Questions about Electrolysis of Water
Electrolysis of Water – Multiple Choice Questions
Questions
1. What are the products of the electrolysis of pure water?
A) Oxygen and hydrogen gases
B) Carbon dioxide and oxygen
C) Hydrogen peroxide and oxygen
D) Hydrogen and chlorine gases
E) Water and oxygen
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2. During the electrolysis of water, which gas is produced at the cathode?
A) Oxygen
B) Hydrogen
C) Chlorine
D) Carbon dioxide
E) Nitrogen
3. What is the role of the electrolyte (e.g., sulfuric acid) added to water in electrolysis?
A) To slow down the reaction
B) To increase water's conductivity
C) To produce extra gases
D) To prevent gas formation
E) To change the water's color
4. At the anode, what happens to water molecules during electrolysis?
A) They gain electrons and form hydrogen gas
B) They lose electrons and form oxygen gas
C) They remain unchanged
D) They combine to form hydrogen peroxide
E) They evaporate
5. Which electrode is connected to the negative terminal in water electrolysis?
A) Anode
B) Cathode
C) Salt bridge
D) Reference electrode
E) None of the above
6. What is the balanced half-reaction at the cathode during water electrolysis?
A) 2H₂O + 4e⁻ → 2H₂ + 4OH⁻
B) 2H₂O → O₂ + 4H⁺ + 4e⁻
C) 4OH⁻ → O₂ + 2H₂O + 4e⁻
D) H₂O → H₂ + O
E) 2H⁺ + 2e⁻ → H₂
7. What volume ratio of hydrogen to oxygen gas is produced in water electrolysis?
A) 1:1
B) 2:1
C) 1:2
D) 3:1
E) 1:3
8. Why is pure water a poor conductor of electricity?
A) It has no ions
B) It has many ions
C) It is a strong acid
D) It is highly viscous
E) It is solid at room temperature
9. Which of the following increases the rate of water electrolysis?
A) Using pure distilled water
B) Adding an electrolyte like Na₂SO₄
C) Decreasing temperature
D) Using non-conductive electrodes
E) Removing the power source
10. Which gas produced during water electrolysis is explosive when mixed with air?
A) Oxygen
B) Hydrogen
C) Nitrogen
D) Carbon dioxide
E) Chlorine
11. What is the overall chemical equation for the electrolysis of water?
A) 2H₂O(l) → 2H₂(g) + O₂(g)
B) H₂O → H₂ + H₂O₂
C) H₂O → O₂ + CO₂
D) 2H₂O → 2H₂O + O₂
E) H₂O → H⁺ + OH⁻
12. What happens at the anode during water electrolysis?
A) Reduction of H⁺ ions
B) Oxidation of water molecules to oxygen gas
C) Hydrogen gas formation
D) Electron gain
E) Water formation
13. How many moles of electrons are needed to produce 1 mole of oxygen gas in water electrolysis?
A) 2
B) 3
C) 4
D) 6
E) 8
14. Electrolysis of water is an example of:
A) Spontaneous redox reaction
B) Electrolytic cell process
C) Galvanic cell operation
D) Combustion
E) Neutralization
15. Which of these statements is true about electrodes in water electrolysis?
A) Both electrodes lose mass
B) The cathode is the site of oxidation
C) The anode is connected to the positive terminal
D) The cathode produces oxygen gas
E) No electrons flow through the external circuit
16. What effect does increasing voltage have on the electrolysis of water?
A) Slows the reaction
B) Increases the rate of gas production
C) Changes product ratios
D) Stops the reaction
E) Converts oxygen into ozone
17. What gas is formed at the electrode connected to the positive terminal?
A) Hydrogen
B) Oxygen
C) Chlorine
D) Carbon dioxide
E) Nitrogen
18. Which ions migrate towards the cathode during electrolysis of water with an acid electrolyte?
A) OH⁻ ions
B) H⁺ ions
C) Cl⁻ ions
D) O²⁻ ions
E) Na⁺ ions
19. Why are inert electrodes preferred for water electrolysis?
A) They react with water to form products
B) They do not participate in the chemical reaction
C) They increase resistance
D) They decompose water chemically
E) They increase hydrogen solubility
20. What is the molar ratio of electrons consumed to hydrogen gas produced in water electrolysis?
A) 1:1
B) 2:1
C) 2:2
D) 2:1/2
E) 2:1
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Answers and Explanations
1. A – Electrolysis of water produces oxygen and hydrogen gases.
2. B – Hydrogen gas is produced at the cathode (reduction site).
3. B – Electrolytes like sulfuric acid increase water’s conductivity.
4. B – Water molecules lose electrons (oxidation) at the anode to form oxygen.
5. B – The cathode is connected to the negative terminal.
6. A – Cathode half-reaction: 2H₂O + 4e⁻ → 2H₂ + 4OH⁻ (in basic solution).
7. B – Hydrogen is produced in twice the volume of oxygen (2:1).
8. A – Pure water has very few ions, so it is a poor conductor.
9. B – Adding electrolyte increases ion availability, speeding up the reaction.
10. B – Hydrogen gas is highly explosive when mixed with air.
11. A – Overall reaction: 2H₂O(l) → 2H₂(g) + O₂(g).
12. B – Oxidation at the anode forms oxygen gas.
13. C – 4 moles of electrons produce 1 mole of O₂ gas.
14. B – Water electrolysis is an electrolytic process, requiring external energy.
15. C – Anode is connected to the positive terminal where oxidation occurs.
16. B – Increasing voltage increases gas production rate.
17. B – Oxygen gas is formed at the anode (positive electrode).
18. B – H⁺ ions move toward the cathode for reduction.
19. B – Inert electrodes do not react and allow electron flow.
20. E – 2 electrons are needed per molecule of H₂ produced (2:1 ratio).
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