KS3 · Chemistry

Displacement of metals

Drop grey magnesium into blue copper sulfate solution and orange-brown copper starts to appear. But nobody added any copper. So where did it come from?

Chemistry · Reactivity

Will it react? Predict before you mix

Reactivity series, most reactive first: magnesium, zinc, iron, copper. Calcium is also more reactive than copper. For each pair, compare the element you add with the metal in the compound. Will a displacement reaction happen?

Still to sort

Displacement happens (0)

The element added is more reactive than the one in the compound.

Where the line is: It doesn't matter which one you add. All that matters is which is more reactive. The more reactive one ends up in the compound.

No reaction (0)

The element added is less reactive than the one in the compound.

Where the line is: A less reactive element cannot take the place of a more reactive one. We write 'no reaction'.

7 of 7 still to sort.

Watch out: Swap the pair round and the answer flips. Iron + copper sulfate reacts; copper + iron sulfate doesn't.

Predict, then check

Copper sulfate dissolved in water makes a blue solution. It's called aqueous copper sulfate, or copper sulfate solution. 'Aqueous' just means dissolved in water.

A strip of grey magnesium is dropped into blue copper sulfate solution. What will you see?

Where did the copper come from?

The iron nail puzzle

An iron nail is left in blue copper sulfate solution. Later, the nail is coated in an orange-brown solid: copper.

Which is closest to what you think right now?
How sure are you?

Writing word equations

Your turn to fill the gaps

Zinc is added to copper sulfate solution. Does a displacement reaction happen? If it does, write the word equation. Then try it the other way round.

  1. Find the two metals: zinc (added) and copper (in the compound, copper sulfate). Zinc is above copper in the reactivity series.
  2. missing step
Which line is step 2?

The spotting-tile practical

Rank four metals from your own results

In the practical, each row of a spotting tile gets a different metal salt solution, added with a dropper pipette. The same metal is added with a spatula to every solution in a column, and you record careful observations. A spotting tile is ideal: the chemicals combine easily and accurately, it uses less reactant than test tubes, and it's easy to label if you stand it on a sheet of paper. Best of all, you can watch every reaction at the same time. In this grid each metal has its own row. Tick every salt solution that metal displaces a metal from. (A metal added to its own salt has nothing to swap with, so that one's a 'no'.)

Magnesium
Zinc
Iron
Copper

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The copper that appears on the nail was never in the nail.

What you need to know

  • A displacement reaction happens when a more reactive element takes the place of a less reactive element in a compound. Metals do this, and so do other elements.
  • A more reactive metal displaces a less reactive metal from its salt, in solution. A less reactive metal cannot displace a more reactive one, so there is no reaction.
  • The higher a metal is in the reactivity series, the more reactive it is. Use the series to predict whether displacement will happen.
  • An aqueous solution is a substance dissolved in water. Copper sulfate dissolved in water is a blue solution: aqueous copper sulfate, or copper sulfate solution.
  • Word equations: more reactive metal + metal salt → new salt + displaced metal, e.g. magnesium + zinc sulfate → magnesium sulfate + zinc.
  • The displaced metal is made from the metal salt in the solution, not from the metal you added.
  • Signs of displacement: a colour change in the solution or the metal, a temperature change, and effervescence (fizzing).
  • Carbon can extract some metals from their ores when carbon is more reactive than the metal. This is also a displacement reaction.

The big picture

In a displacement reaction, a more reactive element takes the place of a less reactive one in a compound, so magnesium + copper sulfate → magnesium sulfate + copper. The reactivity series predicts it: the higher a metal, the more reactive, and a less reactive metal cannot displace a more reactive one (copper + zinc sulfate → no reaction). The displaced metal comes from the salt in the solution, not the metal you added. Colour changes, temperature changes and fizzing are the signs, and a spotting-tile practical confirms the order magnesium, zinc, iron, copper.

Key points

1More reactive takes the place of less reactive, never the other way round.
2Magnesium + copper sulfate: the blue fades to colourless magnesium sulfate solution, grey magnesium becomes orange-brown copper, and the temperature rises slightly.
3That temperature rise happens because the reaction is exothermic: energy is released to the surroundings.
4Iron nail in copper sulfate solution: the copper that appears comes from the solution, not the nail.
5Copper + zinc sulfate → no reaction, and copper will not displace calcium from calcium nitrate solution.
6In the spotting-tile practical, magnesium displaced all three other metals, zinc displaced copper and iron, and iron displaced only copper. So the order is magnesium, zinc, iron, copper, which agrees with the reactivity series.

Worked example

Problem

Two test tubes each hold iron sulfate solution. Magnesium is added to tube A and copper to tube B. Which tube reacts? Write the word equation for the reaction that happens.

⚠ Watch out

Thinking the displaced metal comes from the metal you added, as if the iron nail turned into copper. It's the other way round: the copper was in the copper sulfate solution, and the iron has taken its place in the compound.

🧠

Memory hook

Think of the compound as a seat. The more reactive element takes the seat, and the less reactive one is left standing as the free metal. A weaker element can't take a stronger one's seat. It's only a picture, but it tells you who ends up where.

✓

Check yourself

Copper is added to magnesium sulfate solution. Predict what you'd see, and explain your prediction using the reactivity series.

Flashcards

(14)
What is a displacement reaction?
A reaction where a more reactive element takes the place of a less reactive element in a compound.
Which metal can displace which from a salt solution?
A more reactive metal displaces a less reactive metal from its salt in solution. A less reactive metal can't displace a more reactive one.
What does the position of a metal in the reactivity series tell you?
The higher it is, the more reactive it is. That lets you predict whether a displacement reaction will happen.
What does 'aqueous' mean?
Dissolved in water. Aqueous copper sulfate is copper sulfate dissolved in water, and it's blue.
What do you see when magnesium is added to copper sulfate solution?
The blue fades, grey magnesium turns into orange-brown copper, and the temperature rises slightly.
Why does the temperature rise when magnesium reacts with copper sulfate?
The reaction is exothermic: it releases energy to the surroundings.
Name three signs that a displacement reaction has happened.
A colour change (of the solution or the metal), a temperature change, and effervescence (fizzing).
An iron nail in copper sulfate solution goes orange-brown. Where did the copper come from?
From the copper sulfate in the solution. Iron took copper's place in the compound. The nail didn't turn into copper.
What is the pattern for a displacement word equation?
More reactive metal + metal salt → new salt + displaced metal.
Complete: iron + copper sulfate → ?
Iron sulfate + copper.
How is a spotting tile set up for the displacement practical?
Each row gets a different metal salt solution (dropper pipette). The same metal goes into every solution in a column (spatula). Then record careful observations.
Give two reasons a spotting tile suits this practical.
Any two: uses less reactant than test tubes; chemicals combine easily and accurately; easy to label on a sheet of paper; you can watch all the reactions at once.
What order did the spotting-tile results give, and how?
Magnesium, zinc, iron, copper (most reactive first). Magnesium displaced three metals, zinc two, iron one, copper none.
When can carbon be used to extract a metal from its ore?
When carbon is more reactive than the metal. Carbon displaces the metal from its compound.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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