YouTube Transcript: Chapter 7.1b Equations for Ionic Reactions
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This content explains how to represent chemical reactions in different formats, focusing on writing and balancing molecular, complete ionic, and net ionic equations, especially for reactions occurring in aqueous solutions.
hi class today we are learning about
writing and balancing Chemical Equations
our learning goals are to derive
chemical equations from narrative
descriptions of chemical reactions and
in this video we'll focus on writing and
balancing these equations in the total
ionic and net ionic formats
so in the previous video we learned
about writing chemical equations that
are balanced we talked about reactants
and products and what the arrow meant
and how we needed to balance them
there's some more information that we
can gather from chemical reactions that
we're going to talk about in this video
to start off with we'll learn that the
physical states of reactants and
products are usually indicated on a
chemical reaction and you'll find these
in parentheses following the the
molecules that they refer to so there
are going to be several different
descriptors right we might see a g in
parentheses to indicate that that
species is a gas we might have L for a
liquid s for something that's in the
solid form and then also AQ which stands
for aqueous so if we have a reactant
that is dissolved in water
so here we have an example showing the
reaction of solid sodium metal with
water to give sodium hydroxide this is a
base that is aqueous so that will be
dissolved in the water and also will
generate hydrogen gas
so this reaction is very specific and
that it's written for that particular
reaction that the sodium is a solid that
the water is liquid that the sodium
hydroxide is dissolved in the water and
that hydrogen gas is evolved
there are some other information we can
find on chemical reactions for example
there might be some sort of symbol or
writing that is indicated on top of the
arrow that separates the reactants and
products and in this example we see
carbon calcium carbonate decomposing to
calcium oxide and carbon dioxide and
you'll see this Delta symbol the
triangle that is above the arrow tells
us that heat is required in order for
the reaction to proceed
so that's just another example of some
information you might find written in
the chemical equation
for the rest of this video we're going
to focus on reactions in aqueous media
and in particular we're going to think
about how ionic species
will can be written in different formats
right so in this case we're going to
focus on the equation written here and
what we call the molecular format
you'll see that molecular equations
don't explicitly represent the ionic
species that are present in solution and
what that means is that ions are written
together so that the cation is with its respective
respective
anion and it's written together as in
the molecular format so in this example
we have calcium chloride and silver
nitrate reacting to give us calcium
nitrates and silver chloride and you'll
see that just as we discussed previously
the phase is written for each of these
species so we have aqueous aqueous for
our reactants and our product we have
something that is aqueous but then
because several of these species are
aqueous we can think about what happens
when those ionic compounds are dissolved
in the water right to give the aqueous form
form
many ionic compounds they will
dissociate into their constituent ions
so the reaction for calcium chloride and
this dissociation is written explicitly
here we know that the calcium chloride
will break apart so that the ions
dissociate to give us the calcium two
plus ion which is aqueous right so it's
dissolved in the water as well as
chloride ions and based upon the
Stoichiometry of the ionic compound we
know that we should have two chloride
ions for every one calcium ion and so we
need to make sure to indicate that with
the reaction Stoichiometry for the dissociation
dissociation
let's try another example so here we
have silver nitrate which is another
ionic compound which should dissociate
when it's dissolved in water
now we know that nitrate should have a
minus one overall charge and if we think
about how we indicate the the subscripts
right based upon the charges of these
cancelings so that our overall form is
neutral we know that this would give us
nitrate ions
foreign which has the negative one
charge and then the silver ions should
have a plus one charge so that overall
this would be neutral
we need to keep up with the
Stoichiometry of the reaction so if we
have two silver nitrates on our reactant
side we're going to generate two silver
ions and also two nitrate ions
in our last example and the silver
should be aqueous in our last example we
have calcium nitrate
if we think about how the charges are
used to give us the molecular formula we
know that this two comes from the plus
two charge on the calcium right and
though the one right the assumed
subscript there gives us the negative
one charge on the nitrate so we would
write this as
nitrate ions
and calcium
two plus ions
both dissolved in water so they have the
AQ in parentheses and then we need to
write in our coefficients we can see in
our molecular formula there's only one
calcium so I don't need to write in the
one coefficient since it's assumed
but I have two nitrate ions in my
molecular formula so I will write this
as a 2 in front of the nitrate as my coefficient
coefficient
and we see the last species that was
involved in our molecular equation is
silver chloride and that was a solid so
that indicates that it does not dissolve
in water to a significant extent
so what we're going to do now is take
our molecular equation for this reaction
and use it to write what's called the
complete ionic equation and to do that
it says that we are going to represent
any ionic compound that dissolves in
water as the dissociated ions so we're
going to represent represent them
explicitly as having dissociated so what
we did in the previous slide was write
what happens when our three aqueous
species dissolve and dissociate into
their ions
so what I'm going to do now is write in
what we had written on the previous
slide so for calcium chloride we said it
dissolves to give us calcium and
chloride ions so we're going to write
that in so we had calcium two plus
and that was aqueous and we also had two
chloride ions
right that were aqueous when the calcium
chloride dissolves now we have the
silver nitrate
on the previous slide we showed that
this is going to dissociate to give us
two silver ions and two nitrate ions
so we'll write that in we have to
silver ions
so that takes care of our reactant side
and now for our product side we saw on a
previous slide that we showed that the
calcium nitrate will dissolve to give us
calcium two plus ions and two nitrate ions
ions
so let's write that in for the calcium nitrate
nitrate
and two
nitrate ions
when the calcium nitrate dissolves and
then finally we have the silver chloride
which we know is in solid format because
of this s in parentheses so because it's
solid it tells us that it does not
dissolve in the water it's going to stay
in that phase so we cannot assume that
it's going to dissociate into its ions
we have to write it as the solid silver
chloride and we write it and keep it in
the molecular format
another format that we can write it in
is What's called the net ionic equation
so at the top of the slide I have the
equations we've previously written we've
written the molecular equation where we
keep our ionic compounds written as
molecular species
then I have my complete ionic equation
where I've taken any ionic compound that
dissolves and written it as its
constituent ions so we we broke those
apart into their separate ions so this
is what we did in the previous slide and
now what we're going to do is write the
net ionic equation to do that we need to
recognize the species that are called
spectator ions sort of ions are that are
just hanging around and watching the
reaction proceed these are ions whose
present is required to maintain charge
neutrality but they're not really
involved in the chemical or physical
change of the reaction
so I can identify spectator ions as ions
that are present on both sides of my
chemical equation so for instance I can
see that I have calcium 2 plus as a
reactant and also as a product and I can
also see that I have nitrate ions
present as reactant and also product
so to achieve my net ionic equation I
simply need to eliminate these spectator
ions I'm going to eliminate them and not
write them as I write my net ionic equation
equation
you'll also notice that when you tend to
write the net ionic equation you're
going to want to start with your cation
so I'm going to start by writing my
or should have been an AQ here for my
silver ions
and then I'm going to look at my
products and all I have left here
would be the silver chloride solid
so this is my net ionic equation now you
may remember from a previous video that
we want to write our coefficients for
the reaction with the smallest possible
integers and you'll see that for each
species involved in this equation I have
a coefficient of 2. so I can divide all
of my coefficients by 2 and write my net
ionic equation with the smallest
coefficients so then it would just be
silver ions with a coefficient of one
right my chloride ions also a
coefficient of one and that would form
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