Precipitation Titration - Electrochemistry Video Lecture | Physical Chemistry

alright so in this video I'm going to go
over precipitation reactions so in my
last video I was talking about the
solubility of ionic compounds and the
rules for determining whether an ionic
compound is soluble or insoluble or
insoluble and those rules are going to
come in handy when we do precipitation
reactions so in general what you do in
two precipitation reactions is you mix
two or more solutions together so these
solutions could be ions or they could be
soluble ionic compounds and we mix these
solutions together and upon mixing those
two solutions together we get a solid
that forms out of the solution this
solid is called a precipitate or we say
that the solid precipitates out of the
solution so the word precipitate could
be used as a noun or verb anyway the
solutions that we start with those are
always going to be soluble ionic
compounds and we express the fact that
they're soluble by attaching aq in
parenthesis at the very end of it so
these little things in parenthesis are
very important when you do precipitation
reactions and you can't leave them out
anyway for a soluble ionic compound
you're going to use the aq for aqueous
so both of these are soluble ionic
compounds however if the product of the
reaction is an insoluble ionic compound
then you would use s in parentheses for
solid otherwise if it was soluble you
would use aqueous just like the
reactants so let's go through an example
on a just a simple example on
precipitation reactions so suppose I
have silver ion AG plus and that is
aqueous and chloride ion Cl minus that
is also aqueous so suppose I have a
solution of silver ion and I have a
solution of chloride ion and they're
both dissolved so they're both equates
now upon mixing these two solutions
together
see I of a silver ion that's my cation I
have a chloride ion that's my aunt ion
so the formula for the product is going
to be AG CL and now we have to assign
the state of matter that AG CL is in so
is it solute is it solid or is it
aqueous and that's where the solubility
rules are going to come in handy so
let's look at the solubility rules for
this compound and see if we can't find
out see if we can't find out whether
this is soluble or insoluble let me just
find my appropriate solubility rule here
okay here's a good one alright it says
so we're talking about AG CL here it
says that compounds containing chloride
ion bromide ion and iodide ion are
soluble so it's looking like this is
going to be soluble but then read the
rest of the rule it says chloride
bromide ayodhya die these are soluble
except when these ions pair with silver
ion mercury 2 2 plus ion or lead 2 ion
so this is actually going to be
insoluble because of the exception of
this rule silver is one of the
exceptions so that means I'm going to
attach an S in parenthesis instead of a
Q that means that this did not dissolve
in water like the ions did so that's an
example of a very simple precipitation
reaction now we're going to move on to
another one that's a little complex and
the reason why is because it involves
ionic compounds instead of just free
ions so suppose I have the following
reactants of my reaction so I have
silver nitrate so that's AG no.3 and I
have sodium bromide and a br
so what are the products going to be of
this reaction well first we have to
realize that in our initial condition we
said that both of these are soluble both
of the solutions that we mix together
are soluble so that means both the
silver nitrate and the sodium bromide
must be soluble so now we have to apply
the solubility rules again to see if
these these two things are even soluble
so if you look up the solubility rules
there's one for okay it says that all
compounds containing lithium sodium
potassium nitrate okay so nitrate that's
going to be soluble so that means this
is going to be a Q and then I'll go back
to my same rule that had the chloride
ion it that also applies to the bromide
ion it says that these are generally
soluble except when they pair with
silver mercury to two plus or lead to
ion and it's not it's pairing with
sodium ions so both of these are going
to be soluble so we're going to write
aqueous so indeed we may have a
precipitation reaction because we start
out with two soluble ionic compounds so
now we're going to look and see what our
products are going to be and generally
what we're going to do is we're just
going to write out all the possible
products and then we're going to figure
out using the solubility rules whether
they're solid or aqueous so what I like
to do is I like to identify my cations
in my anions in the reactants so our
cations are positively positively
charged ones are going to be the silver
ion and the sodium ion and our
negatively charged ions are the hand
ions those are going to be the nitrate
ion and the bromide ion so what we're
going to do is we're going to take we're
going to criss cross we're just going to
swap out the the cations and anions so
whatever anion was paired with the
silver ion we're going to pair it with
the other anion which is the bromide ion
and we're going to do same thing with
these two ions so I'm going to have
silver ion and bromide together and then
I'm going to have sodium ion and nitrate
ion together so let's write out those
products
so silver is going to pair with bromides
that's going to be AG BR and we don't
know the state of it yet we'll figure
that out in just a second and then we
have the sodium ion and the nitrate ion
pairing together so that's in a no.3 and
we don't know the state of that either
you still have to look at the solubility
rules so let's start with AG BR is that
soluble or insoluble well remember I'll
pull out the rule again if you have
bromide ion it's soluble except if you
have silver ion or these other two ions
pairing with it so that means that this
is going this is not going to be aqueous
this is going to be a solid because this
is an insoluble ionic compound and then
I'll pull out my other rule regarding
the nitrate ion so it says this rule
says that if you have the nitrate ion
then it's soluble and there's no
exceptions to that rule so that means
that we're going to attach aqueous to
this side of the equation so this is
indeed a precipitation reaction because
we do have a solid precipitating out
when we mix these two solutions together
note you could also add more than two
solutions you could have three solutions
together and then there would be a host
of other possible products but in
general I think most problems there's
just two so let's go on to another
example on these precipitation reactions
and see what happens given another
scenario okay so suppose I have sodium
sulfate so that's na2so4 and if you look
at the solubility rules this is aqueous
I'm not going to pull them out just
trust me that's aqueous and KCl
potassium chloride that's also going to
be aqueous if you refer to the rules
which I do recommend doing refer to the
rules and you know see check for
yourself that that's aqueous anyway so
we have na2so4 that's sodium sulfate and
then we have KCl that's potassium
chloride so
going to write out the possible products
if we identify our cations those are
sodium ion and potassium ion and we
identify our anions those are going to
be the sulfate ion and the chloride ion
so now that we've identified cations and
anions we're going to mix them up so
that means one of our possible products
is going to be looks like the sodium and
the chloride ion go together and a CL
and we don't know the state of matter
we'll say that the very end and then our
other possible product is going to be
from the potassium and the sulfate ion
so we'll get k2so4 remember because each
potassium ion is positively charged at a
positive 1 charge
we need two of them to balance out with
the negatively charged sulfate ion and
we don't know the state of that either
we have to determine it from the
solubility rules in that and now let's
look at sodium chloride it says that all
solubility rules say that all compounds
that have the sodium ion are going to be
soluble that's according to this rule
here and they hold it up a little better
so that's going to be aqueous now
because we just determined that soluble
potassium sulfate if you apply the
solubility rules you'll also find out
that this is aqueous as well hopefully
your teacher professor whatever which is
just going to have the solubility rules
on the back of your test otherwise you
know good luck it's a lot of
memorization but you kind of have to do
it so notice that we have two aqueous
reactants but then we have two aqueous
products so this is not a precipitation
reaction so if both of your products are
aqueous and neither of them are solid
then we say that there is no reaction we
did not get a precipitate out of the
solution so there you go
that's just a basic lesson on
precipitation reactions good luck