Electrochemistry
Cambridge O Level Chemistry 5070 — 4.1 Electrolysis — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions
5 min read 5 parts Lesson 1 of 2
Lesson summary
4.1 Electrolysis — electroplating: identify the anode, cathode and electrolyte, and explain how a metal coating forms through oxidation and reduction. Electroplating coats an object with a thin layer of another metal using an electric current.
- Anode: oxidation
- Cathode: reduction
Section 1 of 5Set up electroplating
Set up electroplating
Electroplating coats an object with a thin layer of another metal using an electric current. The object to be coated is connected as the cathode, the negative electrode. The anode, the positive electrode, is usually made of the pure coating metal.
The electrolyte is an aqueous solution containing ions of the coating metal, often made from a soluble salt. The ions can move through the solution to the cathode. If an object is to be coated with tin, it can be the cathode, a tin strip the anode, and tin(II) chloride solution the electrolyte.
Section 2 of 5What happens at each electrode
Anode
Anode oxidation
Atoms of the coating metal lose electrons at the anode and enter solution as positive ions. For tin: Sn(s) → Sn2+(aq) + 2e−. The tin anode gradually loses mass.
Cathode
Cathode reduction
Positive metal ions gain electrons at the cathode and become metal atoms on its surface. For tin: Sn2+(aq) + 2e− → Sn(s). A layer of tin builds up on the object, so the object gains mass.
Section 3 of 5Why electroplate an object
Why electroplate an object
A suitable coating can protect the underlying metal from corrosion or surface damage. Chromium or nickel plating is used when a durable, resistant surface is wanted. A thin coating can provide useful surface properties without making the entire object from the more expensive metal.
Electroplating can also improve appearance. Silver plating is used on some cutlery and jewellery. The coating metal is chosen for the intended function; the core metal supplies most of the shape and strength.
Section 4 of 5Worked identification
Worked identification
To silver-plate a metal spoon, connect the spoon as the cathode and a silver strip as the anode. Use an electrolyte containing silver ions. Silver ions gain electrons and deposit on the spoon; silver atoms at the anode lose electrons to replace ions in the solution.
Remember
- oxidation is loss of electrons and reduction is gain of electrons.
- At the anode oxidation occurs;
- at the cathode reduction occurs.
- Check that each half equation balances both atoms and electrical charge.
Section 5 of 5Check your understanding
- 1.
- Which electrode is the object being plated?
- 2.
- What must the electrolyte contain?
- 3.
- Write the two half equations for tin plating.
- 4.
- Explain why the tin anode loses mass.
- 5.
- Give one protective and one decorative reason for electroplating.
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