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C. Chemical Reactions · Chemical Reactions

Five Types of Chemical Reactions

C3 C2 A1.12About 65 minutes
Learning goals
  • I can describe synthesis, decomposition, single displacement, double displacement and combustion reactions with general patterns.
  • I can classify a reaction from its balanced equation, and explain my reasoning.
  • I can recognize reactions that fit more than one category, such as a metal burning in oxygen.
  • I can use evidence of chemical change to describe what was observed in a reaction.

Words to know

synthesis reaction
Two or more reactants combine to form one product: A + B → AB. Also called a combination reaction.
decomposition reaction
One compound breaks down into two or more simpler substances: AB → A + B.
single displacement reaction
An element replaces another element in a compound: A + BC → AC + B.
double displacement reaction
Two ionic compounds exchange ions: AB + CD → AD + CB.
combustion reaction
A substance reacts rapidly with oxygen, releasing energy as heat and light.
neutralization
A double displacement reaction between an acid and a base that produces a salt and water.
exothermic / endothermic
A reaction that releases energy to the surroundings / absorbs energy from the surroundings.

Sorting reactions like sorting music

There are millions of known chemical reactions. Nobody memorizes them all. Instead, chemists sort them into a few types, the way a music app sorts songs into genres. Once you know a reaction's type, you can often predict its products without ever having seen it. That is the main skill of this unit.

In Grade 10 you met five types. In Grade 11 you need to recognize them instantly from formulas, handle the tricky cases, and then (in the next lessons) predict products for each type.

The five patterns

Use letters to stand for elements or ions. A and C are usually metals or positive ions; B and D are usually non-metals or negative ions.

  1. Synthesis: A + B → AB. Several reactants, one product.
  2. Decomposition: AB → A + B. One reactant, several products.
  3. Single displacement: A + BC → AC + B. An element and a compound react to give a new element and a new compound.
  4. Double displacement: AB + CD → AD + CB. Two compounds swap partners.
  5. Combustion: fuel + O₂ → oxides + energy. For a hydrocarbon (compound of C and H) or a compound of C, H and O, burned completely, the products are always CO₂ and H₂O.
SynthesisA+B→ABDecompositionAB→A+BSingle displacementA+BC→AC+BDouble displacementAB+CD→AD+CBCombustionfuel+O₂→oxides+energy
The five reaction patterns. Watch where each lettered particle ends up: in a displacement reaction the partners change, but every particle on the left still appears on the right.
TypeExampleWhat you might observe
Synthesis2Mg(s) + O₂(g) → 2MgO(s)Bright white light; white powder forms
DecompositionCaCO₃(s) → CaO(s) + CO₂(g)Strong heating needed; gas given off
Single displacementZn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)Reddish-brown copper forms on the zinc; blue solution fades
Double displacementAgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)White precipitate (solid) forms in clear solutions
CombustionCH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)Blue flame; heat; water vapour condenses on a cold surface
One example of each type. Every one of these will appear again in this unit.

Clues hidden in the formulas

Look at the reactant side first:

  • O₂ plus a fuel (a hydrocarbon such as CH₄, or a compound of C, H and O such as ethanol C₂H₅OH) → combustion.
  • One compound alone (often with "heat" or "electricity" over the arrow) → decomposition.
  • Two elements (or two simple compounds) joining into one product → synthesis.
  • One element + one compound → single displacement. Check that a different element comes out on the right.
  • Two ionic compounds, usually both (aq) → double displacement. Look for a precipitate (s), a gas (g) or water (l) among the products.

An acid + base reaction (HCl + NaOH → NaCl + H₂O) is a special double displacement called neutralization: the H⁺ from the acid and the OH⁻ from the base end up together as water.

Is O₂ a reactant, burning a fuel?CombustionyesnoIs there only ONE reactant?DecompositionyesnoIs there only ONE product?SynthesisyesnoElement + compound on the left?Single displacementyesnoTwo ionic compounds on the left?Double displacementyes
A quick decision chart for classifying a reaction. Start at the top and stop at the first "yes". Then check your answer against the general pattern.

When a reaction fits two types

Some reactions belong to two categories, and that is fine as long as you can explain why.

  • Magnesium burning in air: 2Mg + O₂ → 2MgO is a synthesis (one product) and also a combustion (rapid reaction with oxygen, giving heat and light).
  • Carbon (charcoal) burning: C + O₂ → CO₂ is both synthesis and combustion.
  • Hydrogen burning: 2H₂ + O₂ → 2H₂O is both, too. This is the reaction in a hydrogen fuel cell, although there the energy is released as electricity instead of a flame.

When a question asks for one type, give the one that best describes the pattern and mention the other if you are unsure. Many teachers accept "synthesis (also combustion)".

Energy is part of every reaction

Every reaction either releases energy (exothermic: burning, neutralization, most synthesis reactions) or absorbs it (endothermic: most decomposition reactions, such as heating limestone). That is why decompositions usually need heat, electricity or light to keep going. You will study energy changes in reactions in Grade 12 Chemistry (SCH4U).

Worked example

Classify and explain (1)

Classify: 2KClO₃(s) → 2KCl(s) + 3O₂(g)

  1. Count reactants: there is only one reactant, potassium chlorate.
  2. It breaks into two simpler substances: potassium chloride and oxygen.
  3. The pattern AB → A + B fits (here, a compound gives a simpler compound and an element).

Decomposition. One compound breaks down into simpler substances.

Worked example

Classify and explain (2)

Classify: Cl₂(g) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)

  1. Reactants: chlorine is an element; sodium bromide is a compound.
  2. Products: a new compound (NaCl) and a new element (Br₂).
  3. Chlorine took bromine's place beside sodium: A + BC → AC + B (here the non-metals are swapped).

Single displacement (a halogen displaces another halogen).

Worked example

Classify and explain (3)

Classify: H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

  1. Reactants: two compounds, an acid (H₂SO₄) and a base (KOH).
  2. The partners swap: K⁺ joins sulfate, and H⁺ joins OH⁻ to make water.
  3. Acid + base → salt + water is the neutralization pattern.

Double displacement, specifically neutralization.

Worked example

Classify and explain (4)

Classify: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

  1. Ethanol is a compound of C, H and O, used as a fuel.
  2. It reacts with oxygen, and the products are carbon dioxide and water.
  3. Check the balance: C 2 = 2, H 6 = 6, O 1 + 6 = 7 = 4 + 3. ✔

Combustion (complete combustion of ethanol).

Hands-on activity

Reaction hunt at home

You need: a notebook and a grown-up's permission to watch cooking or cleaning.

Over one or two days, look for chemical changes around your home and record each one in a table with four columns: what happened, evidence of a chemical change (new colour, gas, light, heat, precipitate, new smell), likely reactants, and reaction type (if you can tell).

Ideas: a gas stove or furnace flame (combustion of natural gas, mostly methane); toast browning; a rusty bike chain or shovel (iron combining with oxygen and water); baking soda and vinegar fizzing (a double displacement whose product breaks down into carbon dioxide gas and water); an antacid tablet fizzing in water.

Safety: only watch flames and stoves; do not light or adjust them. Never mix cleaning products to "see what happens": some mixtures (for example bleach with ammonia or with acids) release toxic gases.

What to look for: at least five entries, with honest "not sure" answers where the type is unclear. Cooking reactions (like toast browning) are very complex and do not fit the five simple types, which is a good thing to notice.

Just for fun

Why did the sodium ion leave the chloride for the nitrate in a double displacement?

It was a swap of convenience. (Don't worry, chloride found a new partner in the very same equation. In chemistry, nobody stays single for long, except in a single displacement.)

Your turn

1

Classify: N₂(g) + 3H₂(g) → 2NH₃(g)

2

Classify: 2H₂O₂(aq) → 2H₂O(l) + O₂(g)

3

Classify: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

4

Classify: Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq)

5

Classify: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

6

Classify: HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + H₂O(l)

7

Classify: S₈(s) + 8O₂(g) → 8SO₂(g) and explain why two answers are possible.

More practice: warm-up

1

Which type has only one reactant?

2

Which type has only one product?

3

Which type always has an element and a compound on each side?

4

What two products form when a hydrocarbon burns completely?

5

What is the general pattern for double displacement?

More practice: on your own

1

Classify and explain: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

2

Classify and explain: CuCO₃(s) → CuO(s) + CO₂(g)

3

Classify and explain: SO₃(g) + H₂O(l) → H₂SO₄(aq)

4

Classify and explain: BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)

5

A student says C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (cellular respiration) is a combustion. Is that reasonable? Explain.

Challenge

1

Write a balanced example of your own for each of the five types, using substances not shown in this lesson. Label each.

2

Explain why 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) is classified as single displacement, even though water does not look like an ionic compound.

3

Most decomposition reactions are endothermic and most synthesis reactions are exothermic. Use the idea of bonds to explain why these two facts fit together.

Check your understanding

Quiz

1.

Which type of reaction is 2HgO(s) → 2Hg(l) + O₂(g)?

2.

Which equation shows a single displacement reaction?

3.

Neutralization is a special case of which reaction type?

4.

What are the products of the complete combustion of any hydrocarbon?

5.

2Mg + O₂ → 2MgO with a dazzling white flame can be called:

6.

A clue that a reaction is double displacement is:

For parents and teachers

This lesson is about pattern recognition. A good way to help is a quick-fire game: write an equation on a card, and your teen names the type and says why in one sentence ("one reactant, so decomposition").

Common difficulties:

  • Mixing up single and double displacement. Ask: "Is there a free element on the left?" If yes, it is single displacement.
  • Thinking every reaction with oxygen is combustion. Burning needs a fuel; SO₃ + H₂O → H₂SO₄ has no oxygen gas at all and is synthesis.
  • Worrying when a reaction fits two types. Reassure them that this is normal.

The home activity is observation only. Remind your teen never to mix cleaning products.