2dReactivity series

The reactivity series ranks metals by how readily they react. Almost every question in this sub-topic depends on knowing the order, so it is worth learning properly.

Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, Zinc, Iron, Copper, Silver, Gold

Most reactive at the top, least at the bottom.

Two non-metals are placed in it as reference points, and both matter:

• Carbon sits between aluminium and zinc.
• Hydrogen sits between iron and copper.

Reaction with water

Only the top of the series reacts with cold water: potassium, sodium, lithium and calcium.

Metals further down do not. Magnesium reacts with steam; iron rusts slowly in air and water; copper does nothing at all.

Reaction with dilute acid

metal + acid → salt + hydrogen

A metal only produces hydrogen if it is above hydrogen in the series.

So magnesium fizzes vigorously in dilute hydrochloric acid, zinc fizzes slowly, and copper does not react at all — because copper is below hydrogen.

That last result is evidence, not a failed experiment. "No reaction" places copper below hydrogen just as surely as fizzing places zinc above it.

Comparing rates fairly

If you are comparing metals with acid, only the metal may change. Keep the same:

• volume and concentration of acid
• mass and surface area of metal
• temperature

Surface area matters as much as mass. Powder reacts far faster than a single lump of the same mass, so comparing powder with ribbon tells you about particle size, not reactivity.

A more reactive metal displaces a less reactive one from its compound.

That single rule covers both types of displacement you need.

Metal with a metal salt solution

Put zinc into blue copper(II) sulfate solution and two things happen:

• A brown solid forms on the zinc — that is copper.
• The blue colour fades — the copper ions are leaving the solution.

Zn + CuSO₄ → ZnSO₄ + Cu

Zinc is above copper, so it takes copper's place. The sulfate group stays intact throughout and simply changes partner.

A good answer accounts for both observations, because they come from opposite halves of the same reaction: the solid appearing is the copper deposited, the colour fading is the copper ions removed.

Metal with a metal oxide

Heat magnesium powder with copper(II) oxide and a vigorous reaction gives magnesium oxide and copper.

Heat copper powder with magnesium oxide and nothing happens, because copper is less reactive than magnesium and cannot take its oxygen.

Both results together prove the order. Questions often give you the pair for exactly that reason, and an answer covering only the reaction that worked has used half the evidence.

Predicting

Compare the metal being added with the metal in the compound:

• Added metal higher in the series → displacement happens.
• Added metal lower → no reaction.

Magnesium added to zinc sulfate reacts. Copper added to zinc sulfate does not.

Where metals are found

Gold is found in the ground as the uncombined element; iron only as an ore.

That is displacement logic applied to geology. An unreactive metal never combined with anything in the first place, so it is still sitting there as the metal. Where a metal is found is direct evidence of its reactivity.

What rusting needs

Iron rusts only when oxygen and water are both present. Remove either one and rusting stops.

That is why the classic three-test-tube experiment works:

Each control tube removes exactly one requirement. When answering, say which substance is missing in each tube — that is where the marks are.

Salt does not cause rusting but it speeds it up, which is why cars rust faster near the sea.

Barrier methods

Painting, oiling, greasing and coating with plastic all work the same way: they keep oxygen and water away from the iron surface.

Their weakness is obvious once stated — scratch the coating and the iron underneath is exposed, and rusting begins there.

Sacrificial protection

A more reactive metal is attached to the iron. Because it is more reactive, it corrodes in preference to the iron, and the iron is protected even where it is exposed.

Blocks of magnesium on a ship's steel hull work this way. They are consumed over time and must be replaced — which is what "sacrificial" means.

Galvanising

Coating iron with zinc gives both protections at once:

1. A barrier, while the coating is intact.
2. Sacrificial protection, because zinc is more reactive than iron — so even a scratched surface is protected.

Why tin plating behaves differently

Tin is less reactive than iron. A tin-coated can works as a barrier, but scratch it and the iron corrodes preferentially, concentrated at the small exposed area — so it rusts faster than bare steel would.

The coating metal's position in the reactivity series is everything. More reactive protects; less reactive makes it worse.

There are two definitions of oxidation and reduction, and questions use both. Read which one is being asked for.

In terms of oxygen

In Mg + CuO → MgO + Cu, the magnesium gains oxygen so it is oxidised; the copper(II) oxide loses oxygen so it is reduced.

In terms of electrons

OIL RIG — Oxidation Is Loss, Reduction Is Gain — of electrons.

In Zn + Cu²⁺ → Zn²⁺ + Cu, zinc loses two electrons so it is oxidised; the copper ions gain two so they are reduced.

Redox

Both happen together, always. Electrons lost by one substance are gained by another, so there is no such thing as oxidation on its own.

A reaction where both occur is a redox reaction — and a full answer identifies both halves, not just one.

Oxidising and reducing agents

This is where most marks are lost, because the naming feels backwards:

In Mg + CuO → MgO + Cu, magnesium is the reducing agent — it removes the oxygen from the copper(II) oxide, and is itself oxidised in doing so.

The test: work out what happens to each substance first, then name the agents. Trying to name them directly is how the reversal creeps in.

In the blast furnace

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Follow the oxygen. Iron(III) oxide loses it, so it is reduced. Carbon monoxide gains it, so it is oxidised — making carbon monoxide the reducing agent.

Dilute hydrochloric and sulfuric acids are reacted with magnesium, zinc and iron, and the rates compared.

acid + metal → salt + hydrogen

Apparatus

Test tubes or small conical flask, measuring cylinder, splints, balance.

Method

1. Measure equal volumes of acid of the same concentration into each tube.
2. Add the same mass of each metal, in the same physical form.
3. Compare how fast bubbles are produced.
4. Test the gas with a lighted splint.

Results

The order of vigour is the reactivity series being demonstrated directly. Copper is below hydrogen, so it cannot displace it from an acid — a negative result carrying as much information as the positive ones.

Testing the gas

A lighted splint gives a squeaky pop: hydrogen.

(Note the contrast with the carbonate reaction, which gives carbon dioxide and turns limewater milky. Fizzing alone does not identify a gas.)

Naming the salt

The acid decides the salt, the metal supplies the name:

• Hydrochloric acid → chlorides (magnesium chloride)
• Sulfuric acid → sulfates (zinc sulfate)

Mg + 2HCl → MgCl₂ + H₂

Zn + H₂SO₄ → ZnSO₄ + H₂

Making it a fair test

Only the metal may change. Everything else must be held constant:

• Same volume and concentration of acid
• Same mass of metal
• Same surface area — powder reacts far faster than a strip of the same mass
• Same starting temperature

Surface area is the one most often overlooked. Comparing zinc powder with magnesium ribbon tests the particle size, not the metals, and the conclusion drawn about reactivity would be worthless.

Measuring rather than watching

"Bubbles faster" is a comparison, not a measurement. For a numerical result, collect the hydrogen in a gas syringe and record volume against time, or stand the flask on a balance and record the mass lost.

Safety

Eye protection throughout. Hydrogen is flammable, so keep the lighted splint away until you are testing deliberately.