Electrolysis Principles and Electrode Reactions

Understanding Electrolysis

Electrolysis involves the decomposition of a substance using electricity. For this to occur, the substance must be an electrolyte, which is a compound that contains mobile ions. This is why electrolysis only works with ionic compounds that are either molten (liquid) or in an aqueous solution. In a solid state, the ions are held in a rigid lattice and cannot move to carry a current.

The setup consists of a power supply, usually a battery or DC source, connected to two electrodes submerged in the electrolyte. The electrodes are:

1. The Cathode, which is the negative electrode.
2. The Anode, which is the positive electrode.

The Movement of Ions

When the circuit is complete, the power source acts like an electron pump. It pulls electrons away from the anode and pushes them towards the cathode. This creates a potential difference that causes the ions in the electrolyte to migrate:

1. Cations, which are positively charged ions, are attracted to the negative cathode.
2. Anions, which are negatively charged ions, are attracted to the positive anode.

It is helpful to remember the acronym PANC, which stands for Positive Anode, Negative Cathode.

Reactions at the Cathode: Reduction

At the cathode, the positively charged cations receive electrons from the electrode. In chemistry, the gain of electrons is defined as reduction. When these ions gain electrons, they are 'discharged' and change into neutral atoms or molecules.

Worked Example: Lead(II) Bromide

In the electrolysis of molten lead(II) bromide ($PbBr_{2}$), the lead ions ($Pb^{2+}$) migrate to the cathode. Each lead ion gains two electrons to form a neutral lead atom:

$Pb^{2+}(l) + 2e^{-} \rightarrow Pb(l)$

The lead atoms collect at the bottom of the container as molten metal. This gain of electrons confirms that reduction has taken place at the cathode.

Reactions at the Anode: Oxidation

At the anode, the negatively charged anions lose electrons to the electrode. The loss of electrons is defined as oxidation. Once they lose their extra electrons, they become neutral atoms, which often bond together to form diatomic molecules.

Worked Example: Lead(II) Bromide

In the same electrolysis of molten $PbBr_{2}$, the bromide ions ($Br^{-}$) migrate to the positive anode. Each bromide ion loses one electron to become a bromine atom. These atoms immediately pair up to form bromine gas molecules ($Br_{2}$):

$2Br^{-}(l) \rightarrow Br_{2}(g) + 2e^{-}$

Brown vapours of bromine gas are observed at the anode. This loss of electrons confirms that oxidation has taken place at the anode.

The Redox Nature of Electrolysis

Because oxidation and reduction are happening simultaneously in the same system, electrolysis is a type of redox reaction. The overall equation for the lead(II) bromide example shows the decomposition of the compound into its elements:

$PbBr_{2}(l) \rightarrow Pb(l) + Br_{2}(g)$

In all electrolytic processes, the flow of electrons through the external circuit (from the positive anode back to the negative cathode) matches the transfer of electrons occurring at the electrode surfaces. No electrons flow through the electrolyte itself, only ions move within the liquid.