Experiment Overview

Electrolysis is a process in which electrical energy is used to drive a non-spontaneous chemical reaction. It is commonly used to decompose compounds into their elements or simpler substances.

What This Experiment Demonstrates

• Conversion of electrical energy into chemical energy
• Redox reactions occurring at electrodes
• Movement of ions in an electrolyte

Apparatus Required

• Power supply (DC source)
• Electrodes (graphite or metal)
• Electrolyte solution or molten compound
• Beaker or electrolysis cell
• Connecting wires and clips

Chemicals Required

• Electrolyte (e.g. copper sulfate solution, sodium chloride solution)
• Distilled water (if required)

Theory

Electrolysis involves the flow of electric current through an electrolyte, causing chemical changes at the electrodes. The electrolyte must contain mobile ions, either in molten form or dissolved in a solvent such as water.

The electrode connected to the positive terminal of the power supply is called the anode, while the electrode connected to the negative terminal is called the cathode.

Oxidation occurs at the anode (loss of electrons), while reduction occurs at the cathode (gain of electrons). This can be remembered using the mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

The products formed during electrolysis depend on several factors, including the nature of the electrolyte, the concentration of ions, and the type of electrodes used (inert or reactive).

The amount of substance produced during electrolysis can be calculated using Faraday’s laws of electrolysis:

\[Q = It\]

\[n = \frac{Q}{zF}\]

where \(Q\) is the charge, \(I\) is the current, \(t\) is time, \(n\) is the number of moles of substance, \(z\) is the number of electrons transferred, and \(F\) is Faraday’s constant.

Method

1. Set up the electrolysis apparatus by placing the electrolyte in a beaker.
2. Insert the electrodes into the electrolyte.
3. Connect the electrodes to the DC power supply using wires.
4. Ensure the circuit is complete and switch on the power supply.
5. Observe the reactions occurring at each electrode.
6. Record any gas evolution, color changes, or deposition of substances.
7. Turn off the power supply after sufficient observation.

Observations

Gas bubbles may form at one or both electrodes, and solid may deposit on the electrodes depending on the electrolyte used. Color changes in the solution may also be observed.

Calculations

• Measure the current flowing through the circuit.
• Record the time for which electrolysis occurs.
• Calculate the total charge passed using \(Q = It\).
• Determine the amount of substance produced using Faraday’s law.

Observation: identifying products

The products formed during electrolysis can be identified using simple tests:

• Hydrogen gas can be identified using a lighted splint, producing a “pop” sound.
• Oxygen gas can be identified by relighting a glowing splint.
• Chlorine gas has a characteristic greenish color and a pungent smell, and can bleach damp litmus paper.
• Metal deposition can be observed as a coating on the cathode.

Result

The electrolyte undergoes decomposition, producing substances at the electrodes. The identity and quantity of the products depend on the electrolyte composition, electrode type, and the amount of charge passed through the system.

Video Demonstration

Video courtesy of STEM nation (YouTube). Video used for educational purposes under YouTube’s embedding policy.