Displacement as Redox

The Redox Nature of Displacement

Why does a piece of shiny silver-coloured zinc turn into a red-brown sludge when dropped into blue copper sulfate solution? This happens because of an invisible swap of electrons between the elements involved.

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in one of its compounds.
These are a sub-type of redox reaction because both oxidation and reduction happen simultaneously.
For a displacement to occur, the added metal must be higher in the reactivity series. The higher a metal is, the greater its tendency to form a positive ion (cation).

The Electron Transfer Mechanism

In a displacement reaction, electrons are transferred directly from the more reactive metal to the ions of the less reactive metal.
To remember which process is which, use the OIL RIG mnemonic: Oxidation Is Loss, Reduction Is Gain of electrons.
The more reactive metal atoms lose electrons to become positive ions. This loss of electrons is oxidation. Because this metal causes the other species to be reduced, it acts as the reducing agent.
The less reactive metal ions (which are dissolved in the solution) gain those electrons to become neutral solid atoms. This gain of electrons is reduction. By taking electrons, these ions act as the oxidising agent.
These reactions are typically exothermic, releasing heat energy to the surroundings.

Visual Indicators of Displacement

When zinc is added to copper(II) sulfate ( $CuSO_4$ ), the blue colour of the solution gradually fades.
This happens because the blue $Cu^{2+}$ ions are reduced to solid copper atoms, which precipitate as a red-brown solid.
Meanwhile, the solid zinc atoms are oxidised into colourless $Zn^{2+}$ ions, making the solid zinc appear to dissolve or "disappear" into the aqueous solution.

Ionic Equations and Half-Equations (Higher Tier)

To show the exact mechanism of electron transfer, chemists write an ionic equation, which deliberately omits spectator ions.
Spectator ions are ions that do not change their physical state or their electrical charge during the reaction (such as the sulfate ion, $SO_4^{2-}$ ).
The general ionic equation for a displacement reaction is:

A(s) + B^{n+}(aq) \rightarrow A^{n+}(aq) + B(s)

(Where metal A is more reactive than metal B).

We can split the overall ionic equation into half-equations to show the oxidation and reduction components separately. The total charge on the left of a half-equation must perfectly equal the total charge on the right.

Worked Example

Write the ionic and half-equations for the reaction between solid magnesium and aqueous copper(II) sulfate.

Step 1: Identify the reacting species and spectator ions.

Magnesium ( $Mg$ ) is more reactive than copper, so it displaces copper ions ( $Cu^{2+}$ ).
The sulfate ion ( $SO_4^{2-}$ ) is the spectator ion and is omitted from the equations.

Step 2: Write the full ionic equation with state symbols.

Mg(s) + Cu^{2+}(aq) \rightarrow Mg^{2+}(aq) + Cu(s)

Step 3: Write the oxidation half-equation.

Magnesium atoms lose two electrons ( $2e^-$ ) to form $Mg^{2+}$ ions:

Mg \rightarrow Mg^{2+} + 2e^-

Step 4: Write the reduction half-equation.

Copper ions gain two electrons to form neutral copper atoms:

Cu^{2+} + 2e^- \rightarrow Cu

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Exam Tips

Common Mistake
Students often state that "copper is reduced" instead of "copper ions are reduced". You must specify that the metal ION gains electrons, not the neutral atom.
2
When asked to explain why a displacement is a redox reaction, examiners expect you to explicitly identify which specific atom loses electrons (oxidation) and which specific ion gains electrons (reduction).
3
Always remember to include the charges on your ions when writing half-equations (e.g., writing $Cu^{2+} + 2e^-$ instead of just $Cu + 2e^-$ ), as forgetting the charges will lose you marks.
4
When explaining the reactivity series, use the exact Edexcel phrase: more reactive metals have a "greater tendency to form their positive ion".

Key Terms(15)

Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive element in one of its compounds.
Redox: A reaction where both reduction and oxidation occur at the same time.
Oxidation: The loss of electrons by a substance.
Reduction: The gain of electrons by a substance.
Reducing agent: The species that loses electrons, thereby reducing another species.
Oxidising agent: The species that gains electrons, thereby oxidising another species.
OIL RIG: A mnemonic used to remember electron transfer: Oxidation Is Loss, Reduction Is Gain.
Ionic equation: A chemical equation that shows only the reacting particles that undergo a change in oxidation state or physical state.
Spectator ions: Ions that do not change their physical state or charge during a reaction and are excluded from ionic equations.
Half-equation: An equation that describes only the oxidation or reduction component of a reaction, explicitly showing the movement of electrons.
Cation: A positively charged ion, formed when an atom loses electrons.
Exothermic: A reaction that releases energy to the surroundings, usually in the form of heat.
Precipitate: An insoluble solid that emerges from a liquid solution during a reaction.
Aqueous: A substance dissolved in water, indicated by the state symbol (aq).
Reactivity series: A list of metals ordered by their reactivity (their tendency to lose electrons and form positive ions).

Previous Subtopic: Relative Reactivity of MetalsRelative Reactivity of Metals Next Subtopic: Reactivity SeriesReactivity Series

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Key Terms(15)

Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive element in one of its compounds.
Redox: A reaction where both reduction and oxidation occur at the same time.
Oxidation: The loss of electrons by a substance.
Reduction: The gain of electrons by a substance.
Reducing agent: The species that loses electrons, thereby reducing another species.
Oxidising agent: The species that gains electrons, thereby oxidising another species.
OIL RIG: A mnemonic used to remember electron transfer: Oxidation Is Loss, Reduction Is Gain.
Ionic equation: A chemical equation that shows only the reacting particles that undergo a change in oxidation state or physical state.
Spectator ions: Ions that do not change their physical state or charge during a reaction and are excluded from ionic equations.
Half-equation: An equation that describes only the oxidation or reduction component of a reaction, explicitly showing the movement of electrons.
Cation: A positively charged ion, formed when an atom loses electrons.
Exothermic: A reaction that releases energy to the surroundings, usually in the form of heat.
Precipitate: An insoluble solid that emerges from a liquid solution during a reaction.
Aqueous: A substance dissolved in water, indicated by the state symbol (aq).
Reactivity series: A list of metals ordered by their reactivity (their tendency to lose electrons and form positive ions).

Displacement as Redox

The Redox Nature of Displacement

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in one of its compounds.
These are a sub-type of redox reaction because both oxidation and reduction happen simultaneously.
For a displacement to occur, the added metal must be higher in the reactivity series. The higher a metal is, the greater its tendency to form a positive ion (cation).

The Electron Transfer Mechanism

In a displacement reaction, electrons are transferred directly from the more reactive metal to the ions of the less reactive metal.
To remember which process is which, use the OIL RIG mnemonic: Oxidation Is Loss, Reduction Is Gain of electrons.
The more reactive metal atoms lose electrons to become positive ions. This loss of electrons is oxidation. Because this metal causes the other species to be reduced, it acts as the reducing agent.
The less reactive metal ions (which are dissolved in the solution) gain those electrons to become neutral solid atoms. This gain of electrons is reduction. By taking electrons, these ions act as the oxidising agent.
These reactions are typically exothermic, releasing heat energy to the surroundings.

Visual Indicators of Displacement

When zinc is added to copper(II) sulfate ( $CuSO_4$ ), the blue colour of the solution gradually fades.
This happens because the blue $Cu^{2+}$ ions are reduced to solid copper atoms, which precipitate as a red-brown solid.
Meanwhile, the solid zinc atoms are oxidised into colourless $Zn^{2+}$ ions, making the solid zinc appear to dissolve or "disappear" into the aqueous solution.

Ionic Equations and Half-Equations (Higher Tier)

To show the exact mechanism of electron transfer, chemists write an ionic equation, which deliberately omits spectator ions.
Spectator ions are ions that do not change their physical state or their electrical charge during the reaction (such as the sulfate ion, $SO_4^{2-}$ ).
The general ionic equation for a displacement reaction is:

A(s) + B^{n+}(aq) \rightarrow A^{n+}(aq) + B(s)

(Where metal A is more reactive than metal B).

We can split the overall ionic equation into half-equations to show the oxidation and reduction components separately. The total charge on the left of a half-equation must perfectly equal the total charge on the right.

Worked Example

Write the ionic and half-equations for the reaction between solid magnesium and aqueous copper(II) sulfate.

Step 1: Identify the reacting species and spectator ions.

Magnesium ( $Mg$ ) is more reactive than copper, so it displaces copper ions ( $Cu^{2+}$ ).
The sulfate ion ( $SO_4^{2-}$ ) is the spectator ion and is omitted from the equations.

Step 2: Write the full ionic equation with state symbols.

Mg(s) + Cu^{2+}(aq) \rightarrow Mg^{2+}(aq) + Cu(s)

Step 3: Write the oxidation half-equation.

Magnesium atoms lose two electrons ( $2e^-$ ) to form $Mg^{2+}$ ions:

Mg \rightarrow Mg^{2+} + 2e^-

Step 4: Write the reduction half-equation.

Copper ions gain two electrons to form neutral copper atoms:

Cu^{2+} + 2e^- \rightarrow Cu

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Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often state that "copper is reduced" instead of "copper ions are reduced". You must specify that the metal ION gains electrons, not the neutral atom.
2
When asked to explain why a displacement is a redox reaction, examiners expect you to explicitly identify which specific atom loses electrons (oxidation) and which specific ion gains electrons (reduction).
3
Always remember to include the charges on your ions when writing half-equations (e.g., writing $Cu^{2+} + 2e^-$ instead of just $Cu + 2e^-$ ), as forgetting the charges will lose you marks.
4
When explaining the reactivity series, use the exact Edexcel phrase: more reactive metals have a "greater tendency to form their positive ion".

Key Terms(15)

Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive element in one of its compounds.
Redox: A reaction where both reduction and oxidation occur at the same time.
Oxidation: The loss of electrons by a substance.
Reduction: The gain of electrons by a substance.
Reducing agent: The species that loses electrons, thereby reducing another species.
Oxidising agent: The species that gains electrons, thereby oxidising another species.
OIL RIG: A mnemonic used to remember electron transfer: Oxidation Is Loss, Reduction Is Gain.
Ionic equation: A chemical equation that shows only the reacting particles that undergo a change in oxidation state or physical state.
Spectator ions: Ions that do not change their physical state or charge during a reaction and are excluded from ionic equations.
Half-equation: An equation that describes only the oxidation or reduction component of a reaction, explicitly showing the movement of electrons.
Cation: A positively charged ion, formed when an atom loses electrons.
Exothermic: A reaction that releases energy to the surroundings, usually in the form of heat.
Precipitate: An insoluble solid that emerges from a liquid solution during a reaction.
Aqueous: A substance dissolved in water, indicated by the state symbol (aq).
Reactivity series: A list of metals ordered by their reactivity (their tendency to lose electrons and form positive ions).

Previous Subtopic: Relative Reactivity of MetalsRelative Reactivity of Metals Next Subtopic: Reactivity SeriesReactivity Series

Back to Displacement as Redox

Put your knowledge into practice — try past paper questions for Chemistry

Key Terms(15)

Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive element in one of its compounds.
Redox: A reaction where both reduction and oxidation occur at the same time.
Oxidation: The loss of electrons by a substance.
Reduction: The gain of electrons by a substance.
Reducing agent: The species that loses electrons, thereby reducing another species.
Oxidising agent: The species that gains electrons, thereby oxidising another species.
OIL RIG: A mnemonic used to remember electron transfer: Oxidation Is Loss, Reduction Is Gain.
Ionic equation: A chemical equation that shows only the reacting particles that undergo a change in oxidation state or physical state.
Spectator ions: Ions that do not change their physical state or charge during a reaction and are excluded from ionic equations.
Half-equation: An equation that describes only the oxidation or reduction component of a reaction, explicitly showing the movement of electrons.
Cation: A positively charged ion, formed when an atom loses electrons.
Exothermic: A reaction that releases energy to the surroundings, usually in the form of heat.
Precipitate: An insoluble solid that emerges from a liquid solution during a reaction.
Aqueous: A substance dissolved in water, indicated by the state symbol (aq).
Reactivity series: A list of metals ordered by their reactivity (their tendency to lose electrons and form positive ions).