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Electrolytic cell

An electrolytic cell.
An electrolytic cell.

Electrolysis is the use of electric current to drive chemical reactions.

Many chemical reactions cannot take place (they are non-spontaneous) without an input of energy, typically in the form of an applied electric current.

Electrolysis requires three main components:

  • An electric current supply: Provides energy for the reaction through a circuit (e.g. a battery).
  • An electrolyte: A liquid containing dissolved ions that are to be separated. It is often an ionic compound that is either molten or dissolved in a liquid (usually water).
  • Two electrodes: Conductors that transfer the current between the supply and the electrolyte. They usually do not take part in the reaction (inert).

The device containing these three components is called an electrolytic cell.

Electrolysis is used to separate ionic compounds into their constituent elements.

An electrolyte is a liquid or a gel that conducts electricity in electrolysis reactions because it contains mobile charged particles such as mobile ions.

Molten ionic compounds are electrolytes because the ions are free flowing in liquid form (e.g. melting solid table salt ($$\ce{NaCl}$$) gives molten table salt).

Aqueous ionic compounds are also electrolytes because the ions are dissolved (and thus mobile) in water (e.g. dissolving table salt in water gives aqueous salt).

Solid ionic compounds are not electrolytes and do not conduct electricity.

The ions in solid ionic compounds are held together tightly in a lattice structure by electrostatic forces and are therefore not mobile.

Solid ionic compounds (such as solid table salt) thus cannot be separated in electrolysis reactions.

In a solid ionic compound, the ions are locked in the lattice structure.
In a solid ionic compound, the ions are locked in the lattice structure.

A battery is often the electric current supply in electrolytic cells. An electric current is a flow of electrically charged particles.

A battery contains a positive terminal and a negative terminal.

Electrons generated from reactions in the battery flow out of the negative terminal. Electrons required for the reactions flow into the positive terminal of the battery.

Upon leaving the negative terminal, the electrons travel to the cathode (the negative electrode) through a metallic wire.

The cathode is therefore reduced (electrons are gained).

Electrons flow away from the anode (the positive electrode) through a metallic wire toward the positive terminal of the battery.

The anode is therefore oxidised (electrons are lost).

A battery therefore creates a one-way flow of electrons from the anode through the connecting wire and back to the cathode.

Electrodes are conductors that connect the current-carrying metallic wire to the non-metallic electrolyte solution. Inert electrodes are usually used during electrolysis.

Two electrodes are used: the anode and the cathode.

The anode becomes positively charged due to the loss of negatively charged electrons to the battery. The anode thus attracts negatively charged anions in the surrounding electrolyte.

The cathode becomes negatively charged due to the gain of negatively charged electrons from the battery. The cathode thus attracts positively charged cations in the surrounding electrolyte.

The excess electrons at the cathode reduce the cations in the surrounding electrolyte. These reduced cations become neutral atoms or molecules that leave the electrolyte solution as either solids or gases.

The positively charged anode oxidises the anions in the surrounding electrolyte. These anions become neutral atoms or molecules that usually leave the electrolyte solution as gases.