Types Of Half Cell - Electrochemistry Video Lecture - Physical Chemistry

welcome to this video in the
electrochemistry topic we're going to
look in this particular video on the
different types of half-cell that we can
have and we want to know the standard
conditions that each of these operate
under in order to measure these standard
electrode potentials so let's just start
off with the simplest type of 1/2 cell
which we're quite familiar with if
you've been watching the videos from the
beginning which is these metal metal ion
half cells and in these half cells we've
typically got an electrode made out of a
particular metal in this case it's a
copper and a piece of copper that's
acting is the electrode and essentially
remember what the electrode is doing is
it is acting as a conductor of electrons
to connect with an external circuit an
external circuit so we need it some
object which will carry the electrons to
and from the electrode in the operation
of the cell that's what the electrode is
and in this case because there's a metal
involved in the redox equilibrium we can
simply just use that metal as the
electrode now in every single one of
these cells what you'll notice is that
there is all the components of the redox
equilibrium are present so in this case
we've accounted for the copper we need a
solution then that contains copper
two-plus ions and the concentration
concentration of 1 mol per decimeter
cubed and that's the standard
concentration that you'll need to make
sure that you know 1 mol per decimeter
cube and we can get that for example
that's this solution here so for example
we might want to use copper sulfate
solution and the reason for that we
could use copper nitrate something like
that we just want to counter ion that
will will be a readily soluble salt of
copper so it will release all the copper
two-plus ions into
solution and we want the counter ion to
be fairly unreactive well very
unreactive now all these cells are going
to operate under the same standard
temperature which is 298 Kelvin said 25
degrees here sort of room temperature
now we may want to instead of having a
metal and a metal ion have something a
bit like this where we've actually got a
gas chlorine and an ion of the gas
that's being formed so again we need the
same type of thing we need both chlorine
supplied this times the gas and we're
going to have that at a pressure of one
atmosphere
this time it's chlorine
so the gas is present and that's usually
piped in here and then we have this sort
of glass shield sometimes the most usual
e holes here to allow the gas to bubble
out into the solution you'll often
sometimes in diagrams see this type of
thing coming out because there'll be
some holes there the gas is being
supplied through a pipe here and in the
solution we're going to need some cl-
ions so we need CL - and concentration
again of 1 mol per decimeter keep this
the standard concentration for these
solutions now the electrons produced
here have got to be able to get out to
an external circuit so we need an
electrode neither colouring nor chloride
ions are a suitable electrode because
neither of them conduct electrons and
therefore what we need is an inert
electrode which is just going to act for
as a place for the electrons to stay if
we've got a very high resistance
voltmeter connected or for the electrons
to travel up it to the external circuit
and our choice for the electrode if in
doubt is always platinum platinum
chemical symbol PT and so we've got a
platinum wire coming down here and then
we've all
at the bottom here with a porous
platinum and and this is what we come
times call platinum black
so there's porous platinum end here we
can find it's down the bottom here and
that's just to ensure rapid
equilibration for measurement of
standard electrode potentials again the
temperature of this cell standard
conditions necessitate at 298 K
temperature and the third type final
type of hops that we'll look at here is
if we've got two ions involved in the
redox equilibrium so for example in this
case we've got an iron three-plus
gaining an electron and for me iron
two-plus this is the equilibrium we've
got so this time we've got two regions
that are in the same phase so they both
going to be in the solution so we need
both fe 2 plus and their fee 3 plus in
the solution so we could have chosen
perhaps a sulphate of iron 2 and a
sulphate of iron 3 plus it doesn't
matter too much and but we've got both
of these present in the solution and in
this case we could either have both at 1
mol per decimeter cubed but sometimes
for technical reasons it might not be
possible to make all iron's present to
such a high concentration or we've got
to have equal concentrations and this is
known as equimolar solutions so it's
sufficient to have equal concentrations
because if you think about they're both
present at 1 mol per decimeter cubed if
a birth president of the half a mole per
decimeter cubed doesn't affect the
position of the equilibrium as long as
they're present in equal amounts equal
concentrations it won't affect the
position of the equilibrium so equal
molar solutions are sufficient in these
ion ion half selves again neither of
these is a conductor so we're going to
need a platinum electrode Platinum
electrode and we've got this porous
platon at the bottom here the platinum
black usually is just sufficient to say
a platinum electrode and the platinum
black is assumed and finally at 298 K
temperature as before now one for you to
try
what apparatus and conditions would be
needed to measure the standard electrode
potential for the following reaction so
pause the video and see if you can work
that out okay so what are we going to
need well first thing just see is that
we've got aqueous solutions here so
we're going to have solutions and what
are the eight grade substances that
we're going to have to have in our
solutions or we need some manganese ions
- probably from something like potassium
manganate so we're gonna need mno4 -
we're also going to need H+ maybe from
hydrochloric acid or sulfuric acid and
we're going to need em in two plus again
perhaps a sulphate or a chloride it
depends on the solubility you're in a
highly soluble
mn 2 plus salt and remember that we want
these in equal molar concentrations so
whatever we can get and that's just an
important thing to note here is that
even though there are eight H pluses in
this equation we still go for equimolar
solutions we don't take account of the
stoichiometry of the reaction here the
other thing we've not included is the
water and that's for the same reason
that we mentioned when we're looking at
different conditions in these redox
equilibria solids and liquids don't take
part in the in equilibrium at least
their concentrations don't change and
there's a pet so much water around
anyway that we don't need to include
water there's always going to be enough
water around now in addition we need an
electrode and that's going to have to be
a platinum electrode because there's
nothing else here that could act as a
conductor and we need a temperature of
298 Kelvin
so in summary we need all the species
involved in a redox equilibrium to be
present in order to measure the half
cell potential if we want to make it a
standard half-cell potential we need a
standard concentration of 1 mole per
decimeter cubed for everything in
solution a standard pressure for gases
is one atmosphere under standard
temperature for everything is children
98 Kelvin if a metal is not present in
the half cell you should use a platinum
electrode and if several species are
present installations then it's
equivalent to have it's ok to have equal
molar concentrations the same molarity
same mole concentration present that's
okay if you can't find solutions as
concentrated as 1 mole per decimeter
cubed