YouTube Transcript: Chapter 7.2a Precipitation Reactions and Solubility
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This content explains precipitation reactions, where dissolved substances react to form solid products, and introduces solubility rules to predict whether a compound will dissolve or precipitate.
hello everyone this video is going to be
on precipitation reactions and
solubility our learning goals are to
Define what a precipitation reaction is
and to be able to identify whether a
reaction is such a reaction
precipitation reaction and then we also
want to be able to predict the
solubility of some common inorganic
compounds by learning the solubility rules
rules
so first we need to know what a
precipitation reaction is
this reaction is one in which we have
some dissolved substances which react to
form one or more solid products so this
is the important feature that one of our
reactants is solid whereas all of our
reactants Right started in the aqueous phase
phase
these reactions are also called double
displacement or double replacement or
metathesis reaction so you may hear this
called by these other names
when we think about what's happening in
these reactions it's really that it
involves an exchange of ions and we when
we look at some examples of these
precipitation reactions you'll see why I
say that
reactions that are cause precipitates
are common in nature and in Industry so
in your textbook it talks about
precipitation of lead iodide which is
shown over here on the left you see that
bright yellow precipitate which is the
lead iodide formerly this solid was used
to form artist pigment right because of
that vibrant color I can think about
kidney stones right which are very
painful as stones that are formed
um and these are made from calcium
oxalate within the body and then another
example would be from Coral so the
species of coral can add calcium
carbonate precipitate to their reefs to
slowly extend their structure and grow
so those are just some examples and
there are certainly many others
so some vocabulary for you to learn when
talking about precipitation reactions
the first is solubility and we will deal
more with calculating solubility and
doing more calculations with it but
solubility is the maximum concentration
of a substance that can be achieved
under the conditions right that that are
being used so how much of a substance
can be dissolved in solution under a
given set of conditions
um substances that have a large
solubility are called soluble
so if something can be dissolved in a
large extent we call it soluble
another vocabulary word will be
precipitate so a substance will
precipitate when
it is dissolved or the conditions are
such that its concentration exceeds its solubility
solubility
so in this example we have thallium
chloride right it has an aqueous
solubility of 3.2 grams per liter at 20
degrees Celsius so if you have a liter
of water and you try to dissolve four
grams this will be above the solubility
and so not all of it will dissolve and
you'll be left with a precipitate at the
bottom of your beaker
um because you have exceeded the solubility
solubility
so anytime your reaction or your
solution conditions exceed the
solubility then you'll have precipitation
precipitation
substances that have a low solubility
are called insoluble right so in
contrast to Solutions or to substances
that are called soluble we have
substances that are called insoluble
because they have a low solubility and
these substances will readily
precipitate from solution
so in order to know whether substances
are going to dissolve and be soluble or
if they are going to precipitate from
solution we need to know how to use the
solubility rules and your textbook
summarizes these in two tables you can
see that this first table shows us
ionic compounds that are soluble
so you'll see that it lists compounds
that contain certain ions so that's
what's in this First Column and then in
the second column it tells us some
exceptions to these so let's go about
this systematically in our first box it
lists cations which are going to be
soluble and we can see by looking at the
exceptions column that there are none
right there are no exceptions to this so
it means that if I have a compound that
has the ammonium ion I know it will be
soluble there are no exceptions no
anions that I can combine this with that
are going to be insoluble right and the
same are true for these other group one
cations right so sodium
and a plus there's nothing that I'm
going to combine this with that will
give an insoluble compound
moving down then we have some of our
halogens right so the chloride bromide
iodide ions these are going to be
soluble except there are some exceptions
so when we combine it with silver ions
Mercury as well as lead then these would
become insoluble right so all compounds
with chloride bromide and ion are going
to be soluble except these exceptions
right so if I have silver chloride or
silver bromide or silver
iodide right those would be the
exceptions with silver I could have mercury
mercury
right so Mercury chloride
chloride Mercury
Mercury
bromide Mercury
iodide right those would be exceptions
and then lead
right so lead chloride
lead iodide lead
lead
bromide is the one I'm missing
so these would be the insoluble exceptions
for these normally soluble
um anions
next we have fluoride we can see that
that is going to be soluble except when
it makes compounds with group two metal
cations lead as well as iron we've got
lead two plus and iron three plus so the
exceptions here would be the group two
metal cations
so something like calcium
which would give us cif2 this would be
an insoluble exception we could have
lead right so lead
fluoride that would be an insoluble
exception and with the iron three plus
ion we would have f e f 3 right this
moving down further we've got some
molecular ions so this would be the
acetate ion this would be bicarbonate
nitrate and chlorate these are always
going to be soluble because we have no exceptions
exceptions
and then lastly we have sulfate so4 2
minus you can see that this is always
going to be soluble except when combined
with these different ions so for example
right and this would be insoluble or for
example the barium
ion so barium sulfates
would be insoluble I could also write
the strontium compounds
compounds
ionic compounds so this would be
insoluble as well so these are just some examples
examples
of the exceptions that would form
insoluble precipitates with sulfate
and now this table shows us the
compounds that are usually insoluble
carbonate we have
chromate phosphate and sulfide these are
always going to be insoluble and then we
list only some exceptions now because we
have switched the way we are writing
this table right it's telling us that
these are always
insoluble and the exceptions will be the
ones that are soluble so a group one
cation right so say we have sodium that
forms an ionic compound with the carbonate
this will be soluble
because this represents the exception
ion that can form an ionic compound with
right and then this would be soluble
so be careful what table you're looking
at and understanding what the exceptions represent
represent
and then our last one on this table is
hydroxide so hydroxide is always going
to be insoluble again and our exceptions
will tell us when it is soluble so here
we can see that barium hydroxide will be
so that will be soluble Group 1 cations
so we could have lithium for example
lithium hydroxide sodium hydroxide right
so now let's use this information to
write some precipitation reactions our
first example is a reaction in which we
have sodium chloride aqueous right so
that's dissolved in water mixing with
silver and nitrate silver nitrate is
actually used to test solutions to see
if they contain the halides so it will
form a precipitate if you have a
chloride ion bromide or iodide and so
you'll see this type of reaction right
the precipitate forming and then falling
to the bottom of the test tube
so let's write our molecular our
complete ionic and net ionic equations
for this example
what I'd like to do is start with
showing what happens when these
dissolved ionic compounds dissociate in
solution as we learned in a previous
video so we know that they'll break up
into their constituent ions so this
sodium chloride will break up to give us
sodium ions
and the silver nitrate will also break
up into its constituent ions right to
give us silver ions
now we talked about how these
precipitation reactions are also called
double replacement or double
displacement it's the idea that our
cations and anions are switching
Partners so before the sodium and the
chloride were together now let's pair
the sodium with the nitrate ion and
we'll pair the chloride and the silver
together so they're just switching their partners
partners
when we do that we'll have
sodium nitrate
and we'll have silver chloride
now what we want to do is go to those
solubility tables and decide whether our
products are going to be soluble or
insoluble if they're soluble then we'll
get to write the aqueous symbol after
them if they are precipitate then we'll
write as a solid
so if we go back to our tables we'll see
that we're looking for silver chloride
silver chloride was one of those
insoluble exceptions that we had written
so this is actually going to be a solid
a precipitate the other species that
we're looking for is our sodium nitrate
we said that sodium is one of those
group one cations that is always going
to be soluble there are no exceptions so
sodium nitrate should be something that
dissolves so we can write it as being an
aqueous species
so this will be our aqueous species and
the silver chloride will be our solid
so we've written our molecular equation
now let's write our complete ionic
equation right the product side we know
that the silver I'm sorry the sodium
nitrate is soluble so it will break
apart or dissociate into its ions so
but the silver chloride solid will remain
remain
together in the solid form
so now we've written our complete ionic
equation and if you remember from last
time to write the net ionic equation we
simply eliminate The Spectator ions so
we look for ions that are present on
both the reactant and product side and
we eliminate them we can see that the
sodium ion is present on both sides and
the nitrate is present on both sides and
so we're left with our net ionic equation
equation
so I wrote the silver first and then
those move on to give us the silver
chloride solid
let's do another example this reaction
yields that lead iodide precipitate the
bright yellow that is used in artist
pigment was our one of our previous
examples this can be generated by
reacting potassium iodide and lead
nitrate so just as before let's start by
writing our reactants for the complete
ionic equation we can see that both of
these species are aqueous and so we're
going to assume that they break apart
into their constituent ions
so when this potassium iodide
dissociates it's going to give me
and iodide ions
and we can see that there is a 2 for my
coefficient so I'm also going to make
sure to put a 2 in front of each of
those ions
and now when I think about how the lead
nitrate breaks apart into its ions
looking at my formula for the lead
nitrate I can see that I should have two
nitrate ions for every one lead ion so I
made sure to put a 2 here for my coefficient
coefficient
so I have dissociated my aqueous
reactants and now just as before they're
going to switch partners and so my
potassium ion is going to reform with
the nitrate and then my iodide is going
to come together with the lead
so let's write those here above right
the spoiler is that this should be
aqueous I have two potassiums and two
nitrates so I should be able to write a
2 as my coefficient
and then when the lead and the iodide
come together
this is going to give me lead iodide
pbi2 and if I go back to look at my
I can see that iodide is normally
soluble so here's the iodide ion but one
of its exceptions is when it gets
together with lead ions so lead iodide
is insoluble
so here we're going to write it with an
S to show that it will be the solid precipitate
precipitate
so this is my molecular equation if I
want to complete my complete ionic
equation right I'm going to write my
product side now so my lead nitrate is
going to dissociate to give me the I'm
sorry my potassium nitrate is going to
dissociate to give me potassium ions
and nitrate ions
I see that I have two of those so that
means I will have two potassium ions as
well as two nitrate ions
and then finally the lead iodide will
stay together in the solid form
and so that is our complete ionic
equation finally to write our net ionic
equation we eliminate The Spectator ions
we see that we have potassium on both
sides and we also have nitrate on both
sides and so rewriting what is left we
see that lead 2 plus
is going to react with two iodide ions
to yield our precipitate which is lead
iodide and we make sure to write that it
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