Upper secondary · 10 min

Electrolysis of water in a Hofmann voltameter

Split water into hydrogen and oxygen and check Faraday’s law and the 2 : 1 volume ratio.

Goal

Check that $n(\ce{H2}) = It/2F$ , that hydrogen and oxygen are produced in a 2 : 1 ratio, and how the volume depends on $I$ and $t$ .

Apparatus and reagents

Hofmann voltameter with water made conductive by a little dissolved salt or dilute acid; a low-voltage DC supply and an ammeter.

Procedure

Set I = 1 A and t = 60 s and read both volumes.
Double the time, then double the current instead. Compare the volumes.
Set I = 1 A and t = 300 s and compare the ratio of the two volumes.

What to observe

At 1 A, 60 s gives about 7.6 mL of H₂; 300 s gives 38.0 mL of H₂ and 19.0 mL of O₂.
The volume is proportional to the charge I·t: doubling either one doubles it (2 A for 300 s: 76.1 mL of H₂).
Hydrogen collects at the negative electrode and oxygen at the positive one.

Explanation

Cathode: $\ce{2H2O + 2e- -> H2 + 2OH-}$ . Anode: $\ce{2H2O -> O2 + 4H+ + 4e-}$ . Overall: $\ce{2H2O -> 2H2 + O2}$ . The charge $Q = It$ carries $Q/F$ moles of electrons ( $F = 96485$ C/mol), giving $n(\ce{H2}) = It/2F$ and $n(\ce{O2}) = It/4F$ , hence the 2 : 1 ratio. Volumes here use $V_m = 24.47$ L/mol (25 °C, 1 atm) and ignore gas solubility and water vapor, so real readings are slightly lower.

History of the experiment

Weeks after Volta announced his pile, Nicholson and Carlisle split water with it in May 1800 — and Davy's electrolyses soon tore potash and soda open to reveal potassium and sodium (1807). Faraday's laws of 1834 then made the bubbles quantitative: the volume of gas is a direct count of coulombs passed,

n(\ce{H2}) = It/2F

, exactly the bookkeeping this lab performs.

Chemists behind it

Michael Faraday Humphry Davy