PH Curves and Endpoints - Mole Concept Video Lecture | Physical Chemistry

so we're going to look at now a specific
type of titration called which is
between acids and bases so we're gonna
look at an acid-base titration okay we
spoke in the introductory video about
the introductory titration video about
the idea of an equivalence point and we
know that the equivalence point is when
neither reactant is in excess that is to
say that the the reactant that was in
the burette and the reactant that was in
the conical flask are present in the
ratio given by the chemical equation so
if we have a chemical equation that says
that to 30 minds are going to react with
one carbonate ion to produce some some
products and we have sodium in the
burette we have sodium ions in the
burette and carbonate ions in the
conical flask then the equivalence point
we reach when there when we have added
twice as many sodium ions to the conical
flask as katas carbonate ions that are
already in there so in the equivalence
point is reached
sodium ions and carbonate ions will
exist will be present in a two-to-one
ratio okay so that's how we defined the
equivalence point now similarly we
define something called the end point of
a titration now the end point is
identified by a color change so the end
point occurs when the the solution in
the conical flask suddenly changes car
and when that's when that solution in
the conical flask suddenly changes color
that is when we end the titration so
while the equivalence point is the point
at which the two reactants are present
in the ratio given by their chemical
equation the end point is when we
actually stopped high training's when we
stop the process and measure the titer
that we've used to measure the amount of
solution we've released from the burette
and this end point this end point is I
don't is the way we identify this end
point we were the way that we decide
when to end the titration is when the
color of
solution in the conical flask changes
okay now in order to ensure that the
equipment we want to get the equivalence
point and the end point as close as
possible to one another we want to stop
the titration when these are present in
the ratio given by the reaction that's
when we want to stop the titration in
order to really calculate to a high
degree of accuracy the concentration of
the unknown solution so in order to
ensure that the color change occurs
close to or equivalence point we want to
choose an indicator that is going to
change color at or close to the
equivalence point now if we have a
titration between a strong acid and a
strong base then we're gonna have a
we're gonna house we're gonna draw
something that's called a pH a pH curve
it's a pH curve shows the pH with
respect to the volume of acid that's
been added from the pure air assuming
that the acid is in the burette and the
base is in the for asked already so a
strong acid and a strong base so if we
have a strong acid instruc then the pH
curve is going to look a little
something like this so as we as we begin
to add acid to the base it's gonna start
decreasing from 14 okay and then as we
go closer the equivalence drop very
sharply and then peter out close to 1 as
we continue adding strong acid so we
know that when they are so in this case
we can see that the equivalence point is
around there okay when we when we react
a strong acid with a strong base we know
that we produce a salt and we produce
water so when this reaction has occurred
to a conflation when the reaction
between the strong acid and this hurts
of completion there is that means that
neither the strong acid all the strong
base is present in excess so the only
thing that we have in the solution is
salt
and water so that means that the pH is
going to be seven it's going to be
neutral so therefore we can tell at the
equivalence point the pH at the
equivalence point is going to be seven
now we want to choose an indicator to
add to the solution in the conical flask
such that the solution changes color at
the equivalence point
so we want to make the ette we want to
choose an appropriate indicator such
that the endpoint is close to the
equivalence point now by looking at this
graph we can see that the equivalence
point occurs at a pH of seven now the pH
of this solution the pH of the solution
in the conical flask drops very sharply
as we get close to the equivalence point
you know we might have a pH of 10 we
might add one more drop of acid to get
to a pH of 7 then we might add one more
drop of acid and get to a pH of about 4
so for that reason it makes it a lot
easier because of the sharp nature of
this change in pH makes a lot easier to
choose an appropriate indicator so we
know that the pH is 7 at the equivalence
point so we want to choose an indicator
that changes color at a pH of 7 so this
pH range is listed here for the three
indicators we've got our the pH ranges
for which each of these indicators
change color so we can see that methyl
red changes color between a pH of 4.2
and 6.3 a pH of 7 lies outside this
range and so methyl red is not going to
be appropriate now we've got bromothymol
blue and phenol red left now a pH of 7
lies in the pH ranges of both of these
indicators so basically phenol red if we
color code this if we have phenol red
color coded there okay phenol red is
gonna change change color between here
and about here if we have bromothymol
blue color coded like that bromothymol
blue is going to change color between
here and there yeah
okay so either of these is really
appropriate because there's so little
difference in the volume of acid that
were adding with this pet with the sharp
pH change either of these indicators
will be appropriate for this titration
because the pH changes at about seven
each of there as we as we add a drop to
get to seven and another drop to go past
seven and go to a pH of about four both
of these are indicators are gonna change
color very quickly and very close to the
equivalence point and that's for that
reason they're both or either of these
would be appropriate for the titration
of a strong acid and a strong base now
this is the pH curve of a strong acid
and a strong base however if we have for
example a sweet base if things are going
to look a little bit different all right
so we're gonna draw a weak acid and a
strong base here now if we have a weak
acid and a strong base okay and so pH of
seven just list that on the axes it if
you have a weak acid and a strong base
and we're starting out the strong base
we're gonna start close to 14 we're
gonna add acid it's gonna suddenly start
to drop pretty quickly as before however
it's gonna plateau out at a higher pH
than previously because we've only got a
weak acid when the solution is
ultimately quite acidic because we've
added excess acid then the pH is gonna
be a bit higher than it was with this
track now closer to six rather than you
know two or three so here
the equivalence point is in this region
all right so while bromothymol blue and
phenol red were both appropriate for
hydration between a strong acid and the
strong base here because the equivalence
point occurs at a slightly higher pH you
know in this sort of range here we want
well this is the sort of range that we
want the endpoint to occur in this is
the sort of range we want the solution
in the conical flask to change color so
for that reason because it's a bit
higher than it was before
phenol red would be a more appropriate
choice of indicator for this situation
because we've got a weak-ass in the
strong base the equivalence point occurs
at a higher pH and since color at a
slightly higher pH range than
bromothymol blue and is therefore more
appropriate than bromothymol blue to
signify the end point of this titration
similarly if we would have a a strong
acid in a weak base then this whole then
this whole pH curve would be shifted
down the same way that this is shifted
up as a result of the acid only being
weak if we had a strong acid in a weak
base and the whole thing would be
shifted down in a similar fashion so if
we had a strong acid and a weak base
then it would look a little bit more
like this so we have a weak base and we
can start with a lot lower pH we're
gonna go down to here
okay so in this situation me file red
would probably be more appropriate
because we're looking at lower pH range
at which the equivalence point occurs
okay we're looking at sort of generally
a lower region which the pH changes okay
so basically when we look when were
choosing the indicator that we want to
use to identify the endpoint of an
titration and so when we're choosing the
indicator with which we want to ensure
that the endpoint is close to the
equivalence point we looking for an
indicator that's gonna change change
color sharply around the equivalence
point so there may be more than one
option however we're gonna choose the
most appropriate in this case the most
appropriate is a methyl trip one final
thing to add on to this discussion of pH
curves so we're on bases strong acids
and weak bases and weak acids strong
bases however if we were to do a weak
acid and a weak base then the pH curve
would look a little bit more like this
it will rather than having that sharp
point that sharp equivalence point
that's easy to for which it is easy to
find any indicator that's gonna change
color okay this is what the pH curve is
going to look like for a weak acid and a
weak base so rather than having that
sharp shape for which it's easy to
choose and in
cater that will change color in this
region this the pH changes much more
gradually for the titration between a
weak acid and a weak base through this
region here and for that reason it's
indicator that changes color sharply
around the equivalence point so for that
reason if we have a weak acid or a weak
base for which of which we want to know
the concentration to a to a high degree
if we want to standardize a weak acid or
a weak base as in make a weak acid or a
weak base a standard solution by finding
out its concentration to a high degree
of accuracy then what we're going to
have to do is if we have a weak acid
we're gonna have to titrate it with a
strong base in order to ensure we get
that sharp region at the equivalence
point and similarly if we have a weak
base and we want to find out its
concentration via titration then we're
gonna have to use a strong acid to
ensure that we don't get this very
gradual you know difficult to deal with
shape on the pH curve okay if we have a
weak ace with a strong acid we can get a
knife and everything's gonna be a little
bit easier to deal with so that's what
we're dealing with when we have pH
curves and endpoints so it's basically
just we need we want to look for each
indicator that's gonna change color
sharply around the equivalence point of
a titration so that we can identify the
equivalence point so we define the
equivalence point as the point at which
the reactants are present in the rate in
the ratio given by the chemical equation
and we define the end point as the point
at which the color of the solution in
the conical flask changes so the end
point is also the point at which we end
the titration so we want to make the
endpoint as close as possible to the
equivalence point in order to ensure
that our results are accurate and we do
that by selecting the right indicator as
I have shown
you