Electrolysis: Electrical Conduction Through Liquid, and Gases. – SS3 Physics Lesson Note (First Term)

Electrolysis is the process in which an electric current passes through a liquid by the movement of positive and negative ions, causing chemical changes at the electrodes.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define electrolysis, electrolyte and non-electrolyte, and give examples of each;
2. state what is deposited at the anode and at the cathode;
3. state Faraday’s first and second laws of electrolysis;
4. use m = ZIt to calculate mass, current, time or electrochemical equivalent;
5. list the industrial applications of electrolysis and explain why gases normally do not conduct.

What are the key terms in electrolysis?

The key terms in electrolysis are electrolysis, electrolyte, non-electrolyte, electrode, anode, cathode and ion.

• Electrolysis: the process whereby a liquid conducts electricity by the movement of ions, while undergoing chemical change.
• Electrolyte: a liquid that allows electricity to pass through it and is decomposed by it.
• Non-electrolyte: a liquid that does not allow electricity to pass through it.
• Electrode: a metal or carbon plate through which current enters or leaves the electrolyte.
• Anode: the electrode connected to the positive terminal of the battery.
• Cathode: the electrode connected to the negative terminal of the battery.
• Ion: a charged particle formed when an atom or group of atoms gains or loses electrons.

What is the difference between an electrolyte and a non-electrolyte?

An electrolyte is a liquid that conducts electricity and is chemically changed by it. A non-electrolyte is a liquid that does not conduct electricity.

What is deposited at each electrode?

Metals and hydrogen are deposited at the cathode, while non-metals and oxygen are deposited at the anode.

• At the cathode: positive ions gain electrons. Metals such as copper, and hydrogen, are liberated.
• At the anode: negative ions lose electrons. Non-metals such as chlorine, and oxygen, are liberated.
• Active anode: the anode may dissolve into the solution, as copper does in copper(II) sulphate solution.

What happens when water is electrolysed?

Water containing a little acid decomposes into hydrogen and oxygen when current passes through it.

• Cathode: hydrogen gas is given off.
• Anode: oxygen gas is given off.
• Volumes: the volume of hydrogen is twice the volume of oxygen.
• Meaning: electrolysis splits compounds using electricity. It does not make electric charges.

What are Faraday’s laws of electrolysis?

Faraday’s laws link the mass of a substance liberated during electrolysis to the quantity of electricity passed.

• First law: the mass of a substance liberated during electrolysis is proportional to the quantity of electricity passed through the electrolyte.
• Second law: the relative masses of substances liberated by the same quantity of electricity are proportional to their chemical equivalents.
• Chemical equivalent: the relative atomic mass of the element divided by its valency.

How do you calculate the mass deposited in electrolysis?

The mass deposited is found from the formula m = ZIt, where Z is the electrochemical equivalent of the substance.

If a current I flows for a time t, the quantity of electricity is Q = It, in coulombs. By the first law, m is proportional to Q, so:

m = ZIt

Z = m / It

• Z: electrochemical equivalent, the mass deposited by one coulomb (g/C or kg/C).
• I: current in amperes (A).
• t: time in seconds (s).
• m: mass of substance in grams (g) or kilograms (kg), matching the unit of Z.

What are the steps for solving an electrolysis problem?

1. Write down the quantities given and the one required.
2. Change time to seconds (minutes × 60, hours × 3600).
3. Check that mass and Z use the same unit (grams or kilograms).
4. Choose m = ZIt, or rearrange it for Z, I or t.
5. Substitute, calculate and state the answer with its unit.

Worked examples

1. Finding Z: a cathode of mass 5 g weighs 5.01 g after 5 A flows for 50 s. Gain in mass m = 0.01 g. Q = It = 5 × 50 = 250 C. Z = m / It = 0.01 / 250 = 0.00004 g/C.
2. Finding mass: Z = 0.126 × 10^-6 kg/C and 5 A flows for 1 hour. t = 3600 s. m = ZIt = 0.126 × 10^-6 × 5 × 3600 = 2.268 × 10^-3 kg.
3. Finding time: Z for platinum = 5 × 10^-7 kg/C. To plate 1.0 kg with 100 A: t = m / ZI = 1.0 / (5 × 10^-7 × 100) = 2 × 10⁴ s, which is about 5.6 hours.

What are the applications of electrolysis?

Electrolysis is used in industry for electroplating, purifying metals and extracting metals from their compounds.

• Electroplating: coating an object with a thin layer of metal to prevent rusting or improve appearance, as in chromium-plated or nickel-plated objects.
• Purification of metals: impure copper is made the anode and pure copper the cathode in copper(II) sulphate solution. Pure copper builds up on the cathode.
• Extraction of metals: reactive metals such as sodium and aluminium are obtained by electrolysis of their molten compounds.

How is an object electroplated?

1. Clean the object to be plated.
2. Connect the object to the negative terminal, so it is the cathode.
3. Connect a plate of the plating metal to the positive terminal, so it is the anode.
4. Dip both in a solution of a salt of the plating metal.
5. Pass current for the required time. Metal ions are deposited on the object.

How do gases conduct electricity?

Gases conduct electricity only when they are ionised, because at normal pressure they have very few free charged particles.

• Normal conditions: air and other gases are poor conductors, so they act as insulators.
• Ionisation: a very high voltage, flames, X-rays or radioactive radiation can split gas atoms into ions and electrons.
• Conduction: once ionised, the charged particles move towards the electrodes and a current flows.
• Low pressure: in a discharge tube at low pressure, a gas conducts at a lower voltage and glows.
• Lightning: this is a natural discharge of electricity through air at very high voltage.

Evaluation

1. What is electrolysis? Mention at least two uses of electrolysis.
2. What is an electrolyte? Give two examples.
3. State Faraday’s laws of electrolysis.
4. A current of 3 A maintained for 50 minutes deposits 3.048 g of zinc at the cathode. Determine the electrochemical equivalent of zinc.
5. Why do gases not normally conduct electricity?

Assignment

Answer all parts of the following task on electrolysis.

1. What is meant by the statement “the electrochemical equivalent of copper is 0.000 33 g/coulomb”?
2. In the electrolysis of copper tetraoxosulphate(VI) using copper electrodes, 1.53 g of copper is deposited in 30 minutes. Determine the average current used (Z = 3.29 × 10^-4 g/C).
3. (a) Name two industrial applications of electrolysis. (b) Calculate the time to deposit 1.56 g of nickel using a current of 2.34 A in a nickel-plating process. (Assume that 1/20 g of nickel is deposited per ampere-hour.)