Electron transfer reactions

A grey iron nail is dipped into blue copper sulfate solution and immediately withdrawn.

Iron and copper sulfate

The nail is coated in a layer of pink-brown copper metal.The blue Cu2+ ions have been changed into brown Cu atoms.

Grey iron wool is put into blue copper sulfate solution.

The wool becomes coated in brown copper. The blue colour of the solution changes to pale green.

A little of the pale green solution is withdrawn and added to sodium hydroxide solution.

An olive-green precipitate forms, confirming the presence of Fe2+.Grey Fe atoms have been changed into green Fe2+ ions.

Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)

Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)

Cu2+(aq) → Cu(s) Cu2+(aq) + 2e– → Cu(s)

Fe(s) → Fe2+(aq) Fe(s) → Fe2+(aq) + 2e–

Electrons are always added to the more positive side.

When adding half-equations, the electrons should cancel out.

Grey magnesium powder is added to pale green iron(II) sulfate solution.

Magnesium powder and

iron(II) sulfate solution.

The green colour fades, a large amount of black solid forms.

(Some of the magnesium powder reacts with water, producing bubbles of hydrogen gas.)

The black solid is attracted to a magnet. It is iron.The Fe2+ ions have been changed into Fe atoms.

Some of the liquid is withdrawn and added to sodium hydroxide solution. A white precipitate forms, which does not dissolve in excess hydroxide. This is Mg2+.The Mg atoms have become Mg2+ ions.The absence of any dark green gel shows that all the Fe2+ has reacted.

Fe2+(aq) + Mg(s) → Fe(s) + Mg2+(aq)

Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)

Fe2+(aq) → Fe(s) Fe2+(aq) + 2e– → Fe(s)

Mg(s) → Mg2+(aq) Mg(s) → Mg2+(aq) + 2e–

Electrons are always added to the more positive side.