Ionic Compounds and Electrolysis

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons to form ions. Metals lose electrons to become positively charged ions (cations), while non-metals gain electrons to become negatively charged ions (anions).

These oppositely charged ions are held together by strong electrostatic forces in a giant ionic lattice structure. This lattice is a regular, repeating arrangement of ions, which gives ionic compounds their characteristic properties.

Ionic compounds have high melting and boiling points because a lot of energy is needed to break the strong ionic bonds in the lattice. They do not conduct electricity when solid because ions are fixed in place, but they conduct electricity when molten or dissolved in water because the ions are free to move and carry charge.

For example, sodium chloride (NaCl) forms a giant ionic lattice with Na⁺ and Cl⁻ ions. It melts at about 801°C and conducts electricity when molten or in solution.

Electrolysis Basics

Electrolysis is the process of breaking down a compound using electricity. It involves passing an electric current through a liquid or molten ionic substance to cause a chemical change.

The substance that conducts electricity during electrolysis is called the electrolyte. Electrolytes contain free ions that move to the electrodes.

There are two electrodes in an electrolysis setup:

• Cathode: the negative electrode where positive ions (cations) move to gain electrons (reduction).
• Anode: the positive electrode where negative ions (anions) move to lose electrons (oxidation).

Electrons flow through the external circuit from the anode to the cathode, allowing the ions to gain or lose electrons at the electrodes.

For example, in molten lead(II) bromide (PbBr₂), Pb²⁺ ions move to the cathode to gain electrons and form lead metal, while Br⁻ ions move to the anode to lose electrons and form bromine gas.

Electrolysis of Ionic Compounds

Ionic compounds conduct electricity when molten because the ions are free to move. During electrolysis of molten ionic compounds, the compound breaks down into its elements.

The products formed depend on the ions present:

• At the cathode: positive metal ions gain electrons to form metal atoms.
• At the anode: negative non-metal ions lose electrons to form non-metal atoms or molecules.

For example, molten sodium chloride (NaCl) undergoes electrolysis as follows:

• Na⁺ ions move to the cathode and gain electrons to form sodium metal.
• Cl⁻ ions move to the anode and lose electrons to form chlorine gas.

The half equations are:

At the cathode: $\mathrm{Na^+ + e^- \rightarrow Na}$

At the anode: $\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$

This process is used to extract reactive metals like sodium and to obtain chlorine gas.

For instance, if 1 mole of molten sodium chloride is electrolysed, 1 mole of sodium metal and 0.5 moles of chlorine gas are produced because chlorine is diatomic (Cl₂).

Example: Electrolysis of molten lead(II) bromide (PbBr₂)

Lead ions (Pb²⁺) gain electrons at the cathode to form lead metal:

$\mathrm{Pb^{2+} + 2e^- \rightarrow Pb}$

Bromide ions (Br⁻) lose electrons at the anode to form bromine gas:

$\mathrm{2Br^- \rightarrow Br_2 + 2e^-}$

Electrolysis of Aqueous Solutions

When ionic compounds dissolve in water, the solution contains ions from the compound and also H⁺ and OH⁻ ions from water. Electrolysis of aqueous solutions is more complex because water can also be electrolysed.

During electrolysis of aqueous solutions:

• At the cathode, either the metal ion or hydrogen ion is discharged, depending on their reactivity.
• At the anode, either the non-metal ion or hydroxide ion is discharged, depending on the ions present.

The general rules are:

• Cathode: If the metal is more reactive than hydrogen (e.g. sodium, potassium, calcium), hydrogen gas is produced instead of the metal.
• If the metal is less reactive than hydrogen (e.g. copper, silver), the metal is produced.
• Anode: If halide ions (Cl⁻, Br⁻, I⁻) are present, the halogen gas is produced.
• If no halide ions are present, oxygen gas is produced from hydroxide ions.

For example, electrolysis of aqueous copper(II) sulfate produces copper metal at the cathode and oxygen gas at the anode because sulfate ions (SO₄²⁻) are not discharged.

The half equations are:

At the cathode: $\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$

At the anode: $\mathrm{4OH^- \rightarrow O_2 + 2H_2O + 4e^-}$

Example: Electrolysis of aqueous sodium chloride solution

At the cathode, hydrogen gas is produced because sodium is more reactive than hydrogen:

$\mathrm{2H^+ + 2e^- \rightarrow H_2}$

At the anode, chlorine gas is produced from chloride ions:

$\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$

Example: Electrolysis of molten copper(II) chloride produces copper metal at the cathode and chlorine gas at the anode.