10.3 - Reactivity Trends by Displacement

The experiment

Aim

To find the order of reactivity of four metals by testing whether each one displaces the others from solutions of their salts.

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Background

• A more reactive metal takes the place of a less reactive metal in a solution of its salt. This is a displacement reaction.
• A less reactive metal can't displace a more reactive one, so nothing happens.
• You can't see reactivity directly, but you can see whether a displacement reaction happens, from a coating on the metal or a colour change in the solution.
• So in this practical you add each metal to the other metals' salt solutions, record which ones react, and use the pattern to put the metals in order.

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Variables

• Independent variable - Which metal is added to which salt solution.
• Dependent variable - Whether a reaction happens, shown by a coating on the metal, a colour change, or a temperature rise.
• Control variables - The concentration of each salt solution, the size of each piece of metal, and the time before you record a result.

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Method

1. Clean the metals - Rub a small piece of magnesium, zinc, iron, and copper with emery paper to remove the oxide layer.
2. Set out the salt solutions - Put a few drops of magnesium sulfate, zinc sulfate, iron(II) sulfate, and copper(II) sulfate solution into separate dips of a spotting tile.
3. Add one metal - Put a piece of one metal into each of the solutions, and leave it for a few minutes.
4. Record what happens - A coating forming on the metal, the colour of the solution fading, or no change.
5. Test every metal - Rinse the tile and repeat with each of the other metals. A metal doesn't need testing in a solution of its own salt.
6. Rank the metals - The metal that displaces all the others is the most reactive, and the one displaced by all the others is the least reactive.

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Safety

• Eye protection - Wear it throughout.
• Salt solutions - They're irritants, so wash off any splashes straight away.
• Cleaned metal - Edges rubbed with emery paper can be sharp.

Common exam questions

Question 1

A metal is added to a salt solution and a reaction happens. What does this show?

The metal added is more reactive than the metal in the salt, so it has displaced it.

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Question 2

Bubbles form on a piece of magnesium in a salt solution. Does this show displacement?

Not on its own. The bubbles are hydrogen from the magnesium reacting with the water or a trace of acid, not a sign that another metal has been displaced.

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Question 3

How can you tell whether a grey coating on a metal is displaced iron?

Hold a magnet near it. Iron is attracted to a magnet, and the other metals in this practical aren't.

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Question 4

Why does chlorine displace bromine from potassium bromide solution, but bromine can't displace chlorine from potassium chloride solution?

Chlorine is more reactive than bromine, so it can take bromine's place. Bromine is less reactive than chlorine, so it can't take chlorine's place.

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Question 5

Write a balanced symbol equation for the reaction between magnesium and copper(II) sulfate solution.

Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)