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CSEC Chemistry: Electrolysis

When electrolytes conduct electricity, they are decomposed by the current. This is called electrolysis. Said completely, electrolysis id the decomposition of electrolytes due to the passage of an electric current through it.

Electrolysis is carried out using an apparatus known as an electrolytic cell. An electrolytic cell is composed of a source of current, two electrodes and an electrolyte.

Diagram of an electrolytic cell

Electrodes are conductive rods (often made of platinum or graphite) where current enters and leaves an electrolyte. Electrodes are usually inert, and therefore will not take part in the chemical changes occurring in electrolysis.

The positive electrode, known as the anode, is connected to the positive terminal of the power source.

The negative electrode, known as the cathode, is connected to the negative terminal of the power source.


So, as we discussed in this previous post, electrolytes are able to conduct electricity due to having mobile ions. When a current is run through an electrolyte, it ionizes, that is, the positive cations separate from the negative anions. If you've ever heard the phrase 'opposites attract,' a similar thing happens during electrolysis:

  • anions (negative ions) move towards the anode (positive electrode)

  • cations (positive ions) move towards the cathode (negative electrode)

(You could remember this with the first letters of each, anions to the anode and cations to the cathode)


So, if we were to demonstrate the electrolysis of molten sodium chloride:





The sodium cation goes to the cathode, and the chlorine anion goes to the anode.












During electrolysis, a process known as discharge occurs to the ions. Discharge is the process where ions gain or lose electrons (that is, are reduced or oxidized) and become atoms or molecules. Ions lose their charges during discharge.

As you can imagine, the process of discharge would be different at both the cathode and anode, since cations would be losing a positive charge by gaining an electron (reduction) at the cathode while anions would be losing a negative charge by losing an electron (oxidation) at the anode.


Discharge at the Cathode

  1. Positive ions (cations) move towards the cathode.

  2. Cations gain electrons at from the cathode.

  3. Cations become neutral atoms/molecules. (As you know, cations are formed due to a neutral element/compound losing a certain number of electrons. By regaining those electrons from the cathode, the ion becomes neutral again)


This can be shown with the half equation:

Mⁿ⁺ + ne⁻ → M

You probably recognize this as reduction (gain of electrons).



Discharge at the Anode

  1. Negative ions (anions) move towards the anode.

  2. Anions lose/give up electrons to the anode.

  3. The anions become neutral atoms/molecules. (As you know, anions are formed due to a neutral element/compound gaining a certain number of electrons. By giving those electrons to the anode, the anion becomes neutral again)

This can be summarized with the half equation:

Xⁿ⁻ - ne⁻ → X

or

Xⁿ⁻ → X + ne⁻

You probably recognize this as oxidation (loss of electrons).


For example: the electrolysis of potassium iodide:

2K⁺ + 2e⁻ → 2K

Potassium ion → neutral atoms


2I⁻ + 2e⁻ → I₂(g)

Iodide ion → neutral molecules