(d) Reactivity series

Learning objectives

2.15Metal reactivity with water and acidsUnderstand how metals can be arranged in a reactivity series based on their reactions with:• water• dilute hydrochloric or sulfuric acid.2.16Metal displacement reactionsUnderstand how metals can be arranged in a reactivity series based on their displacement reactions between:• metals and metal oxides• metals and aqueous solutions of metal salts.2.17Metal reactivity orderKnow the order of reactivity of these metals: potassium, sodium, lithium, calcium, magnesium, aluminium, zinc, iron, copper, silver, gold 2.18Conditions for rustingKnow the conditions under which iron rusts 2.19Preventing rustingUnderstand how the rusting of iron may be prevented by:• barrier methods• galvanising• sacrificial protection.2.20Oxidation and reductionUnderstand the terms:• oxidation• reduction• redox• oxidising agent• reducing agent in terms of gain or loss of oxygen and loss or gain of electrons.2.21Acid–metal reaction practicalPractical: investigate reactions between dilute hydrochloric and sulfuric acids and metals (e.g. magnesium, zinc and iron)

A more reactive metal reacts faster and more vigorously under the same conditions. Compare like with like: use equal-sized clean metal samples, the same volume and concentration of water or dilute acid, and the same temperature.

Test	Evidence of greater reactivity	Typical product pattern
cold water	faster bubbles, faster disappearance and a larger temperature rise	metal hydroxide + hydrogen
steam	faster hydrogen production and oxide formation	metal oxide + hydrogen
dilute hydrochloric or sulfuric acid	faster hydrogen bubbles and faster metal loss	salt + hydrogen

\ce{Mg + 2HCl -> MgCl2 + H2}\qquad\ce{Zn + H2SO4 -> ZnSO4 + H2}

If magnesium gives hydrogen faster than zinc in the same dilute acid, magnesium is more reactive. A metal below hydrogen, such as copper, does not normally release hydrogen from dilute hydrochloric or sulfuric acid.

Do not compare results from different acid concentrations, temperatures or surface areas. 'More bubbles in total' is not by itself a fair rate comparison, and no visible reaction with cold water does not prove that a metal cannot react with steam.

A metal displaces another metal from its compound only if the added metal is more reactive. A reaction therefore places the added metal above the displaced metal; no reaction places it below, provided the test conditions are suitable.

System	Example	Reactivity conclusion
metal + metal oxide	$\ce{2Al + Fe2O3 -> Al2O3 + 2Fe}$	aluminium is more reactive than iron
metal + aqueous metal salt	$\ce{Zn + CuSO4 -> ZnSO4 + Cu}$	zinc is more reactive than copper

For zinc in copper(II) sulfate, a brown copper coating forms and the blue solution becomes paler as copper(II) ions are removed. Observations identify that a reaction occurred; the equation identifies which metal displaced which.

A metal cannot displace itself. In a salt solution, compare the metals—not the whole salts or ions as if they were metals. Some metal-oxide reactions need heating to overcome activation energy; heating does not reverse the reactivity rule.

The reactivity series runs from metals that lose electrons most readily at the top to the least reactive metals at the bottom.

Position	Metal	Symbol
1	potassium	K
2	sodium	Na
3	lithium	Li
4	calcium	Ca
5	magnesium	Mg
6	aluminium	Al
7	zinc	Zn
8	iron	Fe
9	copper	Cu
10	silver	Ag
11	gold	Au

Use the order to predict displacement: zinc displaces copper from copper(II) sulfate because zinc is above copper; silver does not displace copper because silver is below it. The same order explains why gold is often found uncombined.

Hydrogen and carbon are often inserted as useful reference points, but they are not metals in the specified eleven-metal list. Preserve the exact order of potassium, sodium and lithium at the top, and copper, silver and gold at the bottom.

Iron rusts only when both oxygen and water are present. Rust is hydrated iron(III) oxide; it is a corrosion product rather than a simple layer of pure iron oxide.

Tube	Available conditions	Result	What it shows
iron + water + air	water and oxygen	rust forms	both together permit rusting
iron + boiled water under oil	water, but oxygen excluded	no rust	oxygen is required
iron + dry air with drying agent	oxygen, but water removed	no rust	water is required

Boiling removes dissolved air from water and the oil layer prevents oxygen re-entering. A drying agent removes water vapour from air. Each control changes one required condition while keeping iron present.

Air alone is not a complete explanation: dry air contains oxygen but lacks water. Water alone is also insufficient when dissolved oxygen has been removed and kept out.

Rust prevention either keeps oxygen or water away from iron, or makes a more reactive metal oxidise instead of the iron.

Method	How it protects	What happens if scratched
paint, oil, grease or plastic	barrier blocks oxygen and water	exposed iron can rust
galvanising with zinc	zinc coating is a barrier and zinc is more reactive than iron	zinc still oxidises in preference to iron
sacrificial protection	attached magnesium or zinc loses electrons instead of iron	protection continues while sacrificial metal remains connected

Choose barriers for surfaces that can remain continuously coated. Use galvanising or sacrificial protection where damage is possible, because the more reactive metal continues to protect exposed iron.

Zinc does not protect iron because it is unreactive; it protects because it is more reactive and is oxidised first. Rusting applies specifically to iron and steel, although other metals can corrode.

Oxidation and reduction occur together in a redox reaction. Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons.

Term	Oxygen description	Electron description
oxidation	gains oxygen	loses electrons
reduction	loses oxygen	gains electrons
oxidising agent	supplies oxygen to, or accepts electrons from, another substance	is itself reduced
reducing agent	removes oxygen from, or donates electrons to, another substance	is itself oxidised

\ce{Zn -> Zn^{2+} + 2e^-}\qquad\ce{Cu^{2+} + 2e^- -> Cu}

In $\ce{Zn + Cu^{2+} -> Zn^{2+} + Cu}$ , zinc loses electrons and is oxidised, so zinc is the reducing agent. Copper(II) ions gain electrons and are reduced, so $\ce{Cu^{2+}}$ is the oxidising agent.

The agent causes the other species to change and undergoes the opposite change itself. Do not call an electron donor an oxidising agent, and do not describe reduction only as 'removing a substance'—state oxygen loss or electron gain.

Investigate magnesium, zinc and iron with dilute hydrochloric acid or dilute sulfuric acid by comparing hydrogen-production rates under controlled conditions.

Stage	Action
1	Place equal volumes of the same dilute acid at the same concentration and temperature in labelled vessels.
2	Clean and add equal moles, masses or surface areas of magnesium, zinc and iron; keep the chosen measure consistent.
3	Start timing immediately and measure gas volume at regular intervals with a gas syringe, or time collection of a fixed hydrogen volume.
4	Repeat and compare initial gradients or the time to the fixed volume: faster hydrogen production means greater reactivity.

\ce{Mg + 2HCl -> MgCl2 + H2}\qquad\ce{Fe + H2SO4 -> FeSO4 + H2}

Expected order is magnesium faster than zinc, and zinc faster than iron. Confirm hydrogen only on a small collected sample with a lighted splint: a squeaky pop is positive.

Wear eye protection, use dilute acids and small metal samples, keep flames away while hydrogen is being produced, and point apparatus away from people. Do not stopper a vessel unless gas can escape into a correctly fitted syringe.

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—(e) Extraction and uses of metalsRead

(d) Reactivity series

Learning objectives

Rank metals using water and dilute acids

Use displacement to compare metal reactivity

Know the required metal reactivity series

Identify the two conditions needed for rusting

Prevent rusting by blocking or sacrificing

Track oxidation and reduction by oxygen or electrons

Compare metal reactions with dilute acids safely

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