Required Practical 3

Required Practical 3: Electrolysis of Aqueous Solutions

Revision guide containing method, variables, safety, sample calculations, and model exam answers.

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Scientific Principles & Theory

Scientific Background: Electrolysis is the decomposition of an electrolyte using a direct electric current. In an aqueous solution, water molecules dissociate into H+ and OH- ions alongside the ions from the solute.

The products discharged at the inert electrodes depend on the relative reactivity of the ions:

  • Cathode (Negative Electrode): Hydrogen gas is produced unless the metal ion in solution is less reactive than hydrogen (e.g., copper). In that case, the metal is discharged.
  • Anode (Positive Electrode): Halide ions (Cl-, Br-, I-) are discharged as halogens. If no halide is present, hydroxide (OH-) ions are discharged to form oxygen gas and water: 4OH- -> O2 + 2H2O + 4e-.

Experimental Variables

Independent Variable

The electrolyte solution being electrolysed (copper(II) chloride, copper(II) sulfate, sodium chloride, sodium sulfate).

Dependent Variable

The products discharged and visual observations at the anode (+) and cathode (-).

Control Variables

The concentration of the electrolyte solutions (0.5 mol/dm3), the voltage (4V DC), and the type of electrodes (graphite).

⚠️ Lab Risk Assessment

Hazard Associated Risk Control Measure
Toxic chlorine gas (from chloride solutions) Respiratory irritation and lung damage Electrolyse for a maximum of 5 minutes at low voltage (4V); ensure the laboratory is well-ventilated; do not inhale gas directly.
Electrical current and liquids Risk of short circuit or electric shock Keep electrical connections dry; ensure the graphite electrodes do not touch each other.

Apparatus & Procedure

Required Apparatus

  • Copper(II) chloride solution (0.5 mol/dm3)
  • Copper(II) sulfate solution (0.5 mol/dm3)
  • Sodium chloride solution (0.5 mol/dm3)
  • Sodium sulfate solution (0.5 mol/dm3)
  • Beaker (100 cm3)
  • Petri dish lid with two holes (or bungs)
  • Two graphite (carbon) rods
  • Crocodile clips and connecting leads
  • Low voltage DC power supply (4V)
  • Blue litmus paper and tweezers
  • Stopwatch

Step-by-Step Procedure

  1. Pour about 50 cm3 of copper(II) chloride solution into a beaker.
  2. Place a petri dish lid with two holes on top of the beaker. Insert the carbon rods through the holes. Ensure the rods do not touch.
  3. Attach crocodile leads to the carbon rods and connect them to the DC terminals of the power supply.
  4. Select 4V on the power supply and turn it on.
  5. Observe both electrodes and record any bubbling, colour changes, or solid deposits.
  6. Use tweezers to hold a damp piece of blue litmus paper in the gas bubbles escaping at the anode (positive electrode). If it turns red and then bleaches white, chlorine gas is present.
  7. After 5 minutes, turn off the power supply and check the cathode (negative electrode) for a reddish-brown coating, which indicates copper metal deposit.
  8. Clean the beaker and electrodes thoroughly with distilled water.
  9. Repeat the procedure using copper(II) sulfate, sodium chloride, and sodium sulfate solutions.
Electrolysis of Aqueous Solution Anode (+) Cathode (-) Electrolyte

Fig 1. Laboratory experimental setup for Required Practical 3.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Electrolyte solution Observations at Cathode (-) Observations at Anode (+) Product identified
Copper(II) chloride Red-brown solid deposit formed Bubbling; pale green gas bleaches blue litmus Cathode: Copper; Anode: Chlorine
Copper(II) sulfate Red-brown solid deposit formed Bubbling; colourless gas relights glowing splint Cathode: Copper; Anode: Oxygen
Sodium chloride Bubbling; colourless gas squeaky pop test Bubbling; pale green gas bleaches blue litmus Cathode: Hydrogen; Anode: Chlorine
Sodium sulfate Bubbling; colourless gas squeaky pop test Bubbling; colourless gas relights glowing splint Cathode: Hydrogen; Anode: Oxygen

Data Processing & Analysis

  1. At cathode for NaCl(aq): 2H+(aq) + 2e- -> H2(g) (Reduction - Hydrogen discharged as sodium is more reactive).
  2. At anode for NaCl(aq): 2Cl-(aq) -> Cl2(g) + 2e- (Oxidation - Halide ions discharged).
  3. At cathode for CuSO4(aq): Cu2+(aq) + 2e- -> Cu(s) (Reduction - Copper discharged as it is less reactive than hydrogen).
  4. At anode for CuSO4(aq): 4OH-(aq) -> O2(g) + 2H2O(l) + 4e- (Oxidation - No halide ions present, OH- discharged).

Conclusion & Evaluation

Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Contaminated graphite electrodes from previous runs		 Impurity ions react, producing false results or altering the gas composition. Rinse the electrodes thoroughly with distilled water and rub with fine emery paper before each solution change.
Random Error
Delay in testing chlorine gas with litmus paper		 Chlorine dissolves in solution or escapes into the room, leading to a weak or missed litmus reaction. Hold the litmus paper close to the anode immediately after bubble generation starts.

Exam Practice

Exam-Style Design Question (6 Marks)

A student electrolyses sodium chloride solution. Plan an experiment to identify the products at both electrodes, detailing the apparatus, methods, observations, and gas verification tests.

View Model Answer & Mark Scheme

Model Answer (6/6 Marks):

  1. Apparatus: Set up a small beaker containing sodium chloride solution, two inert graphite rods, connecting leads, and a low voltage DC power supply.
  2. Setup: Support the graphite electrodes in the beaker using a petri dish lid with holes, ensuring the rods do not touch to prevent short circuiting. Connect them to the terminals of the power supply.
  3. Electrolysis: Select 4V and switch on the power supply. Run the process for a maximum of 5 minutes in a well-ventilated room to limit the volume of toxic chlorine gas released.
  4. Cathode Observations: Rapid bubbling is observed. Place an inverted tube over the cathode to collect the gas, then test with a lit splint. A 'squeaky pop' confirms the product is hydrogen gas.
  5. Anode Observations: Bubbles of gas are observed. Hold damp blue litmus paper close to the anode using tweezers. The litmus paper will turn red and then bleach white, confirming chlorine gas.
  6. Chemical Principles: Hydrogen is discharged at the cathode because hydrogen is less reactive than sodium. Chlorine is discharged at the anode because chloride is a halide ion.
Examiner Tip:

Always mention safety considerations such as gas toxicity (chlorine) and ventilation when plan-based questions involve halide electrolysis.