☰ Year plan · Science

Types of reactions and the reactivity series (A)

Week 1 · Unit 1 Chemical Reactions and Their Environmental Impacts, opener; Lesson 1 Types of Chemical Reactions (A): direct combination, decomposition, metals with oxygen, acids and water, the chemical reactivity series · 3 lessons
3

Metals and water, and one metal pushing out another

Science Prep 3 Term 1, Unit 1: pp. 1–8
GoalCompare metals with water, and predict when one metal can replace another from a salt solution.
New words
salt solution · a salt dissolved in water, such as copper sulphate solutiondisplace · to push out and take the place of another element in a compoundless dense · having less mass in the same volume; a less dense solid floats on a liquid
Explanation

Metals high in the series react with water. Sodium floats on water, because it is less dense, and reacts at once and violently: 2Na + 2H₂O → 2NaOH + H₂. This is only to read about: nobody should ever put sodium or any real metal in water. Magnesium needs warm water to react, so it is less reactive than sodium. A second use of the series is replacement from a salt solution: a more reactive metal takes the place of a less reactive metal in its salt. Zn + CuSO₄ → ZnSO₄ + Cu. In a laboratory the blue colour fades and red-brown copper appears on the zinc. Keep three things apart: the metal zinc, its ion Zn²⁺ and its compound zinc sulphate. In the reverse case, copper in zinc sulphate solution, nothing happens, because copper is less reactive than zinc.

Zn + CuSO4→ZnSO4 + Cu
zinc is more reactive than copper, so it takes the place of copper
Worked examples
Fe + CuSO4→FeSO4 + Cu
Will iron nails placed in copper sulphate solution react? Fe is above Cu, so yes: Fe + CuSO₄ → FeSO₄ + Cu. A red-brown coating forms on the nail.
Will copper wire placed in zinc sulphate solution react? Cu is below Zn, so no: no new substances form and the colour stays the same.
MetalSalt solutionReaction?
MgZnSO₄?
ZnMgSO₄?
MgCuSO₄?
Predict with a table: Mg in ZnSO₄, Zn in MgSO₄, Mg in CuSO₄. Mg is above Zn and Cu, and Zn is below Mg, so the answers are yes, no, yes.
Class activity

Who pushes out whom? Make cards for the metals Mg, Zn, Fe, Cu, Ag and for their salt solutions (for example, ZnSO₄ for zinc). Two players draw a metal card and a salt card. Predict yes or no from the series, then write the equation or “no reaction” on the back of the card. A grown-up checks. No real chemicals are used: the point is to practise reasoning from the series.

Practice
1
Name the gas given off when sodium reacts with water, and write the word equation.
2
Magnesium needs warm water to react but sodium reacts at once. Which is more reactive?
3
Will Zn react with CuSO₄ solution? Write the equation.
Zn + CuSO4→?
4
Which is the metal, which is the ion and which is the compound: Zn, Zn²⁺, ZnSO₄?