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.
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.
You can balance equations and follow atoms through a reaction. This lesson sorts reactions into patterns.
Make your prediction first. Help opens once you've locked it in.
Magnesium burning: two substances join into one.
Hydrogen peroxide breaking down: one substance into two.
Zinc and copper oxide: one element swaps into a compound.
Run the synthesis until the target is reached. How many substances go in, and how many come out?
Use the board. This ticks when you get there.
Now the decomposition.
Use the board. This ticks when you get there.
Now the displacement.
Use the board. This ticks when you get there.
In your book: write one example of each kind of reaction as a word equation, and draw the pattern underneath (A + B → AB and so on).
Match each pattern to its name.
Which kind of reaction is it?
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
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.
Open the teacher guide: answers, solutions, slides and a worksheet.
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.
Classifying an unfamiliar equation. When a student guesses, ask them to count the substances on each side and say whether anything swapped, before naming the type.
Stretch: Thermite (aluminium + iron oxide) is used to weld railway lines. Write the word equation and say which type of reaction it is. (aluminium + iron oxide → aluminium oxide + iron: displacement, because aluminium is more reactive than iron.)
Support: Give the three patterns on a card with a box for 'how many in, how many out, did anything swap?'
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.