Electrolysis

Electrolysis: Breaking down an ionic substance by electricity

Electrodes: The conducting rods in an electrolysis cell. One is positive, the other is negative

Electrolyte: A liquid containg free ions that is broken down using electrolysis. A solid must be melted or dissolved in water to become an electrolyte so that the ions are free to move to the electrons

Anode: Positive electrode

Cathode: Negative electrode

OIL RIG

Ions are reduced at the cathode

Ions are oxidised at the anode

If an ionic compound is dissolved in water, we must take the water ions into account

The hydrogen ions will also be attracted to the negative electrode, so only the least reactive ion is reduced. The more reactive ions stay in solution.

Least reactive > Most reactive

Oxygen is given off at the positive electrode from discharged hydroxide ions, unless there is a halide ion present (Any element in group 7), in which case the halide ion is oxidised to form a gas.

Halide > Hydroxide

• Aluminium oxide is extracted from bauxite ore.
• Molten aluminium oxide is mixed with cryolite to lower its melting point
• Aluminium forms at the cathode at the bottom of the tank and is siphoned off
• Oxygen forms at the anode
• The positive carbon electrodes need to be replaced as they start wearing away

2Al O  (l) = 4Al (l) + 3O  (g)

• Brine is a concentrated sodium chloride solution
• Anode: Chlorine gas is formed, a pale green colour
• Cathode: Hydrogen gas is formed, a colourless gas
• Sodium hydroxide is left in solution

Testing:

• Chlorine: Dampl pH paper is bleached white
• Hydrogen: A lit splint makes a squeaky pop sound
• Sodium hydroxide: pH paper turns dark blue- it is an alkali

Uses:

• Chlorine: Kills bacteria, makes bleach & PVC
• Hydrogen: Makes margerine, fertilizers and rocket fuel
• Sodium hydroxide: Makes soap & paper, neutralises acid

2NaCl (aq) + 2H O (l) = 2NaOH (aq) + Cl  (g) + H  (g)

An electroplated object is coated with a thin layer of metal by electrolysis

We use electroplating to:

• Protect the metal from corroding
• To make the object more attractive
• Increase the resistance to scratching
• Save money on expensive metals

The object to be plated becomes the cathode. The anode is made of the plating metal ( nickel)

Anode:

• The nickel atoms lose 2 electrons and form nickel ions which go into the solution
• Zn (s) = Zn  (aq) + 2e

Cathode:

• Nickel ions gain 2 electrons and form nickel atoms that are deposited onto the object
• Zn  (aq) + 2e = Zn (s)