Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Role of sulfuric acid
Dilute sulfuric acid supplies ions so that the solution conducts effectively. Sulfate ions largely remain in solution while water is decomposed overall.
Cathode product
Hydrogen ions gain electrons: 2H+ + 2e- -> H2. Colourless gas forms and gives a squeaky pop with a lighted splint.
Anode product
Hydroxide ions or water lose electrons to form oxygen. The gas relights a glowing splint. A suitable half-equation is 4OH- -> O2 + 2H2O + 4e-.
Volume ratio
The overall equation 2H2O -> 2H2 + O2 predicts twice as many moles and therefore twice the gas volume of hydrogen as oxygen under the same conditions.
Worked example
If 30 cm3 of oxygen is collected, the expected hydrogen volume is 60 cm3, assuming no gas loss and equal conditions.
Practical or data skill
Collect gases in calibrated tubes and compare volume ratios. Identify sources of error such as gas solubility and leakage.
Examination tip
Use the balanced overall equation to explain the 2:1 gas-volume ratio.
Review questions and suggested answers
Question 1
Which gas relights a glowing splint?
Suggested answer
Oxygen.
Question 2
What is the expected H2:O2 volume ratio?
Suggested answer
2:1.
Question 3
What is the overall substance decomposed?
Suggested answer
Water.
