Electrolysis

Electrolysis is the splitting of compounds into their individual elements using electricity. This extraction involves looking at reactivity of elements and whether or not the electrolyte (solution that conducts the electricity) is aqueous (dissolved in water).

In order to be electrolysed (split up using electrolysis), a substance must be molten (liquid) or dissolved in water. This is so that the ions can move and subsequently conduct the electricity. Some substances can be molten for example Aluminium Oxide (Al2O3) and other cannot conduct when molten so they are dissolved, for example Brine (NaCl(aq)).

(Daniel Falcao)

So how do we do electrolysis?

In electrolysis we need a few things to 'electrolise':

• A DC power supply (to power our circuit)
• An anode (positive electrode in the solution)
• A cathode (negative electrode in the solution)
• An electrolyte (the conductive solution)
• Wires to complete the circuit together

My very skillfull diagram

During electrolysis, positive ions are attracted to the cathode because opposites attract. Therefore, the negative ions are attracted to the anode for the same reason. Once the ions move to electrodes, they either gain or lose electrons to become an element molecule/atom again. At the anode, the ion will be oxidised and lose electrons. At the cathode, the ion will be reduced and gain electrons. A good way to remember this is the acronym OILRIG which means "Oxidation is loss, reduction is gain".

To show what happens at each electrode, we can use half-equations which show what happens to the ions as they are oxidised/reduced. An example below shows the half equations for Lead Bromide.

Two half-equations- which make a whole! 

So if our solution is aqueous what effect does this have on our reaction? As we know, water is split into two ions; H+ ions and OH- ions. When these ions are present with our ionic compound, it is dependent on the reactivity of all our ions to what is present at the electrodes. The less reactive element will always be formed at the electrode so for example the electrolysis of aqueous sodium chloride (brine), hydrogen will be formed at the cathode because it is less reactive than sodium. Just picture the more reactive chemicals as stronger so they can hold on against electricity for longer.

Well done if you've made it this far because this is tough to explain and understand by words. So let's try a talk through electrolysis with one of our favourites; the electrolysis of brine (sodium chloride aq). We want to separate this ionic compound but in order for it to conduct electricity, we must dissolve it in water. Once we pass electricity through the solution, the ions will start to become attracted to the electrodes. As the hydrogen is less reactive then the sodium, hydrogen gas will be formed at the cathode. As the chlorine ions are less reactive than the hydroxide (OH-) ions, chlorine gas will be formed at the anode. Remaining in the solution will be sodium and hydroxide ions.