ELECTROLYSIS OF WATER - APPARATUS SETUP

                    6V Battery
                    [+ -]
                     │ │
                     │ │
          ┌──────────┘ └──────────┐
          │                       │
    Test Tube A              Test Tube B
    (filled with            (filled with
     water)                  water)
       │ │                     │ │
       │ │                     │ │
       ↓ ↓                     ↓ ↓
    ┌─────────────────────────────┐
    │    ○                    ○   │ ← Water level
    │    │                    │   │
    │    │    Acidulated      │   │
    │    │      Water         │   │
    │    │   (H₂O + H₂SO₄)   │   │
    │    │                    │   │
    │  [─]                  [+]  │ ← Carbon electrodes
    │ Cathode              Anode  │
    │  (-)                  (+)   │
    └─────────────────────────────┘
         Plastic Mug with
         rubber stoppers at base

OBSERVATIONS:
• Gas A (at cathode): Hydrogen (H₂) - Volume: 2 parts
• Gas B (at anode): Oxygen (O₂) - Volume: 1 part
• Ratio H₂:O₂ = 2:1

⊖ At Cathode (Negative Electrode)

Reduction occurs:

2H⁺ + 2e^– → H₂↑

Hydrogen ions gain electrons and form hydrogen gas, which bubbles up and collects in the test tube.

⊕ At Anode (Positive Electrode)

Oxidation occurs:

4OH^– → 2H₂O + O₂↑ + 4e^–

Hydroxide ions lose electrons and form oxygen gas and water. Oxygen bubbles up and collects in the test tube.

Problem: Very few bubbles forming

Solution: Add 1-2 more drops of H₂SO₄ or check battery connections.

Problem: Water getting hot

Solution: Too much acid added or voltage too high. Use fresh water with less acid.

Marking Scheme Breakdown:

• 1 mark: Apparatus setup and procedure
• 1 mark: Observation about gas volumes
• 1 mark: Chemical equation
• 1 mark: Gas identification tests
• 1 mark: Conclusion about which gas is at which electrode

Aim: To demonstrate the electrolysis of water and identify the gases produced.

Procedure: A plastic mug with two carbon electrodes fitted at the base is filled with water. A few drops of dilute sulphuric acid are added to make it conductive. Two test tubes filled with water are inverted over the electrodes. The electrodes are connected to a 6V battery and current is passed.

Observations:

• Bubbles form at both electrodes and rise up into the test tubes
• The volume of gas collected at the cathode is approximately twice the volume collected at the anode
• The ratio of gases is 2:1

Chemical Equation:

2H₂O(l) → 2H₂(g) + O₂(g)

Gas Tests:

• Gas at cathode: When a burning candle is brought near, it burns with a pop sound. This confirms the presence of hydrogen gas (H₂).
• Gas at anode: When a glowing splinter is brought near, it rekindles (burns more brightly). This confirms the presence of oxygen gas (O₂).

Conclusion: Electrolysis of water produces hydrogen gas at the cathode and oxygen gas at the anode in the volume ratio 2:1, proving that water is composed of hydrogen and oxygen in the ratio H₂O.

✍️ Exam Writing Tips:

• Draw a neat labeled diagram if asked (1-2 extra marks possible)
• Always write the balanced chemical equation
• Mention the 2:1 volume ratio clearly
• Describe both gas tests with observations
• Use proper chemical formulas with subscripts

📊 Summary of Results:

Hydrogen Fuel Production

Electrolysis is used to produce hydrogen gas for fuel cells in electric vehicles and space rockets. It’s a clean energy source that produces only water as a byproduct.

Medical Oxygen

Hospitals use electrolysis to produce pure oxygen for patients with respiratory problems. The oxygen produced is of high purity.

Industrial Chemicals

Electrolysis of brine (salt water) produces chlorine gas, hydrogen gas, and sodium hydroxide—all important industrial chemicals.

Electroplating

Electrolysis is used to coat objects with thin layers of metals like gold, silver, or chromium for decoration and corrosion protection.

1. Why do we add dilute H₂SO₄ to water? Can we use other acids?

Pure water is a poor conductor of electricity because it has very few ions. Dilute H₂SO₄ provides H⁺ and SO₄^2- ions that make water conductive. We use H₂SO₄ specifically because it doesn’t get reduced or oxidized during electrolysis—it only acts as an electrolyte. You could theoretically use other acids like HCl, but HCl would produce chlorine gas at the anode instead of pure oxygen, which would interfere with the experiment.

2. Why is the volume of hydrogen twice that of oxygen?

This directly comes from the chemical formula of water: H₂O. Each water molecule contains 2 hydrogen atoms and 1 oxygen atom. When water decomposes, it produces 2 molecules of H₂ gas for every 1 molecule of O₂ gas, as shown in the equation: 2H₂O → 2H₂ + O₂. Since equal numbers of molecules occupy equal volumes under the same conditions (Avogadro’s Law), the volume ratio is also 2:1.

3. What happens if we reverse the battery connections?

If you reverse the battery connections, the polarity of the electrodes will reverse. The electrode that was previously the cathode (producing hydrogen) will become the anode (producing oxygen), and vice versa. The gases will simply be collected at opposite electrodes, but the chemical process and volume ratio remain the same. This proves that the gas production depends on the electrode polarity, not the physical electrode itself.

4. Is electrolysis of water an endothermic or exothermic reaction?

Electrolysis of water is an endothermic reaction. It requires continuous input of electrical energy to break the strong O-H bonds in water molecules. The energy required to break these bonds (bond dissociation energy) is greater than the energy released when new H-H and O=O bonds form. That’s why we need to keep supplying electricity throughout the process—if we stop, the reaction stops immediately.

5. Can we perform electrolysis with salt water instead of acidulated water?

Yes, you can use salt water (NaCl solution) as it also conducts electricity due to Na⁺ and Cl^– ions. However, the products will be different. At the cathode, you’ll still get hydrogen gas, but at the anode, you’ll get chlorine gas (Cl₂) instead of oxygen because chloride ions are more easily oxidized than water molecules. This is actually how chlorine gas is produced industrially through the chlor-alkali process. For this specific activity demonstrating water decomposition, acidulated water is preferred to get pure oxygen.