In Sorting Five Reactions you were given five reactions and no categories, and asked to group them however the evidence suggested. Groups sorted by whether a gas came off, by whether the mixture got hot, by whether the colour changed. Those are all real patterns and none of them is the one chemists use.
The accepted scheme sorts by what happens to the arrangement of the atoms. It is a less obvious sort, and it earns that by being the one that lets you predict a reaction you have never run — which is what Predicting Products does with it next class.
The five patterns
| Type | General form | What is happening | Balanced example |
|---|---|---|---|
| Synthesis | two substances combine into one | ||
| Decomposition | one substance breaks into two or more | ||
| Single displacement | an element takes another element’s place | ||
| Double displacement | two compounds exchange partners | ||
| Combustion | fuel plus oxygen | a fuel burns, releasing energy |
Read the general forms as shapes rather than as formulas. What makes something a single displacement is that a lone element went in and a different lone element came out; the identity of the compound is beside the point.
Balancing is a claim about atoms, and now about moles
An unbalanced equation is not a rough draft of a balanced one. It is a false statement — it says atoms appeared or vanished, and they did not.
The only thing you may change is the coefficients, the numbers in front. Subscripts are part of the identity of a substance: and are water and hydrogen peroxide, and turning one into the other to make the arithmetic work is not balancing an equation, it is changing what reacted. This is the single most common way a balancing attempt goes wrong.
A method that rarely fails:
- Balance the element that appears in the fewest places first.
- Treat a polyatomic ion that survives the reaction intact — sulfate, nitrate — as a single unit and balance it as one item.
- Leave oxygen and hydrogen until last, since they usually appear in several places.
- If you end up needing a fraction, finish with it and then multiply the whole equation through to clear it.
Here is what changed since Grade 10. Then, balancing meant mass is conserved, and that was the whole of it. Now the coefficients carry a second meaning: they are the ratio in which the particles react, and therefore the ratio in which moles react. Reading , the 3 in front of the carbon dioxide says that burning one mole of propane produces three moles of carbon dioxide — a number you can weigh out. Everything in Stoichiometry rests on that reading, and none of it survives an equation that was not balanced first.
Where the categories overlap, and what they leave out
Two honest complications, because both will come up and both confuse people who were taught the table as though it were airtight.
The categories overlap. Magnesium burning in oxygen is a synthesis reaction and a combustion reaction simultaneously. Both labels are correct, and a question that asks you to pick one is asking a badly formed question. Say both, and say why.
This scheme does not cover all of chemistry. It covers the reactions you meet this year, which is a smaller and more honest claim. Beneath it sits a deeper classification: single displacement and combustion are both cases of electrons moving from one substance to another, which is a scheme you will meet if you continue to Grade 12. Double displacement reactions, by contrast, involve no electron transfer at all — the ions simply swap company. That the five types split into two groups on a criterion nobody mentioned is a sign that the five-type scheme is a useful surface rather than the last word.
Practise identifying and balancing in Reaction Types Practice. Then Predicting Products turns the classification around: given only the reactants, what comes out — and what do you do when the prediction is wrong?
Curriculum connection
C3.1
identify various types of chemical reactions, including synthesis, decomposition, single displacement, double displacement, and combustion
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C2.2
write balanced chemical equations to represent synthesis, decomposition, single displacement, double displacement, and combustion reactions, using the IUPAC nomenclature system [PR, AI, C]
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