this lesson looks at what happens when
brine undergoes electrolysis you may
need to look at our lesson what happens
in electrolysis before going any further
the electrolysis of brine is an
industrial process which is carried out
on a large scale but we can look at the
process on a smaller scale in the
laboratory
brine is a saturated solution of sodium
chloride
we don't want the electrodes to become
part of this reaction and so we use
carbon electrodes
from the sodium chloride we have
positive sodium ions and a plus and
negative chloride ions cl minus and from
the water we have positive hydrogen ions
h plus and negative hydroxide ions o h
minus
when the current flows you will begin to
see tiny bubbles of gas forming at each
electrode
both hydrogen and chlorine gas are
formed but can you work out which
electrode produces which gas
pause the video and see if you can work
it out
the chlorine is formed at the anode
that's the positive electrode
and the hydrogen is formed at the
cathode that's the negative electrode
did you get it right
let's look at what's going on at each
electrode the anode is positive and
attracts the negative chloride and
hydroxide ions
the chloride ions give up their
electrons to the anode and are oxidized
to chlorine molecules two ions need to
be oxidized to make a diatomic chlorine
molecule and two free electrons are
produced the electrons are given up to
the anode and the chlorine gas bubbles
up to the surface of the cell
if you collected the gas from the anode
in a test tube then you would be able to
see the pale green colour of the
chlorine gas it will turn down blue
litmus paper red showing that it is
acidic and then it bleaches it
what about the cathode it is negatively
charged and attracts the positive sodium
and the hydrogen ions
the hydrogen ions each gain an electron
from the electrode to form hydrogen
atoms here's the equation
two hydrogen atoms then bond to form a
diatomic hydrogen molecule
and that's the hydrogen gas given off
the gas here is colorless and can be
collected in a test tube
the test to prove that it is hydrogen is
done with a burning wooden splint and
you'll hear a familiar pop as the
hydrogen burns
the ions left in the solution are the
positive sodium ions and the negative
hydroxide ions which combine to form
sodium hydroxide
overall the electrolysis of brine has
produced hydrogen chlorine and sodium
hydroxide and so we can write a summary
equation
concentrated sodium chloride solution
plus water
when electrolyzed gives sodium hydroxide
solution hydrogen gas and chlorine gas
these products are very useful and the
electrolysis of brine is done on a much
larger industrial scale
chlorine is used to kill bacteria and it
is soluble in water if you've ever
visited a public swimming pool then
you'll be familiar with the smell of
chlorinated water and it's also used to
sterilize water in the mains water
supply
what's more chlorine is one of the main
ingredients of bleach which is used in a
lot of cleaning products but this is
only scratching the tip of the iceberg
as chlorine is used to make dry cleaning
ingredients electrical insulators
building materials refrigerated gases
weed killer pesticides antiseptic
some of the hydrogen and chlorine from
the industrial process is combined to
form hydrogen chloride gas it's a rather
unpleasant gas but it dissolves in water
to produce hydrochloric acid which is a
very useful acid it too can be used for
cleaning metal but it is also used for
producing chloride salts
sodium hydroxide is another cleaning
product and the most common use for it
in the home is for cleaning ovens and
unblocking drains
that's a rather amazing collection of
uses from our original brine
you