Electrolysis

What is electrolysis?

the process of splitting an ionic compound using electricity

What is the electrolyte?

• whatever compound is being broken down

Why must ions be either molten or aqueous?

• a molten ionic compound oraqueous solution contains ions that are free to move, so it conducts electricity and is decomposed by it

• In a solid, ions are fixed in a lattice and cannot move, so no current flows.

• Melting the compound so the ions can move within the liquid.

• Dissolving it in water so the ions separate and move through the solution.

What apparatus is needed for electrolysis?

• Two electrodes (conducting rods) are placed in the electrolyte and connected to a d.c. power supply:

  1. Cathode - The negative electrode, connected to the negative terminal.

  2. Anode - The positive electrode, connected to the positive terminal.

• A direct current (d.c.) is used so each electrode keeps a constant charge and the products form at separate electrodes.

What are electrodes usually made of?

• Electrodes are usually made of graphite or platinum.

• These are good conductors of electricity and are inert (unreactive), so they don't take part in the reaction.

What are the names of the ions?

• A cation is a positive ion, which moves to the cathode.

• An anion is a negative ion, which moves to the anode.

What elements does a molten ionic compound containing two elements break down into?

1. Metal - Forms positive ions, which are attracted to the negative electrode (cathode) and discharged there.

2. Non-metal - Forms negative ions, which are attracted to the positive electrode (anode) and discharged there.

What do non mentals that exist as diatomic molecules (like bromine, chlorine and oxygen) form in molten ionic compounds?

gas molecules at the anode

What happens at each electrode in a molten ionic compound electrolysis?

• At the cathode (negative electrode) - Positive ions gain electrons to become neutral atoms.

• At the anode (positive electrode) - Negative ions lose electrons to become neutral atoms or molecules.

• When an ion loses its charge to become a neutral atom or molecule, it's said to be discharged.

How does charge move through the circuit?

• In the wires and power supply, the current is carried by moving electrons.

• In the electrolyte, the current is carried by moving ions.

What are half equations?

A half equation shows the reaction and electron transfer at a single electrode.

What does the symbol e^- represent?

an electron (a single negative charge)

What is oxidation?

negative ions lose electrons, happens at anode

What is reduction?

positive ions gain electrons, happens at cathode

How to remember oxidation vs reduction and where they occur?

• OIL RIG (Oxidation is Loss, Reduction is Gain)

• Red Cat, An Ox (reduction cathode, oxidation anode)

How do you write a half equation?

1. Write the reactant ion on the left and the neutral product on the right.

2. Balance the atoms. Non-metals that form diatomic molecules (e.g. Cl₂, Br₂, O₂) need two ions on the left.

3. Add electrons to balance the charges:

   • At the cathode (negative electrode), ions gain electrons, so write them on the left side.

   • At the anode (positive electrode), ions lose electrons, so write them on the right side.

What makes an dissolved ionic compound (aqueous solution) different from a molten ionic compound?

A dissolved ionic compound has ions that are free to move, so its solution can be electrolysed without melting it.

What kind of electrodes are used for aqueous solutions?

• inert electrodes

• platnium

• graphite

What ions are present in an aqueous electrolysis solution?

• Water breaks up slightly into hydrogen and hydroxide ions:

  • H₂O(l) ⇌ H⁺(aq) + OH^-(aq)

• This means four kinds of ion are present: the positive and negative ions of the dissolved compound, plus H⁺ and OH^- from the water.

How does electrolysis of aqueous solution work?

• At each electrode, only one of the two competing ions is discharged to form an element, while the other stays in solution.

• Deciding which ion gets discharged depends on specific discharge rules for both the cathode and anode

What ions are attracted to the cathode in an aqueous solution?

The positive ions are the metal ion and the hydrogen ion (H⁺)

What are the discharge rules for aqueous solutions at the cathode?

• The ion of the less reactive element is discharged:

• If the metal is more reactive than hydrogen, hydrogen gas is produced.

• If the metal is less reactive than hydrogen, the metal is deposited.

What ions are attracted to the anode in an aqueous solution?

The negative ions are the non-metal ion and the hydroxide ion (OH^-)

What are the discharge rules for aqueous solutions at the anode?

• If halide ions (Cl^-, Br^-, I^-) are present in a concentrated solution, the halogen (chlorine, bromine, or iodine) is formed.

• If no halide is present (or if the solution is very dilute), oxygen is formed from the hydroxide ions. Sulfate and nitrate ions are not discharged.

What do half equations with inert electrodes look like at the cathode in an aqueous solution?

• 2H⁺ + 2e^- → H₂

• Cu²⁺ + 2e^- → Cu

What do half equations with inert electrodes look like at the anode in an aqueous solution?

• 4OH^- → O₂ + 2H₂O + 4e^-

• 2Cl^- → Cl₂ + 2e^-

What happens when an anode is non-inert copper in an aqueous solution?

When the anode is made of copper, copper atoms lose electrons and dissolve into solution as ions:

Cu → Cu²⁺ + 2e^-

Why do we need to extract aluminium?

• it is a very useful metal used for a lot of things, such as foil, drink cans and electronic devices

• it is useful as it is light, non corrosive, non toxic and malleable

Why does aluminium to be extractedneed electrolysis?

• Aluminium sits above carbon in the reactivity series, so heating its oxide with carbon doesn't work - aluminium holds its oxygen more tightly than carbon can pull it away.

• It must be extracted using electrolysis.

What is aluminium ore and how should it be prepared?

• The ore is bauxite. It's purified to give aluminium oxide, Al₂O₃ (also called alumina).

• The ions (Al³⁺ and O^2-) must be free to move, so the oxide has to be molten.

What is the melting point of aluminium?

• over 2000 because aluminium oxide is an ionic compound so has a high melting point / requires a lot of energy to get ions to move

Why is aluminium oxide dissolved in cryolite?

• since aluminium oxide has such a high melting point

• Dissolving aluminium oxide in molten cryolite solves this:

  • The mixture has a much lower melting point than pure aluminium oxide, so the cell runs at only around 900 to 1,000 °C and far less energy is needed.

  • It makes the mixture conduct better than molten aluminium oxide would on its own.

• The electrolyte is aluminium oxide dissolved in molten cryolite.

What are the electrodes made of in the electrolysis of aluminium

• carbon (graphite)

• The cell lining acts as the negative electrode.

• Carbon blocks dipping into the mixture from above act as the positive electrodes.

What happens at the cathode in the electrolysis of aluminium oxide?

Aluminium ions (Al³⁺) are attracted to the negative electrode and gain electrons to become aluminium atoms. Liquid aluminium sinks to the bottom of the cell and is tapped off.

What happens at the anode in the electrolysis of aluminium oxide?

Oxide ions (O^2-) are attracted to the positive electrodes and lose electrons to form oxygen gas.

What is the overall reaction in the electrolysis of aluminium oxide?

• aluminium oxide → aluminium + oxygen

• 2Al₂O₃(l) → 4Al(l) + 3O₂(g)

Why do positive electrodes wear away in the electrolysis of aluminium oxide?

The hot oxygen reacts with the carbon anodes to form carbon dioxide:

C + O₂ → CO₂

The positive electrodes burn away and must be replaced regularly, which adds to the cost.

Why is aluminium expensive to extract?

Large amounts of electrical energy are needed to keep the mixture molten and to drive the high current through the cell.

What is the half equation for aluminium oxide at the cathode (reduction)?

Aluminium ions gain electrons to become aluminium atoms (reduction):

Al³⁺ + 3e^- → Al

What is the half equation for aluminium oxide at the anode (oxidation)?

Oxide ions lose electrons to form oxygen molecules (oxidation):

2O^2- → O₂ + 4e^-