IMAT Chemistry · Redox Reactions

The Reactivity Series and Displacement Reactions

Metals can be ranked by how easily they lose electrons; a metal higher in the series pushes a lower one out of its compounds, and halogens follow the same rule in reverse.

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Why this matters

No past question has been about the reactivity series alone, but displacement reactions appear among the options of the oxidising and reducing agent questions. It is on the syllabus and links redox to the metals met in other chapters.

Concept 1 of 2: The reactivity series of metals

Every metal reacts by losing electrons, so it acts as a reducing agent. Some give their electrons up very easily (potassium, sodium), others hardly at all (silver, gold). Listing metals in that order predicts how they react with water and acids and how hard they are to extract from their ores.

Definition

The reactivity series lists metals from the most easily oxidised (best reducing agent) to the least.

Order to learn: K, Na, Ca, Mg, Al, (C), Zn, Fe, Pb, (H), Cu, Ag, Au.
Metals above hydrogen release $\mathrm{H_2}$ from dilute acids: $\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$ . Metals below hydrogen do not.
Metals below carbon can be extracted by heating their oxides with carbon; metals above carbon need electrolysis.
Aluminium looks less reactive than it is, because a thin oxide layer protects it.

Metal	With cold water	With dilute HCl	How it is extracted
Potassium, sodium	Violent: hydroxide and $\mathrm{H_2}$	Dangerously violent	Electrolysis of the molten chloride
Calcium	Steady: hydroxide and $\mathrm{H_2}$	Vigorous	Electrolysis of the molten chloride
Magnesium	Very slow (fast with steam)	Vigorous, $\mathrm{H_2}$	Electrolysis
Aluminium	None (oxide layer)	Slow at first, then steady	Electrolysis of molten $\mathrm{Al_2O_3}$
Zinc, iron	None (react with steam when hot)	Moderate, $\mathrm{H_2}$	Heating the oxide with carbon or CO
Lead	None	Very slow	Heating the oxide with carbon
Copper, silver, gold	None	None	Reduction of ores; silver and gold also found as the metal

Higher in the table means a stronger reducing agent; the ions of metals lower down are stronger oxidising agents.

Practice this conceptself-check · 3 quick reps

Concept 2 of 2: Predicting displacement reactions of metals and halogens

Put a more reactive metal into a solution of a less reactive metal's ions and the electrons move downhill: the more reactive metal dissolves as ions, and the less reactive metal comes out as solid. Halogens work the other way round, because they react by gaining electrons: the more reactive halogen takes electrons from the halide ion of a less reactive one.

Definition

Metal displacement: a metal displaces any metal below it from a solution of its salt. The metal is the reducing agent; the metal ion in solution is the oxidising agent.
If the metal is below the one in solution, no reaction occurs.
Halogen displacement: reactivity order $\mathrm{F_2 > Cl_2 > Br_2 > I_2}$ . A halogen displaces the halides below it: $\mathrm{Cl_2 + 2I^- \rightarrow 2Cl^- + I_2}$ . The halogen is the oxidising agent.
The spectator ions (sulfate, nitrate, the alkali-metal ion) do not take part; write the ionic equation without them.

Metal displacement, as an ionic equation

\mathrm{M_{(more\ reactive)} + N^{n+}_{(aq)} \rightarrow M^{n+}_{(aq)} + N_{(less\ reactive)}}

$\mathrm{M}$ the metal higher in the series (reducing agent)
$\mathrm{N^{n+}}$ the ion of the metal lower in the series (oxidising agent)

Worked example

An iron nail is left in blue copper(II) sulfate solution. What happens? What would happen to a copper wire left in iron(II) sulfate solution?

Practice this conceptself-check · 3 quick reps

For halogens, the more reactive one is the oxidising agent

A reactive metal is a strong REDUCING agent because it loses electrons easily. A reactive halogen is a strong OXIDISING agent because it gains electrons easily. In

\mathrm{Cl_2 + 2NaI \rightarrow 2NaCl + I_2}

, chlorine is reduced and the iodide ion is oxidised.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Predicting displacement reactions of metals and halogens
Metal displacement, as an ionic equation
$\mathrm{M_{(more\ reactive)} + N^{n+}_{(aq)} \rightarrow M^{n+}_{(aq)} + N_{(less\ reactive)}}$

Reference tables (1)

The reactivity series of metals7 rows

Metal	With cold water	With dilute HCl	How it is extracted
Potassium, sodium	Violent: hydroxide and $\mathrm{H_2}$	Dangerously violent	Electrolysis of the molten chloride
Calcium	Steady: hydroxide and $\mathrm{H_2}$	Vigorous	Electrolysis of the molten chloride
Magnesium	Very slow (fast with steam)	Vigorous, $\mathrm{H_2}$	Electrolysis
Aluminium	None (oxide layer)	Slow at first, then steady	Electrolysis of molten $\mathrm{Al_2O_3}$
Zinc, iron	None (react with steam when hot)	Moderate, $\mathrm{H_2}$	Heating the oxide with carbon or CO
Lead	None	Very slow	Heating the oxide with carbon
Copper, silver, gold	None	None	Reduction of ores; silver and gold also found as the metal

Higher in the table means a stronger reducing agent; the ions of metals lower down are stronger oxidising agents.

Watch out for (1)

For halogens, the more reactive one is the oxidising agent
→ Predicting displacement reactions of metals and halogens

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