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C. Chemistry: Chemical Reactions · Chemical Reactions

Single Displacement Reactions and the Activity Series

C2.3 C3.5 C3.4 C2.1 A1.1 A1.8About 70 minutes
Learning goals
  • I can recognize a single displacement reaction (A + BC → AC + B).
  • I can use the activity series to predict whether a single displacement reaction will happen.
  • I can write balanced equations for metals reacting with ionic compounds, acids and water, and for halogen displacements.
  • I can describe observations of the reaction between iron and copper(II) sulfate.

Words to know

single displacement reaction
A reaction in which an element replaces another element in a compound: A + BC → AC + B.
activity series
A list of metals ranked from most to least reactive, based on experiments.
reactivity
How easily a substance takes part in chemical reactions.
no reaction
The result when the free element is less reactive than the element it would have to replace.
limiting reactant
The reactant that is used up first, which stops the reaction.

Cutting in

Imagine two dance partners on the floor. A third person cuts in, takes one partner's hand, and the other partner is left standing alone. That is what happens in a single displacement reaction: an element "cuts in" and takes the place of another element in a compound.

Your curriculum's example is a classic: put an iron nail into blue copper(II) sulfate solution, and the nail becomes coated with copper while the solution changes colour. In this lesson you will learn to predict when this kind of reaction happens and write equations for it (C2.3, C3.4, C3.5).

A+BCAC+Belementcompoundnew compoundnew elementA + BC → AC + BA takes B's placeonly if A is morereactive than B
In single displacement, a free element (A) replaces one element (B) in a compound. Element B is pushed out on its own.

Single displacement: A + BC → AC + B

In a single displacement reaction, an element reacts with a compound. The element takes the place of one of the elements in the compound, and the element that was replaced is set free.

  • Usually a metal replaces another metal (or replaces hydrogen in an acid or in water).
  • Sometimes a halogen (nonmetal) replaces another halogen.

Iron and copper(II) sulfate:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Observations: a reddish-brown coating of copper forms on the grey iron, and the blue colour of the copper(II) sulfate solution fades (iron(II) sulfate solution is very pale green). These are evidence of a chemical reaction.

Startgrey iron nail inblue copper(II) sulfatetimeAfter a whilecopper coats the nail;blue colour fadescoating ofcopper metaliron(II) sulfatein solution
Iron displaces copper from copper(II) sulfate. Copper metal forms on the nail, and the solution loses its blue colour as copper ions are replaced by iron(II) ions. (Shading shows the change, not the exact colours.)

Will it react? The activity series

A single displacement only happens if the free element is more reactive than the element it replaces. Chemists have ranked metals by reactivity in an activity series, based on many experiments.

The rule: a metal can displace any metal below it in the series from a compound in solution. It cannot displace a metal above it.

  • Iron is above copper, so iron displaces copper: Fe + CuSO₄ → FeSO₄ + Cu. ✔
  • Copper is below iron, so copper does not displace iron: Cu + FeSO₄ → no reaction.
  • Metals above hydrogen displace hydrogen from acids: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
  • The most reactive metals (potassium, sodium, calcium) even displace hydrogen from cold water: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). (This is why sodium is stored under oil.)
  • Copper, silver and gold are below hydrogen, so they do not react with most acids. That is one reason they are used for coins and jewellery.

Halogens have their own series: F₂ > Cl₂ > Br₂ > I₂ (most to least reactive, the same order as going down group 17). Chlorine displaces bromine: Cl₂(g) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq). But iodine cannot displace chlorine.

potassium Ksodium Nacalcium Camagnesium Mgaluminium Alzinc Zniron Fenickel Nitin Snlead Pb(hydrogen H)copper Cusilver Aggold Aumorereactivereact with acids andeven with cold waterreact with acidsto make hydrogen gasdo not react withmost acids
A simplified activity series. A metal can displace any metal below it. Metals above hydrogen can release hydrogen gas from acids; the top three also react with cold water.

Writing the products

  1. Decide if a reaction happens (activity series).
  2. Swap the element into the compound. Use ion charges to write the new formula: the new compound must be neutral.
  3. Write the displaced element correctly: a metal as a single symbol (Cu), hydrogen and halogens as diatomic molecules (H₂, Br₂).
  4. Balance the equation.

For iron, use Fe²⁺ (iron(II)) in these reactions unless told otherwise.

Worked example

Example 1: Copper and silver nitrate

A copper wire is placed in silver nitrate solution. Predict whether a reaction occurs; if so, write the word equation and balanced equation and describe what you would see.

  1. Copper is above silver in the activity series, so copper displaces silver: a reaction occurs.
  2. Copper forms Cu²⁺; nitrate is NO₃⁻, so the new compound is Cu(NO₃)₂. Silver is released as Ag.
  3. Word: copper + silver nitrate → copper(II) nitrate + silver.
  4. Skeleton: Cu + AgNO₃ → Cu(NO₃)₂ + Ag. Nitrate: 2 on the right, so 2AgNO₃; then 2Ag.
  5. Balanced: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s).

A reaction occurs. You would see shiny grey-white silver crystals grow on the copper wire, and the colourless solution slowly turns blue as copper(II) ions form.

Worked example

Example 2: Predict "no reaction"

Will these react? (a) Ag(s) + HCl(aq), (b) Mg(s) + ZnSO₄(aq), (c) Br₂(aq) + NaCl(aq)

  1. (a) Silver is below hydrogen, so it cannot displace hydrogen from an acid: no reaction.
  2. (b) Magnesium is above zinc: a reaction occurs. Mg(s) + ZnSO₄(aq) → MgSO₄(aq) + Zn(s).
  3. (c) Bromine is less reactive than chlorine, so it cannot displace chloride: no reaction.

(a) no reaction; (b) Mg + ZnSO₄ → MgSO₄ + Zn; (c) no reaction.

Worked example

Example 3: A metal and an acid

Write the balanced equation for aluminium reacting with hydrochloric acid, and name a test for the gas.

  1. Aluminium is above hydrogen, so it displaces hydrogen.
  2. Al³⁺ and Cl⁻ → AlCl₃. Hydrogen is released as H₂.
  3. Skeleton: Al + HCl → AlCl₃ + H₂.
  4. Cl: 3 on the right → 3HCl, but then H = 3 on the left and 2 on the right. Use 6HCl → 2AlCl₃ + 3H₂ (6 H = 6 H; 6 Cl = 6 Cl). Then Al: 2Al.

2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g). The hydrogen gas makes a pop with a burning splint (lab test).

Hands-on activity

Copper-plate a nail

You need: a grown-up, eye protection, about 15 to 20 copper-coloured coins (Canadian pennies or other copper-coloured coins), 1/4 cup of white vinegar, 1 teaspoon of salt, a non-metal bowl or glass, 2 clean steel nails (or steel paper clips), a paper towel, dish soap.

Safety: wear eye protection. Do not drink the solution; it contains dissolved copper. Wash your hands and the coins afterwards.

  1. Wash the nails with dish soap, rinse and dry them, so the surface is clean.
  2. Stir the salt into the vinegar until it dissolves. Add the coins and leave them for 5 to 10 minutes. Notice what happens to the coins. (The vinegar and salt dissolve the dull copper oxide on their surfaces, putting a small amount of copper ions into the solution.)
  3. Remove the coins and rinse them. Keep the liquid.
  4. Put one nail in the liquid so that half of it is under the surface. Leave the other nail on a paper towel as a control.
  5. Check after 10 minutes, 30 minutes and 1 hour. Record what you see on the part of the nail under the liquid and above it, and look for tiny bubbles.

What to look for: the coins become shiny. The part of the nail under the liquid turns reddish-brown or coppery, while the part above stays grey, and the control nail does not change. Iron is above copper in the activity series, so it displaces copper ions from the solution, and copper metal forms on the nail. You may also see tiny bubbles: iron is above hydrogen, so it also slowly reacts with the acid in the vinegar to make hydrogen. Clean-up: pour the solution down the drain with lots of water and wash everything.

Just for fun

Gold sits at the very bottom of the activity series, so it barely reacts with anything. Gold coins can be recovered from centuries-old shipwrecks and still shine. Gold is the cat of the periodic table: completely unbothered by everything around it.

Your turn

1

Write the general pattern for a single displacement reaction.

2

Will zinc react with copper(II) sulfate? If so, write the balanced equation.

3

Will copper react with zinc sulfate? Explain.

4

What gas is produced when magnesium reacts with hydrochloric acid? Write the balanced equation.

5

Will chlorine displace iodine from potassium iodide? Write the equation if it does.

6

Describe two observations when an iron nail is placed in copper(II) sulfate solution.

7

Why are coins and jewellery often made of metals near the bottom of the activity series?

More practice: warm-up

1

Which is more reactive, magnesium or iron?

2

Which metals in the series react with cold water?

3

Does gold react with hydrochloric acid?

4

Which halogen is the most reactive?

5

In Fe + CuSO₄ → FeSO₄ + Cu, which element is displaced?

More practice: on your own

1

Predict and balance: Al(s) + CuCl₂(aq) → ?

2

Predict: Pb(s) + MgCl₂(aq) → ?

3

Predict and balance: Ca(s) + H₂O(l) → ?

4

Predict and balance: Zn(s) + AgNO₃(aq) → ?

5

A student puts a strip of metal X into lead(II) nitrate solution and grey crystals of lead form. She puts metal X into magnesium nitrate solution and nothing happens. Where is X in the activity series?

Challenge

1

Plan an inquiry to put three unknown metals, P, Q and R, in order of reactivity using only solutions of their compounds. What would you do, and how would you interpret the results?

2

Explain why the copper-plated nail in the activity eventually stops getting more copper, even though there is plenty of iron left.

3

Write the balanced equation for iron reacting with silver nitrate, using iron(III), and count the atoms to check.

Check your understanding

Quiz

1.

Which equation shows a single displacement reaction?

2.

Using the activity series, which reaction will occur?

3.

What are the products of Fe(s) + CuSO₄(aq)?

4.

Which metal would NOT produce hydrogen gas with hydrochloric acid?

5.

Which halogen can displace bromine from sodium bromide?

6.

What is the balanced equation for sodium reacting with water?

For parents and teachers

Big idea: C2.3 and C3.5 ask students to investigate displacement reactions and describe their reactants and products from observations; the curriculum's example is "the reaction of iron and copper sulphate". The activity series lets students predict whether a single displacement will happen.

Common difficulties:

  • Ignoring the activity series and assuming every combination reacts.
  • Writing the new compound's formula without checking ion charges (Al with CuCl₂ gives AlCl₃, not AlCl₂).
  • Forgetting that displaced hydrogen and halogens are diatomic (H₂, Br₂).
  • Balancing metal + acid reactions, where the H and Cl must both be tracked.

Safety at home: the copper-plating activity is low-risk, but please supervise: the solution contains dissolved copper and must not be swallowed, everyone wears eye protection, and hands are washed afterwards. Do not try sodium + water or any reaction with strong acids at home.