Single-replacement reaction — Science definition

A reaction where one element replaces another element in a compound (A + BC → AC + B).

Where it is taught

In use

In a single-replacement reaction, one lone element muscles its way into a compound and kicks out another element: A + BC → AC + B.

Types of Reactions

Questions using single-replacement reaction

Problem 1

In the single-replacement reaction Zn + 2HCl → ZnCl₂ + H₂, how many molecules of hydrogen gas (H₂) are produced for every one atom of zinc that reacts?
Answer and explanation

1 molecules

The coefficient in front of H₂ is 1, so one zinc atom releases one H₂ molecule.

Problem 2

Which of these is a single-replacement reaction, in which one element displaces another from a compound?
  1. CaCO₃ → CaO + CO₂
  2. 2H₂ + O₂ → 2H₂O
  3. Fe + CuSO₄ → FeSO₄ + Cu
  4. HCl + NaOH → NaCl + H₂O
Answer and explanation

Fe + CuSO₄ → FeSO₄ + Cu

Iron (a lone element) replaces copper in copper sulfate, matching A + BC → AC + B. Because iron is more reactive than copper, the displacement actually occurs.

Problem 3

In the activity series, a more reactive metal replaces a less reactive one. Which single-replacement reaction will actually occur?
  1. Cu + ZnSO₄ → CuSO₄ + Zn (copper is less reactive than zinc)
  2. 2H₂O → 2H₂ + O₂
  3. N₂ + 3H₂ → 2NH₃
  4. Zn + CuSO₄ → ZnSO₄ + Cu (zinc is more reactive than copper)
Answer and explanation

Zn + CuSO₄ → ZnSO₄ + Cu (zinc is more reactive than copper)

Zinc sits above copper in the activity series, so zinc displaces copper — the reverse would not happen, and the other two are not single-replacement reactions.

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