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# What kind of reaction is it?

Years 9 and 10 Chemistry · Reaction rates

<!-- Speaker note: Students identify synthesis, decomposition and displacement reactions by their patterns, use reactivity to predict whether a displacement happens, and classify unfamiliar reactions and predict products (AC9S10U07, nodes 1 to 4). About 28 minutes (Engage and Explore 9 min · Explain 11 min · Evaluate 3 min · Typical adds 2 min, Wider about 3 min). -->

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## Today

**We are learning to** many reactions follow one of three patterns. Synthesis: two substances join into one (A + B → AB). Decomposition: one substance breaks into simpler ones (AB → A + B). Displacement: a more reactive element takes the place of a less reactive one (A + BC → AC + B). Spotting the pattern lets you predict the products.

- I can recognise synthesis, decomposition and displacement reactions from their patterns.
- I can predict the products of a displacement reaction using reactivity.
- I can classify a reaction I haven't seen before.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace the practicals (magnesium burning, decomposing hydrogen peroxide with a catalyst, metals in copper sulfate). Set the width with `?width=minimum`, `typical` (default) or `wider`. One lesson for four nodes (the three reaction types and classifying them), before Why does heat speed up reactions? (node 5). Node 3 (displacement) is a hinge: students who can't use reactivity to predict which way a displacement goes can't predict products. The prediction and the Typical step target it. Board notes: zinc oxide, copper oxide and magnesium oxide are drawn as small particles, although they are really giant ionic lattices, and copper sulfate is replaced by copper oxide so the board can draw it. The backwards board has no reaction defined at all: the model shows the outcome, not why. Decomposition of hydrogen peroxide is shown without a catalyst. Teacher: In your own words: one example of each type with its pattern is the evidence. -->

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## Why it matters

> Drop an iron nail into blue copper sulfate solution and it comes out coated in copper, while the blue slowly fades. Iron has pushed copper out of its compound.

<!-- Speaker note: You can balance equations and follow atoms through a reaction. This lesson sorts reactions into patterns. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**An iron nail is put in copper sulfate solution. Iron is more reactive than copper. What happens?**

- The copper sulfate turns into iron
- Nothing: metals don't react with solutions
- The iron and copper mix to make an alloy
- Iron takes copper's place: iron sulfate forms and copper coats the nail

<!-- Speaker note: What happens: Iron takes copper's place: iron + copper sulfate → iron sulfate + copper. The more reactive metal displaces the less reactive one from its compound. That is a displacement reaction. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Nothing happens' and 'they mix into an alloy' both miss reactivity. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Freaction-types%2Fstep-2&code=widget%3A+reaction%0Ashow%3A+Mg+x12%2C+O2+x6%0Areact%3A+2+Mg+%2B+O2+-%3E+2+MgO%0Aea%3A+1.5%0Arelease%3A+1%0Acaption%3A+%22Synthesis%3A+magnesium+%2B+oxygen+%E2%86%92+magnesium+oxide.%22%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/reaction-types/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: after each board, ask how many substances went in and how many came out. -->

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## What happened?

* Iron takes copper's place: iron + copper sulfate → iron sulfate + copper. The more reactive metal displaces the less reactive one from its compound. That is a displacement reaction.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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## Three patterns

**Match each pattern to its name.**

| | |
|---|---|
| `A + B → AB` | A. Displacement |
| `AB → A + B` | B. Decomposition |
| `A + BC → AC + B` | C. Synthesis |

* Answer: A + B → AB → C. Synthesis; AB → A + B → B. Decomposition; A + BC → AC + B → A. Displacement

<!-- Speaker note: Join, break apart, swap. -->

<!-- Speaker note: Teacher: the classification sort and the product prediction are the success criteria. -->

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## Classify them

**Which kind of reaction is it?**

Groups: Synthesis · Decomposition · Displacement

iron + sulfur → iron sulfide · 2H₂ + O₂ → 2H₂O · calcium carbonate → calcium oxide + carbon dioxide · 2H₂O → 2H₂ + O₂ (electricity passed through water) · magnesium + copper sulfate → magnesium sulfate + copper · chlorine + sodium bromide → sodium chloride + bromine

* Answer: iron + sulfur → iron sulfide → Synthesis; 2H₂ + O₂ → 2H₂O → Synthesis; calcium carbonate → calcium oxide + carbon dioxide → Decomposition; 2H₂O → 2H₂ + O₂ (electricity passed through water) → Decomposition; magnesium + copper sulfate → magnesium sulfate + copper → Displacement; chlorine + sodium bromide → sodium chloride + bromine → Displacement

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Predict the products

**Zinc is more reactive than copper. What are the products of zinc + copper sulfate?**

- Copper zinc + sulfate
- Zinc copper sulfate
- No reaction
- Zinc sulfate + copper

* Answer: Zinc sulfate + copper

<!-- Speaker note: The more reactive metal (zinc) takes the less reactive one's place: zinc + copper sulfate → zinc sulfate + copper. -->

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## The other way round

**Copper is put into zinc sulfate solution. What happens?**

- Nothing: copper is less reactive, so it can't displace zinc
- The copper dissolves
- Copper sulfate and zinc form

* Answer: Nothing: copper is less reactive, so it can't displace zinc

<!-- Speaker note: Reactivity decides whether a displacement happens. -->

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## Synthesis product

**Aluminium burns in oxygen. It is a synthesis reaction (A + B → AB). What is the product?**

- aluminium and oxygen, mixed together
- aluminium oxide and water
- aluminium oxide

* Answer: aluminium oxide

<!-- Speaker note: Two substances join into one: aluminium + oxygen → aluminium oxide. -->

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## Heating copper carbonate

**Green copper carbonate powder is heated strongly. It turns black (copper oxide) and gives off carbon dioxide. Which kind of reaction is it, and what drives it?**

- Synthesis: the copper carbonate joins with the heat
- Decomposition, driven by heat: one compound breaks into two simpler substances
- Displacement: carbon dioxide takes copper's place

* Answer: Decomposition, driven by heat: one compound breaks into two simpler substances

<!-- Speaker note: copper carbonate → copper oxide + carbon dioxide. Many decompositions need energy put in: heat, as here and for limestone, or electricity, as for splitting water. -->

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## Swapping partners

**In some reactions two compounds swap partners (AB + CD → AD + CB): a double displacement. Neutralisation is one. What are the products of hydrochloric acid + sodium hydroxide?**

- sodium hydroxide chloride
- sodium chloride + hydrogen
- sodium chloride + water

* Answer: sodium chloride + water

<!-- Speaker note: Sodium and hydrogen swap partners: sodium chloride and water (H + OH). acid + base → salt + water. A precipitation, where two solutions make a solid, swaps partners in the same way. -->

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## Elaborate: the other way round

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Freaction-types%2Fstep-4&code=widget%3A+reaction%0Ashow%3A+Cu+x10%2C+ZnO+x10%0Areact%3A+Zn+%2B+CuO+-%3E+ZnO+%2B+Cu%0Aea%3A+1.5%0Agoal%3A+none%0Acaption%3A+Copper+and+zinc+oxide.+Will+copper+take+zinc%27s+place%3F%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/reaction-types/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Running the reverse displacement and seeing nothing happen makes reactivity the deciding factor, not a rule to memorise. -->

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## Elaborate: the other way round

**On this board, copper is mixed with zinc oxide. Watch it for a while. Why does no reaction happen?**

- There isn't enough copper
- Copper is less reactive than zinc, so it can't take the oxygen from zinc
- The temperature is too low

* Answer: Copper is less reactive than zinc, so it can't take the oxygen from zinc

<!-- Speaker note: A displacement goes only from more reactive to less reactive. The reactivity series (potassium, sodium, calcium, magnesium, aluminium, zinc, iron, copper, silver, gold) predicts which way. What this step is for: Running the reverse displacement and seeing nothing happen makes reactivity the deciding factor, not a rule to memorise. -->

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## Iron from rock

**In a blast furnace, carbon takes oxygen from iron oxide: iron oxide + carbon → iron + carbon dioxide. Which kind of reaction is this, and why does it work?**

- Displacement: carbon is more reactive than iron, so it takes the oxygen
- Synthesis: iron and carbon join
- Decomposition: iron oxide breaks apart on its own

* Answer: Displacement: carbon is more reactive than iron, so it takes the oxygen

<!-- Speaker note: Metals less reactive than carbon (zinc, iron, copper) are extracted from their ores this way. -->

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## Lime from limestone

**Builders heat limestone (calcium carbonate) to make lime (calcium oxide) for cement. Which kind of reaction is this?**

- Synthesis
- Decomposition
- Displacement

* Answer: Decomposition

<!-- Speaker note: calcium carbonate → calcium oxide + carbon dioxide. Cement making releases a lot of carbon dioxide for this reason. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Synthesis**: substances join into one (A + B → AB), such as magnesium burning.
- **Decomposition**: one substance breaks into simpler ones (AB → A + B), such as hydrogen peroxide → water + oxygen, often with heat or electricity.
- **Displacement**: a more reactive element takes a less reactive one's place (A + BC → AC + B). It doesn't happen the other way round. In a **double displacement** two compounds swap partners (AB + CD → AD + CB), as in neutralisation: acid + base → salt + water.
- Spot the pattern to classify a reaction and predict its products.

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## Exit ticket

**Classify magnesium + zinc sulfate → magnesium sulfate + zinc, and say why it happens.**

* Displacement: magnesium is more reactive than zinc, so it takes zinc's place.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Why does heat speed up reactions? (collision theory, node 5). In class: metal strips in copper sulfate solution (zinc, iron, magnesium, copper) to build a reactivity series; decompose copper carbonate by heating; burn magnesium ribbon. -->