The Five Kinds of Reaction
Most reactions you will ever meet follow one of five patterns. Recognise the pattern and you can predict the products of a reaction you have never seen.
Builds on: 2.1 Chemical Reactions & Balancing
Patterns, not memorisation
There are millions of known reactions, which sounds hopeless until you notice that the overwhelming majority fall into five structural patterns. Learn the patterns and you can often write the products yourself.
1. Synthesis (combination) — A + B → AB
Two or more simple things join into one more complex thing.
2. Decomposition — AB → A + B
One compound splits into simpler pieces, usually driven by heat or electricity.
3. Single replacement — A + BC → AC + B
One element pushes another out of its compound. Whether it can depends on the reactivity series: a more reactive metal displaces a less reactive one.
Drop a zinc strip into blue copper sulfate and the blue fades while the strip turns coppery — you can watch the swap happen. Reverse it and nothing occurs, because copper is the less reactive of the two. Unit 6 wires exactly this eagerness into a battery.
4. Double replacement — AB + CD → AD + CB
Two compounds exchange partners. Common in solution, and usually driven by something leaving the mixture:
Neutralisation is a double replacement too — acid plus base giving a salt plus water, which is the whole of Unit 4:
5. Combustion — fuel + O₂ → CO₂ + H₂O
Technically a subtype, but so common it earns its own name. Burn any hydrocarbon in plenty of oxygen and you always get carbon dioxide and water:
The energy released is the point: it heats homes, drives engines and — as respiration, which is combustion run slowly and carefully inside cells — powers you.
Starve the flame of oxygen and you get carbon monoxide (CO) and soot instead. CO binds to haemoglobin roughly 200 times more strongly than oxygen does, is colourless and odourless, and is why a blocked flue kills people. A yellow, sooty flame where a blue one belongs is the visible warning.
Predicting products
Recognising the pattern lets you write the products before looking them up. Given “magnesium + hydrochloric acid”: a metal plus an acid is single replacement, so magnesium displaces hydrogen, giving MgCl₂ + H₂. Magnesium is 2+ and chloride 1−, so the formula follows from Unit 1 — and then you balance it. Three lessons combining into a prediction.
⚗️ Lab — Reaction Sorter
Ten reactions, one at a time. Name the pattern, then see why.
- Count how many things are on each side first — that alone separates synthesis from decomposition.
- If a lone element appears on both sides, it is a replacement. One swap = single, two = double.
- O₂ on the left with CO₂ and H₂O on the right is always combustion.