electroplating

Higher and foundation tier- Foundation tier students are NOT required to write ion-electron half equations for reactions happening at the electrodes. You only need to be able to predict the what will be produced at the anode and cathode. You should have a good knowledge of how electrolysis works before reading this page.

chrome plated motorcycle partsOne of the main uses of electrolysis is electroplating objects. Electroplated objects are metal objects which are given a thin layer of metal as an additional coating. Objects are usually electroplated to make them more attractive looking or to help them resist corrosion. Car, motorcycle and bike parts such as mirrors, exhausts, and bumpers for example are often electroplated with chromium metal to make them look shiny and also to resist corrosion. Inexpensive jewellery is made of a cheaper metal which is then silver or gold plated. Rings, braclets, watches are often electroplated.


gold plated electrical connectors

Copper is often used to make wires as electrical connectors to devices such as televisions, xboxs and sound equipment such as electrical guitars, hi-fis and other musical instruments, but in time it will tarnish and its electrical conductivity will fall. Hi-end electrical connectors are gold plated. Gold is an excellent electrical conductor and being very unreactive will not tarnish, however it is far more expensive than copper or even silver which is sometimes used in connectors.


Another common example of electroplated objects are coins. Coins are often made of steel which is copper or zinc plated. Steel will corrode but copper is an unreactive metal and it will prevent the steel from corroding. Steel is much less expensive than copper, so for example if 2 pence coins were made of pure copper, you could make a lot of money by simply melting them down and selling the copper metal produced! The image below shows some everyday electroplated objects.

electroplated objects montage

How to electroplate objects

key being electroplated

Electroplating is very simple to carry out. The object to be plated is made the cathode in a cell, as shown. The object to be plated must be an electrical conductor. In this case a metal key is placed in a copper sulfate solution. The copper sulfate solution contains copper ions, Cu2+(aq) and SO42-(aq) ions. The half equations for the reactions at the anode and cathode are:

cathode half-equation: Cu2+(aq) +2e → Cu(s)
This means that the copper ions in solution will be attracted to the cathode, the key, and when they reach the key the copper ions gain 2 electrons and are reduced to copper atoms. These copper atoms coat or plate the key. Obviously you can swap the key for a ring, bracelet, a car part or in fact any metal object can be plated. By swapping the copper sulfate solution for a silver nitrate or gold nitrate solution you can silver or gold plate objects.

Anode half-equation: Sulfate ions being a stable group ion are not discharged, hydroxide ions from the water are discharged instead to produce oxygen gas.

4OH-(aq)2H2O(l) + O2(g) +4e
Platinum is used for the anode because it is a very unreactive metal and will not interfere with any reactions taking place in the cell. The ammeter is included in the circuit to monitor the size of the current flowing. If too large a current is used the metal plates too quickly onto the key and a poor coating is generally obtained.

Galvanising and tin plating

watercan being galvanised

Galvanising is a common method used to prevent a metal object from corroding. Many cars are galvanised an additional method of preventing corrosion if the paint is damaged or chipped. Galvanising involves coating a metal, normally iron or steel with a thin layer of zinc. Zinc being more a more reactive metal will sacfrice itself to protect the iron or steel. Metal objects are galvanised by either dipping the object in a bath of molten zinc or by electroplating, as shown. Here the steel watering can is given a thin plate of zinc by being electroplated. The watering can is made the cathode and when an electrical current flows the zinc ions in the solution will be attracted to the cathode and reduced:

cathode half-equation: Zn2+(aq) +2e → Zn(s)

If a strip of zinc is used at the anode then it will slowly dissolve away and replace the zinc ions which are reduced at the cathode:

anode half-equation: Zn(s) → Zn2+(aq) +2e
This means that the overall reaction simply involves zinc from the anode dissolving to form zinc ions (Zn 2+) and these ions are then attracted to the cathode where they are reduced and plate onto the watering can.

Tin plating

food cans are tin or chromium plated

Food cans are made from steel, however many foods are acidic or contain substances which can attack the steel can. Most food cans are designed to keep their contents "fresh" for several years and so they need to be constructed in a way that can resist the chemicals in food. The steel can could be plated with tin or even chromium. Tin and chromium are unreactive metal that will protect the steel can and prevent corrosion or oxidation of the metal. The can will also likely have some sort of coating on the inside to help ensure no reaction occurs between the food and the can.

Key Points

Practice questions

Check your understanding - Questions on electroplating.

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