Electrolysis of Water 2
Water is electrolyzed with an apparatus that allows the volumes of oxygen and hydrogen to be quantified.
- Hoffman apparatus
- voltage supply
- platinum electrodes
- After introducing the Hoffman apparatus to the audience, the voltage supply is turned on to provide 2 V between the electrodes in the two burets. Point out the formation of bubbles and the displacement of water at the tops of the inverted burets.
- After 5-10 miniutes, sufficient gas will be evolved that the volumes can be measured and compared for oxygen and hydrogen.
- If desired, the voltage can be turned off, and the hydrogen gas can be collected in an inverted test tube and ignited to give a small pop. Likewise, a smoldering wooden stick can be placed above the oxygen buret and it will reignite to a flame when the O2 is released from the buret.
The overall water electrolysis reaction is:
2 H2O (l) → 2 H2 (g) + O2 (g)
The stoichiometry of this reaction shows that there are 2 moles of H2 gas formed for every 1 mole of O2 gas. Assuming ideal gas behavior and identical water solubilities, there should be twice the volume of hydrogen formed as there was oxygen.
Special Safety Notes
- the power supply is low voltage, but could still give a spark if not adjusted correctly.
- hydrogen is a flammable gas, use care with open flames
- oxygen is an accelerant, use care with open flames