4.3.1a The process of electrolysis
Electrolysis breaks an ionic compound down using electricity
Electrolysis
The decomposition of an ionic compound, when molten or in solution, by passing an electric current through it.
Electrolyte
A molten or dissolved ionic compound that conducts electricity and is broken down during electrolysis.
- Electrolysis uses an electric current to break an ionic compound down into simpler substances.
- The compound being broken down is the electrolyte.
- An electrolyte conducts electricity only when it is molten or dissolved, because its ions are then free to move.
- A solid ionic compound does not conduct, because its ions are locked in a fixed lattice.
- A current can only flow when the ions are free to move and carry the charge.
- Melting or dissolving the compound frees the ions from the lattice.
Ions move to the oppositely charged electrode
Cathode
The negative electrode during electrolysis, where positively charged ions gain electrons.
Anode
The positive electrode during electrolysis, where negatively charged ions lose electrons.
- Two electrodes dip into the electrolyte and are connected to a power supply.
- The cathode is the negative electrode and the anode is the positive electrode.
- Positive ions (cations) are attracted to the negative cathode.
- Negative ions (anions) are attracted to the positive anode.
- Opposite charges attract, so each ion moves to the electrode with the opposite charge.
- This steady movement of ions is what carries the current through the electrolyte.
At the electrodes, ions are discharged into elements
Discharge
The gain or loss of electrons by an ion at an electrode, turning it into a neutral atom or molecule.
- At the cathode, positive ions gain electrons and are discharged as neutral atoms.
- At the anode, negative ions lose electrons and are discharged as neutral atoms or molecules.
- These discharged particles are the elements that make up the compound.
- Because the compound is split into its elements, electrolysis is a decomposition reaction.
- What is an electrolyte?
- Why must an ionic compound be molten or dissolved before it can be electrolysed?
- Which ions move to the cathode, and why?
- What happens to a positive ion when it is discharged at the cathode?
- Why is electrolysis described as a decomposition reaction?
4.3.1b Half equations in electrolysis
Each electrode has its own half equation
Half equation
An equation that shows the electrons lost or gained by a single species in a redox reaction.
Electron
A negatively charged subatomic particle that can be transferred between species during a redox reaction.
- A half equation shows the electrons gained or lost at one electrode.
- At the cathode, ions gain electrons, which is reduction.
- At the anode, ions lose electrons, which is oxidation.
- Balance both the atoms and the charge by adding the right number of electrons.
- Cathode: electrons are on the left, because the ion gains them.
- Anode: electrons are on the right, because the ion loses them.
Cathode half equations: the positive ion gains electrons
Cathode
The negative electrode during electrolysis, where positively charged ions gain electrons.
- Write the positive ion, then add electrons on the left until the charge is zero.
- Lead ions: Pb2++2eāāPb\text{Pb}^{2+} + 2\text{e}^{-} \rightarrow \text{Pb}Pb2++2eāāPb
- Hydrogen ions: 2H++2eāāH22\text{H}^{+} + 2\text{e}^{-} \rightarrow \text{H}_22H++2eāāH2ā
- Aluminium ions: Al3++3eāāAl\text{Al}^{3+} + 3\text{e}^{-} \rightarrow \text{Al}Al3++3eāāAl
- Add one electron for each unit of positive charge on the ion.
- Check the charge is the same on both sides, then the half equation is balanced.
Anode half equations: the negative ion loses electrons
Anode
The positive electrode during electrolysis, where negatively charged ions lose electrons.
Coefficient
A large whole number placed before a chemical formula to show the relative number of particles or moles in a balanced equation.
- Write the negative ion, then add electrons on the right so the charge balances.
- Bromide ions: 2BrāāBr2+2eā2\text{Br}^{-} \rightarrow \text{Br}_2 + 2\text{e}^{-}2BrāāBr2ā+2eā
- Chloride ions: 2ClāāCl2+2eā2\text{Cl}^{-} \rightarrow \text{Cl}_2 + 2\text{e}^{-}2ClāāCl2ā+2eā
- A coefficient of 2 is needed because the halogen forms a molecule of two atoms.
- At which electrode does reduction happen?
- On which side of a cathode half equation do the electrons go?
- Write the cathode half equation for lead ions.
- Write the anode half equation for bromide ions.
- Why is a 2 written in front of the chloride ions in the anode half equation?
