Chemistry · Grades 9–12

Chemical reactions · Quick reference

Reference sheet for Grades 9 to 12: balancing chemical equations, classifying chemical reactions and conservation of mass in reactions. Atoms are rearranged, not created or destroyed, so each element must appear the same number of times on both sides.

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Rule and key idea

Balancing chemical equations

Atoms are rearranged, not created or destroyed, so each element must appear the same number of times on both sides. Change only coefficients, never subscripts, and use the smallest whole numbers.

atoms of each element: left side = right side

Balance this equation using the smallest whole-number coefficients: __ Fe₂O₃ + __ CO → __ Fe + __ CO₂. What is the sum of all the coefficients? → 9

Classifying chemical reactions

Compare the number and kind of reactants and products, then match the pattern.

A + B → AB · AB → A + B · A + BC → AC + B · AB + CD → AD + CB · fuel + O₂ → CO₂ + H₂O

Classify this reaction: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O → combustion

Conservation of mass in reactions

Atoms are only rearranged in a reaction, so mass is conserved. If mass seems to change, a gas has escaped or been taken in from the air.

in a closed system: total mass of reactants = total mass of products

In a sealed container, 28.6 g of substance A reacts completely with 7.7 g of substance B. The only product is substance C. What mass of C forms? → 36.3

Check yourself

Four practice questions on this topic. Try each one before you open the answer.

Balance this equation using the smallest whole-number coefficients: __ SO₂ + __ O₂ → __ SO₃. What is the sum of all the coefficients?
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5 2 SO₂ + O₂ → 2 SO₃: 2 + 1 + 2 = 5
Balance this equation using the smallest whole-number coefficients: __ C₂H₅OH + __ O₂ → __ CO₂ + __ H₂O. What is the coefficient of CO₂?
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2 C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O
Classify this reaction: BaCl₂ + Na₂SO₄ → BaSO₄ + 2 NaCl
- synthesis
- single displacement
- decomposition
- double displacement
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double displacement Double displacement: two compounds exchange partners (AB + CD → AD + CB).
Classify this reaction: Pb(NO₃)₂ + 2 KI → PbI₂ + 2 KNO₃
- double displacement
- combustion
- synthesis
- decomposition
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double displacement Double displacement: two compounds exchange partners (AB + CD → AD + CB).