What you'll learn
Electrochemistry and electrolysis is the topic in CXC CSEC Chemistry that uses electricity to drive chemical change, and it underpins several industries of direct Caribbean importance — aluminium smelting from bauxite, electroplating, and the purification of copper. The topic demands precision rather than difficulty: once you know which ions move where, what happens at each electrode, and the rules for deciding between competing ions in solution, almost every question becomes systematic. By the end of this guide you should be able to define the key terms and distinguish electrolytes from non-electrolytes, predict the products of electrolysis for molten compounds and for solutions, write half equations at both electrodes, explain the effect of using active rather than inert electrodes, and describe the main industrial applications with their conditions.
Key terms and definitions
Electrolysis — the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
Electrolyte — a substance that conducts electricity when molten or in solution and is decomposed by it
Non-electrolyte — a substance that does not conduct electricity in any state
Strong electrolyte — a substance completely ionised in solution, conducting well
Weak electrolyte — a substance only partially ionised in solution, conducting poorly
Electrode — a conducting rod or plate through which current enters or leaves the electrolyte
Cathode — the negative electrode, where reduction occurs
Anode — the positive electrode, where oxidation occurs
Inert electrode — an electrode that does not take part in the reaction, usually graphite or platinum
Active electrode — an electrode that itself takes part in the reaction
Electroplating — coating an object with a thin layer of metal by electrolysis
Selective discharge — the preferential discharge of one ion over another at an electrode
Core concepts
Electrolytes and conduction
An ionic compound conducts electricity only when its ions are free to move. In the solid state the ions are held in fixed positions in the lattice, so no current flows. When the compound is melted or dissolved in water, the lattice breaks apart and the ions become mobile, so the substance conducts and is decomposed.
This distinguishes electrolytic conduction from metallic conduction. In a metal, delocalised electrons move and the metal is unchanged. In an electrolyte, ions move and the substance is chemically decomposed.
Strong electrolytes are completely ionised in solution and conduct well: these include the strong acids such as hydrochloric, sulfuric and nitric acid, the strong alkalis such as sodium hydroxide and potassium hydroxide, and all soluble salts.
Weak electrolytes are only partially ionised and conduct poorly: ethanoic acid, other organic acids, and ammonia solution are the examples required.
Non-electrolytes are covalent substances that do not ionise at all, such as sugar solution, ethanol, petrol and pure water.
Movement of ions and the electrodes
During electrolysis the positive ions, called cations, are attracted to the negative electrode, the cathode. The negative ions, called anions, are attracted to the positive electrode, the anode.
Opposite charges attract, which is the whole reason for the movement, and stating it prevents the commonest error in the topic.
At the cathode, cations gain electrons. Gaining electrons is reduction, so reduction always occurs at the cathode.
At the anode, anions lose electrons. Losing electrons is oxidation, so oxidation always occurs at the anode.
Metals and hydrogen form at the cathode; non-metals other than hydrogen form at the anode.
Electrolysis of molten compounds
With a molten compound the situation is simple, because only two types of ion are present.
Molten lead bromide gives lead at the cathode, as lead ions gain electrons, and bromine at the anode, as bromide ions lose electrons.
Molten sodium chloride gives sodium at the cathode and chlorine at the anode.
Molten aluminium oxide, dissolved in cryolite, gives aluminium at the cathode and oxygen at the anode. This is the industrial extraction of aluminium from Caribbean bauxite, and the cryolite is added to lower the melting point and so reduce the energy required. The carbon anodes react with the oxygen produced at the high temperature and must be replaced periodically.
Electrolysis of aqueous solutions
In solution the situation is more complex, because water itself provides hydrogen ions and hydroxide ions in addition to the ions from the dissolved compound. There is therefore competition at each electrode, and rules are needed to decide which ion is discharged.
At the cathode, hydrogen is produced unless the metal is less reactive than hydrogen, in which case the metal is deposited. So a solution of a copper salt gives copper, because copper is below hydrogen in the reactivity series, whereas a solution of a sodium or potassium salt gives hydrogen.
At the anode, oxygen is produced from hydroxide ions unless a halide ion is present, in which case the halogen is produced. So sodium chloride solution gives chlorine, while sodium sulfate solution gives oxygen.
Concentration also matters at the anode. A concentrated halide solution favours discharge of the halogen, while a very dilute one favours oxygen.
Applying these rules in a fixed order — list the ions present including those from water, decide the cathode product, then decide the anode product — makes every question of this type routine.
Effect of concentration and electrode material
Electrolysis of concentrated sodium chloride solution, known as brine, produces chlorine at the anode, hydrogen at the cathode, and sodium hydroxide remaining in solution. All three products are industrially valuable: chlorine for water treatment and bleach, hydrogen for margarine manufacture and ammonia production, and sodium hydroxide for soap and paper making.
Electrolysis of dilute sulfuric acid, sometimes described as the electrolysis of water, gives hydrogen at the cathode and oxygen at the anode in a volume ratio of two to one, which reflects the formula of water.
The electrode material matters greatly. Inert electrodes of graphite or platinum take no part in the reaction and are used where the products themselves are wanted.
Active electrodes take part. In the electrolysis of copper sulfate solution using copper electrodes, the copper anode dissolves as copper atoms lose electrons and enter the solution as ions, while copper is deposited at the cathode. The anode therefore loses mass and the cathode gains an equal mass, and the concentration of the solution remains unchanged. The same experiment with carbon electrodes instead gives oxygen at the anode and the solution becomes paler as copper ions are removed.
That contrast — same electrolyte, different electrodes, entirely different outcome — is a favourite examination question.
Industrial applications
The purification of copper uses active electrodes. The impure copper forms the anode and a thin sheet of pure copper forms the cathode, with copper sulfate solution as the electrolyte. Copper dissolves from the impure anode and is deposited in pure form on the cathode. The impurities, which include silver and gold, do not dissolve and collect beneath the anode as anode sludge, which is valuable enough to be recovered.
Electroplating coats a cheaper metal with a thin layer of a more attractive or more corrosion-resistant one. The object to be plated is made the cathode, the plating metal forms the anode, and the electrolyte is a solution of a salt of the plating metal. Objects are electroplated to improve appearance, as with silver plating on cutlery, or to prevent corrosion, as with chromium plating on steel.
For an even coating, the current should be low and the object clean and evenly positioned relative to the anode.
The extraction of reactive metals such as aluminium, sodium and magnesium requires electrolysis of the molten compound, because these metals are too reactive to be extracted by reduction with carbon.
Worked examples
Example 1: Predicting products in solution (4 marks)
Predict the products of the electrolysis of aqueous potassium bromide using inert electrodes, and explain your reasoning.
The ions present are potassium ions and bromide ions from the salt, together with hydrogen ions and hydroxide ions from the water.
At the cathode, potassium ions and hydrogen ions compete. Potassium is more reactive than hydrogen, so hydrogen is discharged and hydrogen gas is produced rather than potassium metal.
At the anode, bromide ions and hydroxide ions compete. Because a halide ion is present, the halogen is discharged in preference to oxygen, so bromine is produced.
The products are therefore hydrogen at the cathode and bromine at the anode, with potassium hydroxide remaining in solution.
Example 2: Explaining the effect of electrode material (4 marks)
Copper sulfate solution is electrolysed first with carbon electrodes and then with copper electrodes. Describe and explain the difference at the anode.
With carbon electrodes, the anode is inert and takes no part in the reaction. Hydroxide ions from the water are discharged, losing electrons, so oxygen gas is produced at the anode and the solution becomes paler as copper ions are removed at the cathode without replacement.
With copper electrodes, the anode is active. Copper atoms from the anode lose electrons and pass into the solution as copper ions, so the anode dissolves and loses mass while no gas is produced. Because copper ions are being added to the solution at the same rate as they are deposited at the cathode, the concentration of the solution stays constant.
Example 3: Writing half equations (3 marks)
Write the half equations for the electrolysis of molten aluminium oxide and state at which electrode each occurs.
At the cathode, aluminium ions each gain three electrons to form aluminium atoms, so the three electrons appear on the left of the half equation. This is reduction, and it occurs at the negative electrode.
At the anode, oxide ions each lose two electrons to form oxygen atoms, which then pair to form oxygen molecules, so the electrons appear on the right of the half equation. This is oxidation, and it occurs at the positive electrode.
To combine them, the cathode equation must be doubled and the anode equation tripled so that six electrons appear on each side and cancel.
Common mistakes and how to avoid them
The most frequent error is sending positive ions to the positive electrode. Opposite charges attract, so cations go to the negative cathode. Writing this at the top of the answer space prevents it.
Students often forget that water supplies hydrogen and hydroxide ions in aqueous electrolysis, and predict the products as though the solution were molten. Always list all four ion types for a solution.
Another common slip is stating that the anode is negative because it attracts negative ions. The anode is positive, which is exactly why it attracts negative ions.
In half equations, many candidates balance the atoms but not the charge. Both must balance.
Finally, candidates frequently overlook the role of the electrode material. If copper electrodes are specified rather than carbon, the question is testing the active electrode case.
Exam technique for "Electrochemistry and Electrolysis"
Work in a fixed order every time: list the ions present, identify the cathode product, identify the anode product, then write half equations if required. The mechanical sequence is faster under pressure than reasoning from scratch.
Check whether the electrolyte is molten or aqueous before applying any rule. The two cases have different answers for the same compound.
Note the electrode material explicitly. Carbon or platinum means inert; copper or another named metal means active, and the anode will dissolve.
For industrial questions, state the electrolyte, both electrodes and the product at each, since marks are usually distributed across all four.
When half equations must be combined, multiply them so the electrons cancel exactly before adding.
Quick revision summary
Electrolysis decomposes an ionic compound that is molten or in solution, because only then are the ions free to move. Strong electrolytes such as strong acids, strong alkalis and soluble salts ionise completely; weak electrolytes such as ethanoic acid and ammonia ionise partially; covalent substances such as sugar solution and ethanol are non-electrolytes. Cations move to the negative cathode where they gain electrons and are reduced, giving metals or hydrogen; anions move to the positive anode where they lose electrons and are oxidised, giving non-metals. For molten compounds the elements are simply released. For solutions, hydrogen forms at the cathode unless the metal is less reactive than hydrogen, and oxygen forms at the anode unless a halide is present. Brine gives chlorine, hydrogen and sodium hydroxide, all industrially valuable. Inert carbon or platinum electrodes take no part, while active copper electrodes dissolve at the anode and plate at the cathode, keeping the solution concentration constant. Applications include aluminium extraction from bauxite in molten cryolite, copper purification leaving valuable anode sludge, and electroplating for appearance or corrosion resistance.