decomposition is the reverse of synthesis. That gives us a shortcut: most decomposition rules are just the synthesis rules from the last lesson, run backwards.

Why decomposition needs energy

To break a compound apart, its bonds must be broken, and breaking bonds absorbs energy. So most decomposition reactions are endothermic and only keep going while energy is supplied:

  • Heat (thermal decomposition): heating limestone, baking soda, or a hydrate.
  • Electricity (electrolysis): splitting water or molten salt.
  • Light: hydrogen peroxide and silver compounds slowly decompose in light, which is why hydrogen peroxide is sold in brown or opaque bottles.

A few compounds decompose easily and release energy, such as the sodium azide in older airbag designs: 2NaN₃(s) → 2Na(s) + 3N₂(g).

The decomposition rules

  1. Binary compound → its elements. Remember diatomic elements.
  • 2H₂O(l) → 2H₂(g) + O₂(g) (electrolysis)
  • 2NaCl(l) → 2Na(l) + Cl₂(g) (electrolysis of molten salt; this is how sodium metal is made)
  • 2HgO(s) → 2Hg(l) + O₂(g) (heating; this is the reaction Joseph Priestley used when he isolated oxygen in 1774)
  1. Metal carbonate → metal oxide + carbon dioxide. CaCO₃(s) → CaO(s) + CO₂(g)
  2. Metal hydrogen carbonate → metal carbonate + water + carbon dioxide. 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g) (baking soda in a hot oven)
  3. Metal hydroxide → metal oxide + water. Cu(OH)₂(s) → CuO(s) + H₂O(g). (Sodium and potassium hydroxides do not decompose this way with ordinary heating.)
  4. Metal chlorate → metal chloride + oxygen. 2KClO₃(s) → 2KCl(s) + 3O₂(g)
  5. Oxyacid → non-metal oxide + water. H₂CO₃(aq) → H₂O(l) + CO₂(g). This is why a can of pop goes flat: carbonic acid keeps breaking down and the CO₂ escapes.
  6. Hydrate → anhydrous salt + water. CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g)

And one to know by name: hydrogen peroxide → water + oxygen, 2H₂O₂(aq) → 2H₂O(l) + O₂(g), sped up by a catalyst.

compoundsimpler substancesdecomposition →metal carbonateCaCO₃metal oxide + carbon dioxideCaO + CO₂metal hydroxideCa(OH)₂metal oxide + waterCaO + H₂OoxyacidH₂CO₃non-metal oxide + waterCO₂ + H₂Obinary compound2NaClelements2Na + Cl₂← synthesis (the same reactions run backwards)
Decomposition and synthesis are reverse reactions. Learn the synthesis families from the last lesson, and you can predict most decompositions by reading the arrow the other way.

Spotting the type of compound

To choose the right rule, look carefully at the formula:

  • Only two elements? Binary → elements.
  • Contains CO₃ with a metal? Carbonate → oxide + CO₂.
  • Contains HCO₃? Hydrogen carbonate → carbonate + water + CO₂.
  • Contains OH with a metal? Hydroxide → oxide + water.
  • Contains ClO₃? Chlorate → chloride + oxygen.
  • Starts with H and has O (and it is an acid)? Oxyacid → non-metal oxide + water.
  • Has "· xH₂O" in the formula? Hydrate → anhydrous salt + water.
blue crystalsCuSO₄·5H₂O(s)white powderCuSO₄(s)flameheatwater vapour leaveswaterdroplets
Heating copper(II) sulfate pentahydrate. The blue crystals turn into a white powder as the water of hydration is driven off, and water droplets condense in the cooler top of the test tube.
Worked example

A metal carbonate

Predict the products and write a balanced equation for heating copper(II) carbonate.

  1. Copper(II) carbonate is CuCO₃. It is a metal carbonate, so it gives a metal oxide and carbon dioxide.
  2. Copper keeps its charge: copper(II) oxide is CuO.
  3. CuCO₃ → CuO + CO₂: Cu 1 = 1, C 1 = 1, O 3 = 1 + 2. Balanced.
  4. Observation: the green powder turns black, and the gas turns limewater milky (the test for carbon dioxide).

CuCO₃(s) → CuO(s) + CO₂(g)

Worked example

A metal hydroxide

Predict the products of heating aluminium hydroxide.

  1. Al(OH)₃ is a metal hydroxide → metal oxide + water.
  2. Aluminium oxide: Al³⁺ and O²⁻ → Al₂O₃.
  3. Skeleton: Al(OH)₃ → Al₂O₃ + H₂O. Al: 2 on the right, so 2Al(OH)₃.
  4. Now H on the left is 6, so 3H₂O. Check O: left 6; right 3 + 3 = 6. ✔

2Al(OH)₃(s) → Al₂O₃(s) + 3H₂O(g)

Worked example

A chlorate and a binary compound

Write balanced equations for: (a) heating sodium chlorate; (b) electrolysis of molten magnesium chloride.

  1. (a) Sodium chlorate is NaClO₃ → sodium chloride + oxygen. Skeleton: NaClO₃ → NaCl + O₂. Oxygen 3 vs 2: use 2NaClO₃ and 3O₂, then 2NaCl.
  2. (b) Magnesium chloride is MgCl₂, a binary compound → its elements, magnesium and chlorine gas (Cl₂).
  3. Check (b): Mg 1 = 1, Cl 2 = 2. Balanced as written.

(a) 2NaClO₃(s) → 2NaCl(s) + 3O₂(g) (b) MgCl₂(l) → Mg(l) + Cl₂(g)

Worked example

A hydrate

Write the equation for heating magnesium sulfate heptahydrate (Epsom salt, MgSO₄·7H₂O).

  1. A hydrate decomposes into the anhydrous salt and water.
  2. Hepta- means 7, so 7 water molecules are released per formula unit.
  3. Check: Mg 1, S 1, O 4 + 7 = 11, H 14 on both sides. ✔

MgSO₄·7H₂O(s) → MgSO₄(s) + 7H₂O(g)

Hands-on activity

Kitchen chemistry: decompose baking soda and weigh it

You need: baking soda (sodium hydrogen carbonate), a kitchen scale that reads in grams, an oven-safe dish, an oven, oven mitts, and a grown-up.

Safety: a grown-up handles the oven and the hot dish. The product, sodium carbonate ("washing soda"), is more irritating to skin and eyes than baking soda. Wear gloves to handle it, do not taste it, and label the container if you keep it for laundry.

  1. Weigh the empty dish. Add about 100 g of baking soda and record the exact mass of baking soda.
  2. Bake at about 200 °C for one hour, stirring once halfway (grown-up).
  3. Let it cool completely, then weigh again. Record the mass of the solid.
  4. Put it back in the oven for 20 more minutes, cool and weigh again. If the mass did not change, the reaction is finished.

Analysis: the reaction is 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g). Chemists can calculate (you will learn how in Quantities in Chemical Reactions) that 100 g of baking soda should leave about 63 g of sodium carbonate. How close were you? Where did the "missing" mass go? (It left as water vapour and carbon dioxide gas, so mass was still conserved.) Give two reasons your result could differ from 63 g (for example, the baking soda was not completely decomposed, or the scale is only precise to the nearest gram).

Just for fun

Joseph Priestley, who isolated oxygen by decomposing mercury(II) oxide in 1774, also invented a way to make carbonated water. So the person who helped discover oxygen also gave the world fizzy drinks, which go flat by a decomposition reaction. Chemistry really does come full circle.

Your turn

1

Predict the products and balance: Ag₂O(s) → (heated).

2

Predict the products and balance: heating magnesium carbonate.

3

Predict the products and balance: heating potassium chlorate.

4

Predict the products and balance: heating iron(III) hydroxide.

5

Predict the products: sulfurous acid, H₂SO₃, decomposes.

6

Write the equation for the electrolysis of water. Which gas forms in the larger volume?

More practice: warm-up

1

How many reactants does a decomposition reaction have?

2

Name three ways energy can be supplied for a decomposition.

3

What two products form when a metal carbonate is heated?

4

What does "anhydrous" mean?

5

Why is hydrogen peroxide sold in dark or opaque bottles?

More practice: on your own

1

Predict and balance: heating lithium hydrogen carbonate.

2

Predict and balance: electrolysis of molten aluminium oxide.

3

Predict and balance: heating barium chlorate.

4

Predict and balance: heating copper(II) sulfate pentahydrate. Describe what you would see.

5

Carbonic acid decomposes in an opened bottle of soda water. Write the equation and explain why the drink goes flat faster when warm.

Challenge

1

A student heats 10.0 g of calcium carbonate until the mass stops changing. The solid left has a mass of 5.6 g. Explain where the rest went and identify the reaction type.

2

Electrolysis of water uses electricity to make hydrogen fuel. Explain why this is only an environmental benefit if the electricity comes from low-carbon sources.

3

Predict and balance the decomposition of ammonium carbonate, which on heating gives ammonia, water and carbon dioxide (it is used in smelling salts and some old baking recipes as "baker's ammonia").

Check your understanding

Quiz

1.

What are the products when zinc carbonate is heated?

2.

Which equation shows the decomposition of a metal chlorate?

3.

Most decomposition reactions are endothermic because:

4.

What are the products of heating Ca(OH)₂?

5.

Which is the balanced equation for decomposing silver chloride into its elements?

6.

Baking soda helps cakes rise because, when heated, it:

For parents and teachers

Decomposition rules are easier if your teen links each one to the matching synthesis rule from the last lesson (carbonate ↔ oxide + carbon dioxide, and so on). Flash cards with the compound type on one side and the products on the other work well.

Common difficulties:

  • Not recognizing the compound type from its formula (hydroxide vs hydrogen carbonate, chlorate vs chloride).
  • Forgetting diatomic oxygen and chlorine in the products.
  • Changing a metal's charge during the reaction. In these decompositions the metal keeps its charge.

Activity safety: the oven and hot dish are for a grown-up to handle. The product (washing soda) irritates skin and eyes: gloves on, keep it away from food, and label it.