Electrolysis of Molten Compounds and Metal Extraction

Electrolysis breaks down an ionic compound into its elements by passing an electric current through it. It only works when the ions in the compound are free to move.

In a solid ionic compound the ions are locked in a fixed giant lattice, so they cannot move and the solid does not conduct. Melting the compound or dissolving it in water frees the ions. The liquid that then conducts the current is the electrolyte.

An electrolyte is a molten or dissolved ionic compound whose ions are free to move and carry the charge.

Two electrodes dip into the electrolyte and connect to a power supply:

Electrode	Charge	Attracts
Cathode	Negative	Positive ions (cations)
Anode	Positive	Negative ions (anions)

The electrodes are usually inert (made of graphite or platinum), meaning they carry the current but do not react themselves. Because the compound is split into elements, electrolysis is the reverse of the compound forming, and it needs a continuous input of electrical energy.

Ions move because opposite charges attract. Positive ions are drawn to the negative cathode; negative ions are drawn to the positive anode. At the electrodes they are discharged, meaning they gain or lose electrons and turn into neutral atoms or molecules of an element.

A memory hook keeps the labels straight:

PANIC — Positive Anode, Negative Is Cathode. Cations (positive) go to the cathode; anions (negative) go to the anode.

The direction of electron flow explains what happens:

• At the cathode, positive metal (or hydrogen) ions arrive and gain electrons to become atoms. This is reduction.
• At the anode, negative ions arrive and lose electrons to become atoms, which usually pair up into molecules. This is oxidation.

So a positive ion such as $P{b}^{2+}$ picks up electrons at the cathode to become a lead atom, while a negative ion such as $B{r}^{-}$ gives up an electron at the anode. The overall result is that the compound is separated into a metal at one electrode and a non-metal at the other.

For a molten binary ionic compound (one made of just two elements) with inert electrodes, the rule is simple: the metal forms at the cathode and the non-metal forms at the anode.

Lead bromide, $PbB{r}_2$, is the standard example. Heated until molten, it contains $P{b}^{2+}$ ions and $B{r}^{-}$ ions that are free to move.

Electrode	Ion attracted	Product
Cathode (negative)	$P{b}^{2+}$	Lead (a molten metal)
Anode (positive)	$B{r}^{-}$	Bromine (a brown gas)

The lead ions are reduced to lead metal, which collects as a liquid at the bottom. The bromide ions are oxidised to bromine, which bubbles off as a brown vapour.

The overall reaction is:

$PbB{r}_2\to Pb+B{r}_2$

To predict the products of any molten binary compound, name the metal (cathode) and the non-metal (anode). Molten sodium chloride, for instance, gives sodium at the cathode and chlorine at the anode.

(Higher Tier only) Writing half equations for the electrode reactions is assessed only at Higher tier throughout 4.4.3.

A half equation shows what happens to the ions at one electrode, including the electrons ($e^{-}$) gained or lost. It must balance for both atoms and charge.

At the cathode, positive ions gain electrons (reduction). For molten lead bromide, each lead ion carries a 2+ charge and needs two electrons:

$P{b}^{2+}+2e^{-}\to Pb$

At the anode, negative ions lose electrons (oxidation). Two bromide ions each give up one electron and join to form a bromine molecule:

$2B{r}^{-}\to B{r}_2+2e^{-}$

Check the balance: the cathode gains 2 electrons and the anode releases 2 electrons, so the electrons cancel across the whole cell. To balance charge, always add electrons to the side that makes the total charge equal on both sides.

To turn a discharged non-metal into a molecule you usually need two ions, because gases such as $B{r}_2$, $O_2$ and $C{l}_2$ exist in pairs. Check the balancing number in front of the ion.

Metals more reactive than carbon cannot be extracted by reduction with carbon, so they are extracted by electrolysis of their molten compounds instead. Aluminium is the key example.

The reactivity series decides the method:

Metal reactivity	Extraction method
More reactive than carbon (e.g. aluminium)	Electrolysis of the molten compound
Less reactive than carbon (e.g. iron, zinc)	Reduction by heating with carbon

Electrolysis works because the electric current supplies the energy to force the metal ions to gain electrons, something carbon is not reactive enough to do for these metals. The drawback is cost: melting the compound and driving the current both use large amounts of energy, which is why electrolysis is reserved for the reactive metals that have no cheaper route.

Aluminium is extracted from its ore, which is purified to aluminium oxide ($A{l}_2{O}_3$). The oxide is then electrolysed molten.

Pure aluminium oxide melts at over 2000°C, which would need enormous amounts of energy. To avoid this, the aluminium oxide is dissolved in molten cryolite, an aluminium compound with a much lower melting point. The mixture melts at about 950°C, so far less energy is needed to keep the electrolyte molten. This is the reason a mixture is used rather than pure aluminium oxide.

The molten mixture contains $A{l}^{3+}$ and $O^{2-}$ ions:

Electrode	Ion	Product
Cathode (negative)	$A{l}^{3+}$	Aluminium metal (collects at the bottom)
Anode (positive)	$O^{2-}$	Oxygen gas

Both electrodes are made of carbon (graphite). The oxygen produced at the positive electrodes reacts with the hot carbon to form carbon dioxide, so the positive electrodes gradually burn away and must be replaced regularly. This is a running cost of the process.

(Higher Tier only) The electrode half equations are: $A{l}^{3+}+3e^{-}\to Al$ $2O^{2-}\to O_2+4e^{-}$ The reaction of carbon with oxygen at the anode is $C+O_2\to C{O}_2$.

1. Saying a solid ionic compound conducts

A solid ionic compound does not conduct because its ions are held in a fixed lattice and cannot move. Only when it is molten or dissolved are the ions free to move and carry the charge.

2. Mixing up the electrode names and charges

The cathode is the negative electrode and attracts positive ions; the anode is the positive electrode and attracts negative ions. Use PANIC (Positive Anode, Negative Is Cathode) to keep them straight.

3. Forgetting to balance charge in a half equation

Add electrons to whichever side makes the total charge the same on both sides. For $P{b}^{2+}+2e^{-}\to Pb$, the two electrons cancel the 2+ charge so both sides are neutral. A half equation that does not balance for charge scores no marks.

4. Not explaining why cryolite is used

The mark is for the reason, not just the name. State that cryolite lowers the melting point of the aluminium oxide, so less energy is needed to keep it molten and the process costs less.

5. Vague answers on why the positive electrode is replaced

Link the steps: oxygen forms at the positive carbon electrode, the oxygen reacts with the hot carbon to make carbon dioxide, so the carbon electrode burns away and has to be replaced.

6. Choosing electrolysis for a metal below carbon

Metals less reactive than carbon, such as iron and zinc, are extracted more cheaply by reduction with carbon. Electrolysis is only needed for metals more reactive than carbon, such as aluminium.