Electolysis is the splitting up (lysis) of compounds, using electricity (electro).
It involves two electrodes, the anode (positively charged) and the cathode (negatively charged). The electrolyte is the compound being split up, but it has to be either molten or in solution otherwise the ions wouldn't be able to move.
To remember the charges of the electrodes: PANIC!!
Which Ion goes to which Electrode?
Anions are negatively charged and they are attacted to the positive charge of the anode. Anions are all non-metals. eg. chloride Cl-, oxide O2-, hydroxide OH-
Cations are positively charged and they are attracted to the negative chage on the cathode. Cations are all metals apart from the hydogen ion H+, and the ammonium ion NH4+.
Electrolysis of Molten NaCl
When pue sodium chloride is electolysed, the sodium cations are attracted to the negative cathode, where they become atoms again. The chloride anions are attracted to the positive anode, where they become diatomic molecules again (Cl2) and a gas bubbles can be seen.
From ions to atoms (half equations)
The ions involved ae Na+ and Cl-
Balanced half equation for the Anode:
2Cl- ---> Cl2 + 2e-
Balanced half equation for the Cathode:
Na+ + 1e- ---> Na
The chloride ions lose one electron each to become chlorine molecules (Cl2)
The sodium ions gain 1 electron each to become sodium atoms (Na)
Electrolysis of brine (salt water)
When you electrolyse molten sodium chloride, the only ions involved are Na+ and Cl-, so the products at the electrodes are just sodium and chlorine. However, when sodium chloride has been dissolved in water, it's not that simple, as there are the ions in wate you have to consider.
Water = H20 so the ions involved are H= and OH-.
So altogether the ions in aqueous sodium chloride are Na+, Cl-, H+ and OH-.
The hydrogen ions are attracted to the cathode and the chloride ions are attracted to the anode, where they both become atoms again. Left in the beaker are the Na+ and OH- ions, which make a solution of sodium hydroxide, NaOH.
Diagram of electrolysis of brine
Oxidation and Reduction
Oxidation - For example, when magnesium is burnt in excess air
Mg - 2e- --> Mg2+
Magnesium loses 2 electrons to become a magnesium ion in MgO.
Reduction - for example chlorine reacting with hydrogen
Cl2 + 2e ---> 2Cl-.
Chlorine gains an electron each to become a chloride ion.
Remember: OILRIG! Oxidation Is the Loss of electrons, Reduction Is the Gain of electrons.
Electrolysis to purify copper
The anode is made of impure copper,the cathode is made of pure copper. The electrolyte is copper sulphate solution. The copper ions in the CuSO4 solution are drawn to the cathode and become copper atoms. The copper atoms from the anode then become ions to replace the lost copper ions in the CuSO4 solution. In the process, the pure copper cathode gets bigger and the impure copper anode gets smaller but the concentration of the solution stays the same.