Displacement moves electrons from one metal to another
Definition
Displacement reaction
A reaction in which a more reactive metal takes the place of a less reactive metal in a compound.
Definition
Redox reaction
A reaction in which oxidation and reduction happen at the same time.
- In a displacement reaction the atoms of the more reactive metal lose electrons and become ions.
- The ions of the less reactive metal gain those same electrons and become atoms.
- Loss of electrons is oxidation and gain of electrons is reduction.
- Both changes occur in the same reaction, so a displacement is always a redox change.
- The electrons pass directly between the two species, with no external circuit involved.
Key Idea
The more reactive metal is oxidised and the ions of the less reactive metal are reduced, in every displacement.
Half equations show each electron change on its own
Definition
Half equation
An equation showing the electrons gained or lost by the species reacting at one electrode.
- Each half equation shows only the loss or only the gain, with electrons written as e−\text{e}^{-}e−.
- Magnesium atoms lose two electrons, which is oxidation: Mg→Mg2++2e−\text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^{-}Mg→Mg2++2e−
- Copper ions gain two electrons, which is reduction: Cu2++2e−→Cu\text{Cu}^{2+} + 2\text{e}^{-} \rightarrow \text{Cu}Cu2++2e−→Cu
- The electrons lost in one half equation equal the electrons gained in the other.
- Adding the two half equations together cancels the electrons and rebuilds the overall change.
Common Mistake
- Electrons sit on the right of an oxidation half equation and on the left of a reduction half equation.
- A charge that does not balance across the arrow means an electron has been miscounted.
The ionic equation removes the spectator ions
Definition
Ionic equation
An equation that shows only the ions and substances that change during a reaction, with the spectator ions left out.
Definition
Spectator ion
An ion that is unchanged by the reaction and appears on both sides of the full ionic equation, so it is cancelled out.
- A salt in solution is present as separate ions, so copper sulfate solution holds Cu2+\text{Cu}^{2+}Cu2+ and SO42−\text{SO}_4^{2-}SO42−.
- The sulfate ions are unchanged on both sides, so they take no part in the reaction.
- Leaving them out gives the ionic equation for magnesium and copper sulfate: Mg+Cu2+→Mg2++Cu\text{Mg} + \text{Cu}^{2+} \rightarrow \text{Mg}^{2+} + \text{Cu}Mg+Cu2+→Mg2++Cu
- Stripping the equation this way leaves only the species whose charge changes.
- The same ionic equation describes the reaction whatever copper salt is used, provided it dissolves.
Note
Writing Cu\text{Cu}Cu with no charge on the right records that copper has been reduced to the metal.
A worked example: zinc in copper sulfate solution
- Zinc is above copper, so zinc atoms are the species that lose electrons.
- The oxidation half equation is: Zn→Zn2++2e−\text{Zn} \rightarrow \text{Zn}^{2+} + 2\text{e}^{-}Zn→Zn2++2e−
- The reduction half equation is: Cu2++2e−→Cu\text{Cu}^{2+} + 2\text{e}^{-} \rightarrow \text{Cu}Cu2++2e−→Cu
- Adding them and cancelling the two electrons gives: Zn+Cu2+→Zn2++Cu\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}Zn+Cu2+→Zn2++Cu
- The blue of the copper ions fades and a copper coating forms on the zinc, which is the visible evidence.
Example
- Oxidised: zinc, because its atoms lose electrons to form Zn2+\text{Zn}^{2+}Zn2+.
- Reduced: copper ions, because they gain electrons to form copper atoms.
- Spectator: sulfate, because SO42−\text{SO}_4^{2-}SO42− is unchanged throughout.
Deciding which way a displacement will run
- A displacement happens only when the free metal is above the metal in the compound.
- The atoms of the higher metal give up electrons more readily, so they are the ones that are oxidised.
- Reversing the pair gives no reaction, because the lower metal cannot force electrons onto the higher metal's ions.
- The charge on the ion sets how many electrons move, so a 2+2+2+ ion needs two per atom.
- Balancing the electrons matters when the two charges differ, as with silver's Ag+\text{Ag}^{+}Ag+ and copper's Cu2+\text{Cu}^{2+}Cu2+.
Exam technique
- Naming which species is oxidised means naming the one that loses electrons, not the one that disappears from view.
- An answer in terms of electron transfer earns the marks that an answer about colour alone does not.
- Two half equations are added only after the electrons on each side have been made equal.
Self review
- In terms of electrons, what happens to the more reactive metal in a displacement?
- Write the half equation for iron atoms forming Fe2+\text{Fe}^{2+}Fe2+ ions.
- Why is sulfate described as a spectator ion?
- Why does copper not displace zinc from zinc sulfate solution?
- How many electrons are transferred per copper atom when copper displaces silver?
