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C. Chemical Reactions · Chemical Reactions

Single Displacement: Metals and the Activity Series

C2 C3About 75 minutes
Learning goals
  • I can use the activity series to predict whether a metal will displace another metal, or hydrogen, from a compound.
  • I can predict the products of metals reacting with salt solutions, acids, cold water and steam.
  • I can write balanced equations for single displacement reactions, including metals with more than one ion charge.
  • I can explain a single displacement reaction in terms of atoms becoming ions and ions becoming atoms.

Words to know

single displacement reaction
An element reacts with a compound and takes the place of one of its elements: A + BC → AC + B.
activity series
A list of metals (and hydrogen) in order of how easily they react, built from experiments. A metal can displace any metal below it from a compound in solution.
reactivity
How readily a substance takes part in chemical reactions. For metals, it is how easily the atoms lose electrons to become positive ions.
spectator ion
An ion that is present in the solution but does not change during the reaction, such as sulfate in Zn + CuSO₄.
no reaction (NR)
What you write when the free metal is below the metal (or hydrogen) in the compound, so no displacement happens.

Cutting in on the dance floor

In a single displacement reaction, a free element "cuts in" and takes the place of an element in a compound, leaving that element on its own. Whether it can cut in depends on how reactive it is compared with the element already there.

This lesson is about metals. Metal atoms react by losing electrons to become positive ions. A more reactive metal loses electrons more easily, so it can give electrons to the ions of a less reactive metal and push that metal out of the compound. In the Matter, Chemical Trends, and Chemical Bonding unit you saw where reactivity trends come from (ionization energy and the size of atoms). Here you use the trend to predict reactions.

The activity series

Chemists have compared metals in thousands of experiments and ranked them in the activity series. The most reactive metals are at the top. Hydrogen is included because many metals react with acids by displacing hydrogen.

The rules:

  1. A metal displaces any metal below it from a compound dissolved in water. It cannot displace a metal above it (write NR, no reaction).
  2. A metal above hydrogen displaces hydrogen from an acid: metal + acid → salt + hydrogen gas.
  3. The most reactive metals (lithium to sodium) displace hydrogen from cold water: metal + water → metal hydroxide + hydrogen gas.
  4. Metals from magnesium to iron react with steam (very hot water vapour): metal + steam → metal oxide + hydrogen gas.
  5. Metals below hydrogen (copper, mercury, silver, platinum, gold) do not displace hydrogen from water or from acids like hydrochloric acid.
most reactiveLilithiumKpotassiumBabariumCacalciumNasodiumMgmagnesiumAlaluminiumZnzincFeironNinickelSntinPbleadH(hydrogen)CucopperHgmercuryAgsilverPtplatinumAugoldleast reactivereact with cold watermetal hydroxide + H₂react with steammetal oxide + H₂all above Hdisplace H₂from acidsbelow H: do not displaceH₂ from acids or water
An activity series of metals. A metal can displace any metal below it from a solution of its compound. Where exactly calcium, sodium and a few others are placed differs slightly between textbooks.

What is happening to the particles?

Put a strip of zinc into blue copper(II) sulfate solution. Each zinc atom on the surface loses two electrons and becomes a zinc ion, which moves into the solution. Each copper(II) ion that picks up two electrons becomes a copper atom and sticks to the strip as a reddish-brown coating.

The sulfate ions are there before and after, unchanged. They are spectator ions. If you leave them out you get the net ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). You will write many of these in the Solutions and Solubility unit.

beforeafter a whileZnZnZnCu²⁺Cu²⁺Cu²⁺SO₄²⁻SO₄²⁻SO₄²⁻ZnCuCuZn²⁺Zn²⁺Cu²⁺SO₄²⁻SO₄²⁻SO₄²⁻Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) (sulfate ions do not change)
Two zinc atoms have become zinc ions and left the strip, while two copper(II) ions have become copper atoms on the strip. Count them: 3 Zn and 3 Cu before and after. The blue colour of the solution fades as copper(II) ions are used up. The sulfate ions are spectators.

Predicting the products: a recipe

  1. Find both metals (or the metal and hydrogen) on the activity series. Is the free element higher? If not, write NR.
  2. Swap. The free metal becomes an ion and pairs with the anion. Use its ion charge to write a neutral formula.
  3. Release the displaced element: a metal as its symbol (Cu, Ag), hydrogen as H₂.
  4. Balance and add states.

Multivalent metals: the question will usually name the ion. When it does not, iron forms iron(II) in reactions with acids and with copper(II) sulfate solution, and tin and lead form tin(II) and lead(II).

Water as a reactant: think of it as H–OH. With cold water, the metal pairs with OH⁻: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). With steam, an oxide forms instead: Mg(s) + H₂O(g) → MgO(s) + H₂(g).

Worked example

Will it react? (metal + salt solution)

Predict: nickel metal is placed in aqueous lead(II) nitrate.

  1. Nickel is above lead in the activity series, so a reaction happens.
  2. Nickel forms Ni²⁺; with nitrate, NO₃⁻, the compound is Ni(NO₃)₂.
  3. Lead is released as Pb(s).
  4. Skeleton: Ni + Pb(NO₃)₂ → Ni(NO₃)₂ + Pb. It is already balanced (1 Ni, 1 Pb, 2 nitrate on each side).

Ni(s) + Pb(NO₃)₂(aq) → Ni(NO₃)₂(aq) + Pb(s)

Worked example

Balancing with different charges

Predict: magnesium ribbon is placed in aqueous silver nitrate.

  1. Magnesium is far above silver: reaction happens.
  2. Mg²⁺ with nitrate: Mg(NO₃)₂. Silver is released as Ag.
  3. Skeleton: Mg + AgNO₃ → Mg(NO₃)₂ + Ag. Nitrate: 2 on the right, so 2AgNO₃, then 2Ag.

Mg(s) + 2AgNO₃(aq) → Mg(NO₃)₂(aq) + 2Ag(s). Crystals of silver grow on the ribbon.

Worked example

Metals with acids and water

Predict the products, or write NR: (a) iron + hydrochloric acid; (b) silver + hydrochloric acid; (c) calcium + cold water; (d) zinc + steam.

  1. (a) Iron is above hydrogen. Iron forms iron(II) here: FeCl₂. Hydrogen is released as H₂. Fe + 2HCl → FeCl₂ + H₂.
  2. (b) Silver is below hydrogen: NR.
  3. (c) Calcium is in the cold-water group: hydroxide + hydrogen. Ca²⁺ needs two OH⁻: Ca(OH)₂. Balance H: Ca + 2H₂O → Ca(OH)₂ + H₂.
  4. (d) Zinc is in the steam group: oxide + hydrogen. Zn + H₂O(g) → ZnO + H₂.

(a) Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) (b) Ag(s) + HCl(aq) → NR (c) Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g) (d) Zn(s) + H₂O(g) → ZnO(s) + H₂(g)

Why the activity series matters outside the lab

  • Galvanized steel (used for fence posts, garbage cans and car bodies) is coated with zinc. Zinc is above iron, so the zinc corrodes first and protects the iron.
  • Ship hulls and water heaters use sacrificial anodes of magnesium or zinc for the same reason.
  • Gold, silver, platinum and copper are near the bottom of the series. They resist corrosion, which is part of why they are used in jewellery, coins and electrical connections.
  • Very reactive metals such as sodium and potassium are stored under oil to keep them away from water and air.
Hands-on activity

Copper-plate a nail with pennies

You need: about 15 old, dull Canadian pennies (Canada stopped making pennies in 2012, but many homes still have a jar of them), 60 mL of white vinegar, 5 mL (1 teaspoon) of table salt, a small glass or plastic bowl, two clean plain steel nails or uncoated paper clips (not galvanized, which are zinc-coated), fine sandpaper, paper towel, and a grown-up.

Safety: the solution is not food: do not taste it, and wash your hands afterwards. Pour it down the sink with plenty of water when you finish. Nails are sharp; handle them carefully.

  1. Stir the salt into the vinegar until it dissolves. Add the pennies and wait 5 minutes. (The salty acid dissolves the dull copper oxide on the pennies, so a little copper ends up in the solution as copper(II) ions.)
  2. Remove the pennies, rinse them, and set them on paper towel. Notice how shiny they are.
  3. Sand one nail lightly so it is clean. Put it in the solution. Keep the other nail dry as a control.
  4. Check after 10 minutes, 30 minutes and 1 hour. Compare with the control.

What to expect: a reddish-brown coating of copper forms on the nail. Iron is above copper in the activity series, so iron atoms displace copper(II) ions: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s).

Think: why does a galvanized nail work poorly here? (Its zinc coating reacts instead, and the coating changes how the surface looks.)

Just for fun

Gold sits at the very bottom of the activity series. It is so unreactive that it is often found in nature as the pure metal, which is why prospectors in the Klondike Gold Rush in Yukon could find gold flakes and nuggets in stream gravel. Iron, higher up the series, would have rusted away long before anyone found it.

Your turn

1

Predict the products or write NR: Zn(s) + Pb(NO₃)₂(aq) →

2

Predict the products or write NR: Cu(s) + ZnSO₄(aq) →

3

Predict the products or write NR: Al(s) + HCl(aq) →

4

Predict the products or write NR: K(s) + H₂O(l) →

5

Predict the products or write NR: Au(s) + HCl(aq) →

6

Predict the products or write NR: copper + silver nitrate solution.

7

Predict the products: magnesium + steam.

More practice: warm-up

1

Which is more reactive: zinc or copper?

2

Can lead displace hydrogen from an acid?

3

What gas forms when a metal reacts with an acid?

4

What forms when sodium reacts with cold water?

5

What is a spectator ion?

More practice: on your own

1

Predict or NR: tin + iron(II) chloride.

2

Predict and balance: aluminium + iron(II) sulfate solution.

3

Predict and balance: zinc + sulfuric acid.

4

Predict and balance: barium + cold water.

5

Pieces of metal X, Y and Z are put in each other's nitrate solutions. X displaces Y and Z. Y displaces nothing. Z displaces Y. Rank the metals from most to least reactive.

Challenge

1

A student puts an iron nail in copper(II) sulfate solution and another in aluminium sulfate solution. Predict what happens in each, give equations, and explain using electrons.

2

Explain why an old copper water pipe can safely carry tap water, but a sodium pipe could not exist.

3

Write the net ionic equation for magnesium reacting with silver nitrate solution. How many electrons does each Mg atom lose, and how many Ag⁺ ions does it change into Ag atoms?

Check your understanding

Quiz

1.

Which metal will displace copper from copper(II) sulfate solution?

2.

What are the products of Mg + 2HCl?

3.

Which reaction will NOT happen?

4.

Which metal reacts with cold water to produce a hydroxide and hydrogen gas?

5.

In Zn + CuSO₄ → ZnSO₄ + Cu, which ion is a spectator?

6.

Zinc protects the iron in galvanized steel because zinc:

For parents and teachers

Your teen will need the activity series for this lesson and several later ones. Most textbooks print one; the order in this lesson is fine to use.

Common difficulties:

  • Reading the series the wrong way. The free element must be higher than the one it replaces.
  • Writing the displaced metal with the wrong formula (it is just Cu or Ag, not Cu₂), or forgetting that hydrogen gas is H₂.
  • Using the wrong charge for the new ion. Ask: "What ion does this metal form?"

Activity safety: the vinegar-salt-copper solution should not be tasted; rinse hands and pour it away with lots of water. Use plain steel nails, not galvanized ones. Do not try any reaction of sodium or potassium at home.