What it shows
The mass of copper deposited in electrolysis is proportional to the charge passed, that is current multiplied by time. Weighing the cathode after fixed intervals at a steady current gives a straight line, and the slope lets you estimate Faraday’s constant from school equipment.
The science
Each copper ion needs two electrons to become a copper atom, so the mass deposited depends on the number of electrons supplied. The number of electrons is the charge passed, which is current multiplied by time. Faraday’s first law states mass is proportional to charge, and the proportionality gives mass = (current × time × molar mass) ÷ (2 × F), where F is Faraday’s constant, about 96500 coulombs per mole of electrons. Weighing the cathode before and after fixed times at a constant current therefore gives a straight line whose slope lets you calculate F.
Materials
Cost: under Rs 200, if the school lends the balance and multimeter. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Copper sulphate solution prepared by the teacher, about 1 mol per litre, 250 ml
- Two copper strips or thick copper wires as electrodes
- A 1.5 V to 3 V battery, a rheostat or a fixed resistor of about 10 ohm to set the current, and a multimeter set to measure current in milliamperes
- A lab balance reading 0.01 g (or 0.001 g if available)
- A beaker, connecting wires, crocodile clips and a stopwatch
- Sandpaper, distilled water for rinsing and tissue
- Gloves and goggles
Safety
For this project: Teacher supervision is required; copper sulphate is harmful if swallowed and irritates skin and eyes, so wear gloves and goggles. Use only batteries. Return the used solution to the teacher for disposal.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Sand both copper electrodes clean, rinse with distilled water, dry fully and weigh the one that will be the cathode to 0.01 g.
Set up the cell with the electrodes a few centimetres apart in the copper sulphate solution; connect the weighed strip to the negative terminal of the battery and the other strip to the positive terminal, with the multimeter and the resistor in series.
Connect the battery and read the current; adjust the resistor or the electrode spacing until it is steady at around 100 to 200 mA, within the multimeter range, and record it.
Run for exactly 10 minutes, then disconnect, remove the cathode, rinse gently with distilled water, dry and weigh again.
Put the cathode back and run for another 10 minutes, repeating the weighing after 20, 30 and 40 minutes in total.
Record the current every few minutes and use the average for the calculation.
Plot mass gained against time; it should be a straight line through the origin.
Use the slope and the molar mass of copper, 63.5 g per mol, to calculate F and compare it with 96500; discuss causes of any difference such as copper flaking off.
What you should see
A straight line of mass against time, typically a few hundredths of a gram per 10 minutes at 200 mA, and a calculated Faraday constant within roughly 10 to 20 percent of 96500 C per mol.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01State Faraday’s first law of electrolysis.
The mass deposited at an electrode is proportional to the charge passed.
02Why does copper need two electrons per atom?
A copper ion carries a charge of plus two and must gain two electrons to become neutral copper.
03Why keep the current steady?
The charge passed depends on current, so a changing current would make the calculation wrong.
04What is the main source of error?
Loose copper flaking off during rinsing, or a slightly damp cathode when weighed.
Take it further
Repeat at double the current and show that the slope of the mass against time line doubles.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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