Ch 1 · Electrolysis · AS & A Level Chemistry 9701

Topic 1 of 2 in Electrochemistry — AS & A Level Chemistry 9701 — 47 lessons.

Introduction

Electrolysis uses electrical energy to drive a non-feasible redox reaction, decomposing an electrolyte at two electrodes. Cambridge A2 electrolysis builds on the redox ideas from AS to predict which product forms at each electrode, and adds quantitative calculations linking the charge passed to the mass or volume of product obtained.

Key ideas — Predicting the products of electrolysis

A standard electrode potential, E^ominus, is a number in volts that measures how readily a species is reduced (gains electrons) under standard conditions: the more positive the value, the more easily that species is reduced, and the more negative the value, the more easily the reverse (oxidation) happens instead. Comparing the E^ominus values of the ions and molecules present at an electrode tells you which one reacts preferentially there.

🧑‍🏫 Teacher's explanation

Think of electrolysis as a competitive race. At each electrode, different ions are 'racing' to react. The winner is usually the one that is most 'eager' to change its charge. At the cathode (the negative pole), cations want to gain electrons to become neutral. The ion with the most positive (or least negative) electrode potential wins this race. At the anode (the positive pole), anions want to lose electrons. The ion that is easiest to oxidize wins.

📖 Definitions

🔍 Worked example — Easy

Predict the products of the electrolysis of aqueous CuSO_4 using inert electrodes.

🔍 Worked example — Standard

Calculate the mass of Cu deposited at the cathode if 2.0 moles of electrons pass through the cell. (A_r(Cu) = 64)

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — The Faraday constant

The Faraday constant, F, is the charge carried by one mole of electrons. It is related to the Avogadro constant, L, and the magnitude of the charge on a single electron, e, by:

🧑‍🏫 Teacher's explanation

Imagine you are moving a large shipment of identical small boxes from one warehouse to another. If you know exactly how many boxes are in one crate (the Avogadro constant) and exactly how much each individual box weighs (the charge of one electron), you can easily calculate the total weight of an entire crate (the Faraday constant).

📖 Definitions

🔍 Worked example — Easy

Calculate the total charge Q passed by a current of 0.50 A flowing for 200 s.

🔍 Worked example — Standard

A current of 1.5 A is passed through an electrolyte for 30 minutes. Calculate the total charge passed.

🔍 Worked example — Exam-style

Calculate the mass of Copper (A_r = 64) deposited at a cathode if 96500 C of charge is passed through a solution of CuSO_4. The half-equation is Cu^2+ + 2e^- arrow Cu(s).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Quantity of charge, Q = It

The charge, Q (in coulombs), passed through an electrolytic cell is calculated from the current, I (in amperes), and the time, t (in seconds), for which it flows:

🧑‍🏫 Teacher's explanation

Think of electricity like a flowing river of water. The current, I, is the speed or volume of water passing a certain point every second, while the time, t, is how long you leave the tap running. The total amount of 'stuff' (charge) that has passed through is the product of the two. In chemistry, this 'stuff' is actually a stream of electrons moving through the circuit.

📖 Definitions

🔍 Worked example — Easy

A constant current of 0.50 A is passed through an electrolytic cell for 120 s. Calculate the total charge Q passed.

🔍 Worked example — Standard

Calculate the number of moles of electrons passed when 150 C of charge flows through a cell. (Faraday constant F = 96500 C mol^-1)

🔍 Worked example — Exam-style

A current of 2.00 A is passed through molten CuCl_2 for 30.0 minutes. Calculate the mass of Cu deposited at the cathode. (Relative atomic mass Cu = 64, F = 96500 C mol^-1)

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Calculating mass and volume of product

The number of moles of electrons passed is n(e^-) = Q / F. This is linked to the moles of product using the stoichiometry of the relevant half-equation, for example mathrmCu^2+(aq) + 2e^- arrow Cu(s), which needs 2 mol of electrons per mole of copper. The mass of a solid product is then found from mass = moles × M_r, and the volume of a gaseous product from volume = moles × 24 000\ mathrmcm^3\ mol^-1 (molar gas volume at room temperature and pressure).

Key ideas — Determining the Avogadro constant electrolytically

A known current is passed for a measured time through a cell depositing a metal, for example silver from aqueous silver nitrate at an inert cathode, and the mass of metal deposited is found by weighing the electrode before and after. From the mass and the molar mass, the moles of metal (and hence moles of electrons, using the charge on the ion) are found; dividing the total charge passed, Q = It, by the charge per mole of electrons gives F. Since L = F / e and e is known independently, this experiment gives an experimental value for the Avogadro constant.

🧑‍🏫 Teacher's explanation

Imagine you have a massive pile of identical sand grains, and you want to count how many are in the pile. Instead of counting them one by one, you weigh the whole pile and divide by the weight of a single grain. Electrolysis does the exact same thing, but instead of sand grains, we are counting electrons.

Ch 1 · Electrolysis · AS & A Level Chemistry 9701

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