WATER ELECTROLYSIS — GAS STOICHIOMETRY
Why is Twice as Much Hydrogen Produced? On a volume basis of course.
Follow the electrons through the external circuit,
then follow H₂ and O₂ from their electrodes into
separate gas collection vessels.
From reaction stoichiometry to collected gas
The overall water electrolysis reaction is:
At the same temperature and pressure:
2H₂O
→
2H₂
+
O₂
Therefore, for every 1 mole of O₂,
2 moles of H₂ are produced.
At the same temperature and pressure:
H₂
:
O₂
=
2
:
1
Watch the complete process
Start the electrolyser
Before you press Start Electrolysis, there are no moving electrons or gas molecules. This represents the cell before electrolysis begins.
Follow the electrons
Once electrolysis begins, the moving e⁻ symbols appear in the purple external electrical circuit .
Follow the oxygen
O₂ is produced at the anode. O₂ molecules then travel through the orange gas collection line and enter the oxygen vessel from the bottom.
Follow the hydrogen
H₂ is produced at the cathode. H₂ molecules travel through the blue gas collection line and enter the hydrogen vessel from the bottom.
Compare the volumes
Both vessels use the same physical volume scale. Hydrogen therefore rises at exactly twice the volumetric rate of oxygen.
6. Watch the collection cycle reset
When the vessels reach:
Before you press Start Electrolysis, there are no moving electrons or gas molecules. This represents the cell before electrolysis begins.
Follow the electrons
Once electrolysis begins, the moving e⁻ symbols appear in the purple external electrical circuit .
Follow the oxygen
O₂ is produced at the anode. O₂ molecules then travel through the orange gas collection line and enter the oxygen vessel from the bottom.
Follow the hydrogen
H₂ is produced at the cathode. H₂ molecules travel through the blue gas collection line and enter the hydrogen vessel from the bottom.
Compare the volumes
Both vessels use the same physical volume scale. Hydrogen therefore rises at exactly twice the volumetric rate of oxygen.
6. Watch the collection cycle reset
When the vessels reach:
2 volumes H₂
:
1 volume O₂
the completed 2:1 ratio is shown briefly.
Both vessels then return to zero and a new
collection cycle begins automatically.
ELECTRICAL PATH
Purple wires
e⁻ transport
ANODE GAS PATH
Orange pipe
O₂ transport
CATHODE GAS PATH
Blue pipe
H₂ transport
H₂
:
O₂
=
2
:
1
🎯 Interactive Activity — Predict the Gas Volume
Choose the amount of oxygen collected and
predict the corresponding hydrogen volume.
Choose the volume of O₂ collected at the anode. Then predict the corresponding H₂ volume collected at the cathode.
✓ Gas Collection Takeaway
When the electrolyser is switched on:
⚡ Electrons move through the external electrical circuit.
🟠 O₂ forms at the anode and travels through the oxygen gas line into the oxygen collection vessel.
🔵 H₂ forms at the cathode and travels through the hydrogen gas line into the hydrogen collection vessel.
Because:
⚡ Electrons move through the external electrical circuit.
🟠 O₂ forms at the anode and travels through the oxygen gas line into the oxygen collection vessel.
🔵 H₂ forms at the cathode and travels through the hydrogen gas line into the hydrogen collection vessel.
Because:
2H₂O
→
2H₂
+
O₂
the collected gas volumes at the same
temperature and pressure satisfy:
H₂
:
O₂
=
2
:
1
Once this 2:1 collection point is reached,
the demonstration resets both vessels and
begins another collection cycle.