Lighting up copper – transition metal and chemiluminescence demo

Lighting up copper – transition metal and chemiluminescence demo


Oh! This reaction is an example of
chemiluminescence. Normally you might expect reactions to release or absorb
energy as heat, due to different numbers and strengths of bonds being broken and
the reactants and made in the products. But every now and again the conditions are just right to allow
for a reaction in which most of the energy is released as light rather than
heat energy. We’ve seen luminol doing its thing back
in a previous edition of exhibition chemistry in the luminol fountain
demonstration. But two years ago I saw another luminol demonstration by the
YouTube user NerdRage and wanted to share it with the EIC community. In this
chemiluminescence reaction, an intermediate is formed in an excited
electronic state – like people sledging on a mountain, changing their gravitational
potential energy into movement energy. These electrons can relax down to a
lower energy level in an atom, releasing a particle of light called a photon as they
go. Unlike the sledges though, only certain
movements are allowed leading to very specific frequencies of light being
emitted, in our case blue. Luminol is the molecule that accomplishes this feat. It’s
perhaps best known for its use in forensics, detecting traces of blood at a
crime scene. To get the all-important glow we need to meet three other conditions. We need an oxidising agent – hydrogen peroxide is used both in our experiment
and a crime scenes, but bleach will accomplish the same end. This can make an
investigator’s life more challenging if somebody is trying to clean the scene
with bleach. Now, the reaction works best in an alkali
and here’s why: The first step of the reaction is to
remove hydrogen ions. This is what bases were born to do.
Another important step involves the reaction of oxygen with the luminol,
which kicks off the production of an excited molecule for which we can
release a photon of light energy. We need to get lots of oxygen to the luminol
quickly if we want that all-important glow. Luckily, that’s another speciality of
transition metals: they make for great catalysts. Back at the crime scene, transition metal
ions and blood spatters do the job for us. Specifically, if the iron(II) ions
which will light up any luminol solution sprayed on them. Our experiment skips over a few elements
to copper. Around the copper, a range of complex
ions can form. Complex ion formation is another thing transition metals are
good for. It sounds complex, but it’s not too tough to wrap your mind around. Essentially the metal ions are
surrounded by molecules or ions called ligands, which have lone or non-bonding pairs
of electrons. These form coordinate, also known as dative bonds, to the metal ion
and instead of having a simple metal ion, for example, floating around in solution,
we have a cluster of particles with the surrounding ligands and metal ion acting
as one unit. To some of these complex ions, peroxide can coordinate and react,
liberating oxygen and other oxidising species which can go on to react with
the luminol. The key to this process is the capacity of transition metals to
exist in a range of oxidation states. Essentially they’re pretty good at
shuttling electrons to and from other substances. The YouTube user known
as NurdRage had the awesome idea to put a twist on the typical reaction: a
coin or piece of copper dunked in the luminol solution will make a glow but
the glow quickly just spreads through the whole solution. If only there was a way of shutting off
the glow as soon as it moves away from the metal. Well this is where the EDTA comes in. EDTA
is a polydentate ligand. Polydentate means ‘many toothed’. One molecule can bind
to the transition metal ion in more than one place. These polydentate ligands
display a property called chelation. Chelation describes the way in which the
universe favours the swapping of lots of small ligands for one big one If the strength of the bonds are similar,
and they tend to be because the atoms involved in the coordination or of the
same types, then such a swap means that in the example shown here we go from having
two moles of particles floating around in solution to having seven moles of
particles. This is favoured by entropy: a way of
saying that the universe tends towards a more disordered state. Having seven moles of particles to play with gives you a lot more room for disorder than just
having two moles. As we move away from the piece of copper metal, the ions in
solution will be gobbled up by the EDTA ligands floating around in the solution we prepared. The
hydrogen peroxide just can’t get a look-in so the glow dissipates. So, not a great demo for showing the
magnetic properties of transition metals but everything else is covered.

5 Comments

  1. That's amazing, if that can't capture students imagination, interest and fascination with science, I'd be gob smacked! Thanks for the inspiration!

  2. as with the other more recent videos, the PPT used for the video can be downloaded from under the article at: http://rsc.li/EiC516ec for adapting and using in class 🙂

Leave a Reply

Your email address will not be published.


*