Electrochemistry is a simple topic, for there are no relevant calculations on the current syllabus. An understanding of the Electrochemical Series is important, as is the information that can be derived from it.
Definitions and Laws
The Electrochemical Series
This a list of elements in decreasing order of the ease with which they LOSE electrons.
The higher up an element is, the more readily it loses electrons (and the lower down it is, the more readily it gains electrons).
Since metals react by losing electrons, the list also shows the relative reactivities of metals; the higher up a metal is, the more reactive it is (and the lower down it is, the less reactive it is).
Also, the higher up an element is, the more stable its compounds will be. The compounds on the syllabus are the hydroxides, carbonates and nitrates of the metals.
Displacement reactions: one metal will displace another metal lower down in the series, from a solution of its ions. This is an example of oxidation-reduction.
To describe what happens during an electrolysis:
(i) List all types of ion present in the electrolyte
(ii) Write an equation to show what happens at the cathode
(iii) Write an equation to show what happens at the anode
Doing the above is relatively easy for a molten electrolyte, but for an aqueous solution, it is not so simple, for as well as the electrolyte supplying ions, the water also does so – providing both H+ and OH–.
The electrolyses listed on the syllabus include those of aqueous solutions, and this generally involves two types of ion at both anode and cathode. Which ion is discharged can be predicted from information in the Electrochemical Series. (In Option 1 of the syllabus, the electrolysis of some molten salts are listed. These are included in Materials and Metal Extraction.)
At the cathode, the element that is lower down in the electrochemical series will be released in preference to the other – since it has a greater affinity for electrons. The order of preference for the release of (the most common) ions at the anode is: OH–, Cl–, SO42–. Remember specifically: if OH– ions are present, they will always be released first.
Applications of electrolysis
Electroplating is the covering of one metal (usually a cheap and/or plentiful one) with a thin layer of a more expensive metal, to provide protection and/or a better appearance.
Metal extraction (e.g., aluminium)
Metal purification (e.g., copper)