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Core Theme
Stoichiometry uses balanced chemical equations to establish quantitative relationships between reactants and products, enabling calculations of required amounts for reactions or produced quantities.
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hi everyone welcome to the next video
discussing reactions today we're going
to be learning about reactions to a
geometry we're going to explain the
concept of Stoichiometry as it pertains
to chemical reactions and we're going to
learn how to use balanced chemical
equation to derive stoichiometric
factors which relate the amount of
reactants and products
so in previous videos we've learned
about balanced chemical equations and we
know that there's a great deal of
information that we can learn from them
for example our balanced chemical
equations show the chemical formulas
right of the reactants and products
involved in the reaction and we learned
how our chemical equation would also
include the correct phases for those
reactants and products
we similarly learned that when we have a
balanced chemical equation that the
coefficients the numbers written in
front of the reactants and products tell
us the relative numbers of these
chemical species and today we're going
to learn that using those coefficients
we can derive quantitative relationships
between the amounts of reactants and
products and that this is known as the
reactions stoichiometry
so when learning about Stoichiometry
it's helpful to think about something
familiar which is similar in concept and
that is food preparation or the use of
recipes so let's say that we're trying
to make pancakes and we take our box of
bisquicks and look at the recipe on the back
back
so the recipe might say that for one cup
of pancake mix and three quarter cup of
milk and one egg that we could make
eight pancakes and we can take that
information and write it in the form of
an equation
and this gives us the specific
proportions that are needed of our
reactants and product right our mix milk
and eggs to the product of Pancakes and
we know that that proportion is
necessary so that our pancakes aren't
too runny or aren't too thick so it
gives us the ideal amounts for our
specific product
now let's say that we have our equation
our recipe and instead of eight pancakes
we want to make two dozen pancakes we
know that we need to increase
proportionally the amount of foods in
our recipe right so we could think to
ourselves well since 24 which is two
dozen is equal to eight times three then
this tells me that I can take my recipe
that gives eight pancakes and triple it
right multiply all of my quantities by three
three
so this would tell me that instead of
one cup of mix then I would need three
cups right instead of three-fourths a
cup of milk I would need nine fourths
and instead of one egg I would need
three eggs and that would give me 24
pancakes right so that's one way that we
might think of tripling our recipe in
this case or we could write it
quantitatively by realizing that what we
and I can use the proportions in my
equation to learn that when I generate
and so writing it this way the pancakes
cancel and what I'm left with would be
24 divided by eight or three
and my units here would be eggs right so
tripling our recipe is easy if we know
that we have
you know just triple the pancakes that
we're trying to find but if we're
looking at a different non-integer value
which we might see in our chemical
equations doing the calculation on the
bottom would be would be a better approach
approach
so now let's bring this around to our
chemical equations so we can think of
them again in a similar way to recipes
so from our balanced chemical equation
we can determine the amount of reactant
needed to yield a given amount of
product so in our recipe we knew how
many eggs we would need in order to
generate a certain amount of Pancakes in
a similar way with this chemical
equation we know for example how much
hydrogen gas we would need to react in
order to generate a certain amount of
ammonia gas NH3
we can also learn the amount of one
reactant required to react with a given
amount of another so for example if I
have a certain amount of hydrogen gas I
can calculate how much nitrogen gas I
would need to react with it in order to
use up all of the hydrogen gas for
example right if I have a certain amount
of mix I can figure out how much milk
and eggs I can add to it in order to use
it up completely
so the coefficients in our balanced
equations are going to be used to derive
what we'll call stoichiometric factors
that are going to allow us to do these
calculations to determine to determine
these things so let's look at our
chemical equation that has been on the
slide this tells us that we need one
mole of nitrogen gas to react with three
moles of hydrogen gas to yield two moles
of ammonia gas and that's one way for us
to think of it in terms of moles we
could also think of it in terms of
molecules right one molecule of nitrogen
gas can react with three molecules of
hydrogen gas to yield two molecules of
ammonia gas so it really just depends on
the unit that we're thinking in we just
always have to keep this two to three
ratio for example between the ammonia
and the hydrogen gas
so using this ratio we're able to write
stoichiometric factors that will relate
two different species in our chemical
equation for example we could write that
we would have two ammonia molecules for
every three hydrogen molecules that
react we could also think of this in
terms of dozens right two dozen ammonia
molecules for every three dozen hydrogen
molecules that react you can see that
the unit really doesn't matter as long
as we have the same unit in both the
numerator and the denominator and then
finally and this is probably the
stoichiometric factor that you would use
the most right when we write it in terms
of moles so here we would write two
moles of ammonia are generated for every
three moles of hydrogen gas that are
reacted I want to also point out that
these factors don't always have to have
right for example the product in the
numerator or the hydrogen gas in the
denominator how you write it right which
species you put in the numerator and
which you put in the denominator really
depend on the calculation that you're
trying to do and which species you're
sort of canceling out and which one
you're trying to calculate so that will
depend on your exact equation
so next we'll learn how to use the
stoichiometric factors to perform
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