Le Châtelier’s principle in action

February 04, 2016

Following the mention of the cobalt(II) complex ion system in this post from a couple of days ago, here’s a 30 second video clip of Le Châtelier’s principle in action in the same (with The Smiths).

The equilibrium that is set up is this,

[Co(H2O)6]2+(aq) + 4Cl(aq)[CoCl4]2-(aq) + 6H2O(l)

and the aqueous solution that is added dropwise is 0.1 M silver nitrate.


If you would like an explanation of what you see, then read on…

…the equilibrium starts out with a purplish color, suggesting that a combination of both the pink and blue species are present in the test tube.

Addition of the silver nitrate solution (effectively a source of silver ions), causes the precipitation of solid silver chloride according to the net, ionic equation,

Ag+(aq) + Cl(aq) ➔ AgCl(s)

This reaction causes the removal of free chloride ions from the equilibrium system, thus disturbing the equilibrium, and making Q (the reaction quotient) too large. The equilibrium shifts backwards in order to reduce the value of Q, and to bring it back into agreement with the equilibrium constant, K.

As the reaction goes backwards one sees the pink cobalt species begin to dominate, and the formation of the white precipitate of AgCl.

Neat and simple.


  1. Laura

    Thank you very much for combining the science with The Smiths. (My two preferred things).

  2. ian McMahon

    Quick and to the point however
    * the background of the rack is distracting – white/cream card behind test tubes may have helped colour distinction
    * any thought to a reversal of pink to purple

    the Smiths – a very English choice

    • Adrian

      Well, this was shot on a whim, in 30 seconds, using my phone, as I was doing the lab and I suddenly thought, ‘oh this might be cool to capture’. Studio produced, high quality instructional video, it was not supposed to be.


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