Pass an electric current through a molten salt or a solution and it splits into new substances. This is how metals are plated and refined.
An electrolyte is a substance that conducts electricity in the molten state or in solution and is chemically decomposed by it. The anode is the positive electrode, where anions are discharged. The cathode is the negative electrode, where cations are discharged. Strong electrolytes such as HCl and NaCl ionise almost completely, while weak ones such as acetic acid ionise only slightly.
Which ions move to the cathode, and which to the anode?
Cations (positive ions) move to the cathode; anions (negative ions) move to the anode
Name one non-electrolyte.
Sugar solution or alcohol, which do not ionise
Molten lead bromide gives lead at the cathode and bromine at the anode. Acidified water gives hydrogen at the cathode and oxygen at the anode in a volume ratio of 2 : 1. Copper sulphate solution with inert electrodes gives copper at the cathode and oxygen at the anode. With copper electrodes the anode dissolves and copper is deposited on the cathode.
What is the volume ratio of hydrogen to oxygen in the electrolysis of acidified water?
2 : 1, since 2H₂O → 2H₂ + O₂
Why is a little acid added to water before electrolysis?
Pure water is a very poor conductor; the acid supplies ions so that current can flow
What happens at the anode when copper sulphate solution is electrolysed with copper electrodes?
The copper anode dissolves, sending Cu²⁺ ions into the solution, while copper is deposited at the cathode
In electroplating, the article to be plated is made the cathode and a rod of the plating metal is the anode. The electrolyte is a solution of a salt of the plating metal. Plating protects metals from corrosion and improves their appearance.
To silver-plate a spoon, which electrode is the spoon and which is the anode?
The spoon is the cathode; a rod of pure silver is the anode
1. Define an electrolyte.
2. Which electrode is connected to the positive terminal of the cell?
3. At which electrode are cations discharged?
4. What is formed at the cathode when molten lead bromide is electrolysed?
5. What is the volume ratio of hydrogen to oxygen in the electrolysis of acidified water?
6. In silver-plating a spoon, which electrode is the spoon?
7. Why is acid added to water before electrolysing it?
8. What happens to a copper anode when copper sulphate solution is electrolysed using copper electrodes?
9. Which gas is released at the anode when concentrated sodium chloride solution is electrolysed?
10. Give one reason why articles are electroplated.