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.