S4.7.6 Chapter 6

The reactivity series

Why is gold found shining in the ground, but iron is found as dull red rock?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Gold is heavier, so it sinks out of the rock.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Iron is made by living things, and gold is not.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Gold hardly reacts with anything, but iron reacts with oxygen and is found joined to it.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Order metals by how they react with water and acid
Explain how carbon is used to extract iron from its ore

Some metals react at once with water or air. Others hardly change in a thousand years. The reactivity of a metal is how easily it takes part in reactions. We can find it by watching what a metal does with cold water and with dilute acid.

These four metals react with cold water. They make hydrogen and a metal hydroxide.

These metals react little or not at all with cold water. In dilute acid they make a salt and hydrogen. The faster the fizz, the more reactive the metal.

Put the results together and the metals fall into a list. It is called the reactivity series. Carbon and hydrogen are not metals, but they have a place in the list, because they take part in the same reactions.

Part of the reactivity series. A metal above hydrogen reacts with dilute acid. A metal below hydrogen does not.

Aluminium seems to break the rule, because it reacts little with water and acid although it is high in the list. It is covered by a thin, tough skin of aluminium oxide, which forms within seconds in air. The skin keeps the acid and water away from the metal below.

The series also tells us about displacement. A more reactive metal pushes a less reactive metal out of its compound. Iron put into copper sulfate solution takes the place of the copper: Fe + CuSO₄ → FeSO₄ + Cu. Copper is left as a solid. Copper put into iron sulfate does nothing, because copper is below iron.

Most metals are found in rocks as compounds, mixed with other minerals. A rock with enough of a metal compound to be worth mining is an ore. Iron ore is mostly iron oxide, Fe₂O₃. To get the metal, we must take the oxygen away.

Taking oxygen away is reduction. Adding oxygen is oxidation. Carbon is more reactive than iron. So carbon can take the oxygen from iron oxide and leave the iron. Metals above carbon, such as aluminium, hold their oxygen too tightly for carbon to take it.

See it three ways

As a picture

In words

Carbon is above iron in the series. So carbon wins the oxygen from iron oxide and the iron is left. Carbon is below aluminium, so it cannot do the same for aluminium oxide.

In symbols

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Iron is made in a blast furnace, a tower about 30 metres tall. Iron ore, coke and limestone go in at the top. Coke is nearly pure carbon. Hot air is blown in near the bottom.

The blast furnace. The slag floats on the iron, because it is less dense.

Three things happen in the furnace. The coke burns in the hot air and makes heat: C + O₂ → CO₂. Then the carbon dioxide meets more hot coke and makes carbon monoxide: CO₂ + C → 2CO. The carbon monoxide takes the oxygen from the ore: Fe₂O₃ + 3CO → 2Fe + 3CO₂.

The three reactions. In the last, the iron oxide is reduced and the carbon monoxide is oxidised.

Worked example

Balance the main reaction in the furnace: Fe₂O₃ + CO → Fe + CO₂

Count Fe: 2 on the left. Write 2 in front of Fe.
Fe₂O₃ gives away its oxygen atoms. Each CO takes one and becomes CO₂. Fe₂O₃ has 3 oxygen atoms, so 3 CO are needed.
Write 3 in front of CO and 3 in front of CO₂.
Check: C is 3 and 3. O is 3 + 3 = 6 on the left and 3 × 2 = 6 on the right.

Answer: Fe₂O₃ + 3CO → 2Fe + 3CO₂

Rock has impurities. The main one is sand, which is silicon dioxide. The limestone breaks down in the heat: CaCO₃ → CaO + CO₂. The calcium oxide then joins the sand and makes calcium silicate, called slag. Slag melts and floats on the iron. It is tapped off and used to make roads.

Metals above carbon cannot be extracted this way. They are extracted with electricity. That is the topic of the next chapter.

Try it

Which metal fizzes in vinegar?

You need: two plain iron nails, or a small pad of plain steel wool, a short piece of bare copper wire, if you have one; if not, do the iron cup only, two clear cups, vinegar and a spoon, a hand lens, if you have one.

Be safe: Nails and steel wool can cut skin, so use the spoon for steel wool. Keep vinegar away from your eyes. Taste nothing, and wash your hands afterwards.

Pour vinegar into the cups, about a hand deep.
Put the nails into one cup. If you use steel wool, push it in with the spoon and do not touch it with your fingers.
If you have copper wire, put it into the other cup.
Look closely at both after a few minutes, and again after some hours.

What you will see: Tiny bubbles of hydrogen gather on the iron. The copper wire, if you used it, stays bare. Iron is above hydrogen in the series, so it makes hydrogen with an acid. Copper is below hydrogen, so it does not. The bubbles are small and slow, because vinegar is a weak acid.

If you cannot try it, this is what happens.

A wrong idea many people have

Aluminium does not seem to react, so it must be low in the reactivity series.

Why it feels right: Aluminium pans and foil last for years and do not rust. A metal that does not change looks unreactive.

The test that breaks it: Aluminium is found in rock only as a compound, never as a metal. It is very hard to extract. A really unreactive metal, such as gold, is found as a pure metal.

What is true: Aluminium is high in the series. A thin tough oxide skin forms at once and protects the metal under it.

Key words

reactivity: how easily a metal takes part in reactions.
displacement: a more reactive metal pushing a less reactive one out of its compound.
ore: a rock with enough of a metal compound to be worth mining.
reduction: the loss of oxygen by a substance.
oxidation: the gain of oxygen by a substance.
blast furnace: a tall furnace that makes iron from iron ore, coke and limestone.
slag: the waste that floats on the molten iron, mostly calcium silicate.

Back to your guess

The answer is C: Gold hardly reacts with anything, but iron reacts with oxygen and is found joined to it.

Gold is very unreactive, so it stays a pure metal. Iron reacts with oxygen, so in the ground it is found as a compound, iron oxide, and has to be extracted.

Keep it

Copy this into your exercise book.

A more reactive metal displaces a less reactive one. Carbon is above iron, so it removes the oxygen from iron oxide: reduction. Aluminium is above carbon.

Draw: The column of metals from potassium to gold with carbon and hydrogen in place, and Fe₂O₃ + 3CO → 2Fe + 3CO₂ underneath.