208
81.
Classic chemistry experiments
Quantitative electrolysis
Topic
The mole and electrolysis.
Timing
60 min.
Description
Students perform a quantitative electrolysis using copper electrodes and copper(II)
sulfate solution. The number of moles of metal removed is calculated, and linked to the
amount of electricity passed through the circuit.
Apparatus and equipment (per group)
▼ 6 V DC supply
▼ Ammeter 0–1 A
▼ Five connecting leads
3
▼ 100 cm Beaker
▼ Access to a balance reading to 0.01 g.
Chemicals (per group)
▼ Two copper foil electrodes (approximately 5 cm x 3 cm)
–3
▼ Copper(II) sulfate solution 0.1 mol dm
Teaching tips
Use reduction in mass of the anode to explain the principles. Copper can easily fall off
the cathode and it can be wiped off when drying (especially when deposited at higher
currents or when the cathode is dirty).
The electrodes should be cleaned before use.
For an indication of the theoretical loss of mass: 0.4 A for 30 min removes
0.4 x 30 x 60 x 0.5 x 63.5 = 0.237 g of copper.
Current should be kept constant by direct adjustment of the power supply or by the
inclusion of a rheostat to the circuit. It has been suggested that adjusting the position of
the electrodes is also effective.
Background theory
Students require knowledge of charges on ions, the mole, and the relationship between
quantity of charge, current and time. They can also be introduced to the idea of a mole
of electrons.
Safety
Wear eye protection.
Answers
1,2,3 will depend on experimental results.
4. Main errors are fluctuations in the current and inaccurate measurements.
Classic chemistry experiments
209
Quantitative electrolysis
Introduction
When electrolysis is done on a commercial scale it is important to know how much
current is required and for how long. This experiment relates the amount of metal
removed from an electrode to the electric current and the time the current flows.
Power supply
A
Ammeter
Beaker
Cathode (copper)
Anode (copper)
Copper(II) sulfate
solution
What to record
▼ The masses of the electrodes before electrolysis (identify the electrodes by writing
on them).
▼ The masses of the electrodes after electrolysis.
▼ The current flowing.
▼ The time the current flows.
What to do
1. Clean the electrodes with emery paper (avoid inhaling any dust).
2. Weigh the anode.
3. Immerse the electrodes to a depth of 3–4 cm in the solution of copper(II) sulfate.
4. Allow about 0.4 A to pass for about 30 min.
5. Remove the anode, wash carefully in water and dry gently with a paper towel.
6. Reweigh the anode.
Safety
Wear eye protection.
Questions
1. Calculate the number of moles of copper that have been removed from the
anode.(Mass lost in g / 63.5)
2. Calculate the charge that has flowed through the circuit using the relationship
charge (in Coulombs) = current (in amps) x time (in seconds).
3. Using the answers to questions 1 and 2, calculate the number of Coulombs
required to remove one mole of copper.
4. 193,000 (2 x 96,500) Coulombs is required to remove one mole of copper. The
difference between this and the answer to question 3 is due to errors in the
experiment. What are the main sources of error in this experiment?