If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

ChemWiki: The Dynamic Chemistry E-textbook > Development Details > Approaches > Demos > Additional Demos > The Electrolysis of Water

Copyright (c) 2006-2014 MindTouch Inc.

This file and accompanying files are licensed under the MindTouch Master Subscription Agreement (MSA).

At any time, you shall not, directly or indirectly: (i) sublicense, resell, rent, lease, distribute, market, commercialize or otherwise transfer rights or usage to: (a) the Software, (b) any modified version or derivative work of the Software created by you or for you, or (c) MindTouch Open Source (which includes all non-supported versions of MindTouch-developed software), for any purpose including timesharing or service bureau purposes; (ii) remove or alter any copyright, trademark or proprietary notice in the Software; (iii) transfer, use or export the Software in violation of any applicable laws or regulations of any government or governmental agency; (iv) use or run on any of your hardware, or have deployed for use, any production version of MindTouch Open Source; (v) use any of the Support Services, Error corrections, Updates or Upgrades, for the MindTouch Open Source software or for any Server for which Support Services are not then purchased as provided hereunder; or (vi) reverse engineer, decompile or modify any encrypted or encoded portion of the Software.

A complete copy of the MSA is available at http://www.mindtouch.com/msa

The Electrolysis of Water

Chemical Concepts Demonstrated

  • Electrolysis of water
  • Decomposition of water


The Hoffman electrolysis apparatus is filled with Na2SO4 solution containing universal indicator and is started.



The solution turns blue at the cathode (basic) and red at the anode (acidic). Twice as much gas is evolved at the cathode as at the anode. When the solutions in the two electrodes compartments are mixed, the indicator turns green (neutral).

Explanations (including important chemical equations)

Two reactions can take place at the cathode.

2 H2O (l) + 2 e - → H2 (g) + 2 OH- (aq) E° = -0.8277 V
Na + (aq) + e - → Na (s) E° = -2.7109 V

Two reactions can also occur at the anode.

2 SO4 2- (aq) → S2O8 (aq) + 2 e - E° = -2.05 V
2 H2O (l) → O2(g) + 4 H+ (aq) + 4 e - E° = -1.229 V

It is easier to reduce H2O that Na+ ions at the cathode and it is easier to oxidize H2O than SO4 2- at the anode. Combining the two half reactions so that electrons are conserved, we obtain the following equations.

6 H2O (l) → 2 H2 (g) + O2 (g) + 4 H + (aq) + 4 OH- (aq)


2 H2O (l) → 2 H2 (g) + O2(g)

The decomposition of water produces twice as much hydrogen gas as oxygen gas. Since the number of protons and hydroxide ions formed in the reaction are the same, when the solutions are combined at the end of the electrolysis the indicator has the characteristic green color of a neutral solution.


Last modified
10:30, 2 Oct 2013



(not set)
(not set)

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use