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[Applause]
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the discovery that passing an electric
current through water would split apart
was made quite soon after the discovery
of current electricity itself this
predates Michael Faraday's work in this
area but it is he who introduced most of
the terms that we still use today
electrolysis was named by Faraday though
not an exact translation it means to use
electricity to split something apart
let's remind ourselves of the equipment
that we use in electrolysis almost all
of which was named by Faraday the cell
has a positive anode and a negative
cathode the electrodes connected to the
cell have the same name and charge the
liquid being electrolyzed is the
electrolyte pure water is a very poor
conductor of electricity as free ions
charged species are needed to carry
current in the solution but water
remember is a covalent molecule and at
room temperature
hardly ionizes and so the concentration
of the ions formed is extremely small
pure water acts as an insulator at safe
voltages if however we add a little
sulfuric acid to water this greatly
increases the ability of water to
undergo electrolysis without altering
the products obtained if we use
electrolysis we will see bubbles forming
around both electrodes the most around
the cathode if we use a Hofmann
voltammeter we can collect the gases
made by each electrode the gases are
made in a ratio of two to one but the
initial solubility of oxygen in the
water sometimes masks this testing each
gas is the usual way we'll confirm their
identity let us look closely at what's
happening the ions present in water will
be the hydrogen ion and the hydroxide
ion these ions will be attracted to
the electrode with the opposite charge
to use Faraday's own words I propose to
distinguish these bodies by calling
those anions which go to the anode and
those passing the cathode cations at the
anode we see oxygen being made this is a
loss of electrons which go into the
anode and so is an oxidation reaction at
the cathode we see hydrogen being made
the electrons gained are provided by the
cathode this is a reduction reaction
combining both reactions that are taking
place simultaneously we get a redox
reaction notice that the balanced
equation gives the gases a two to one
ratio
remember oil-rig as a way of tracking
electrons in a redox reaction if water
is acidified with a hydrochloric acid
the reaction is altered due to the
presence of the chloride ions cl- in
this case it's the chloride anions that
are oxidized and not the hydroxide ions
so chlorine and not oxygen is made at
the anode the combined reaction is again
a redox with the gases each being made
in equal amounts as chlorine is very
much more soluble in water than hydrogen
is it will take a while for the water to
become saturated enough to enable any
appreciable amount of gas to be seen and
while you're working with it in the lab
remember chlorine is toxic so take
precautions next