Skip watching entire videos - get the full transcript, search for keywords, and copy with one click.
Share:
Video Transcript
Video Summary
Summary
Core Theme
This content explains the concepts of limiting reactants and theoretical yield in chemical reactions, demonstrating how to identify the limiting reactant and calculate the maximum possible product based on available starting materials.
Mind Map
Click to expand
Click to explore the full interactive mind map • Zoom, pan, and navigate
class this video is going to be on
reaction yields and our learning
objectives are to explain the concepts
of theoretical yield and limiting
reactants or reagents and we're also
going to derive the theoretical yield
for a reaction under specified conditions
conditions
so in the videos thus far we've learned
about Stoichiometry right balance
chemical reactions and the information
we can get from that so when we have the
relative amounts of reactants and
products represented in a balanced
chemical equation right we have them in
their stoichiometric proportions then we
say we have them in stoichiometric
amounts so in all of the exercises and
videos that you've watched so far we've
assumed that the reagents that we need
for a reaction are present in
stoichiometric amounts
however in reality this is not always
the case you may have more of one
reactant and less of another and that's
going to affect how your reaction proceeds
proceeds
so in order to think about this idea of
having a limiting reactant let's think
about making grilled cheese so we know
that if we're going to make grilled
cheese sandwich we need at least one
slice of cheese and two slices of bread
to prepare one sandwich and we can think
of that almost like an equation
right so let's assume that we have 28
slices of bread and 11 slices of cheese
how many sandwiches could we
theoretically prepare with these amounts
well we know that 28 slices of bread
should generate 14 sandwiches but we
only have 11 slices of cheese and if we
need a slice for every sandwich then
we'd only be able to make 11 sandwiches
right so in total we're able to make 11
sandwiches and we're going to have some
bread that's left over right 28 minus 22
would give us six slices of bread that's
so in this case the number of sandwiches
prepared is limited by the number of
cheese slices and so because of this we
call cheese the limiting reactant or
limiting reagent
the limiting reactant is always going to
be completely consumed just like we
would use up all of the cheese in the
previous example if we have a reactant
in a chemical equation that's limiting
it should be completely used up over the
course of the reaction
the bread slices in our example are
provided in excess it's our excess
reagent and so there's going to be some
bread that's left over at the end of the
reaction and the same will be true if we
have a reagent in a chemical reaction
right that is present in excess some of
that reagent will be left over once the
reaction is complete
so now let's consider a chemical example
so in this chemical process we have
hydrogen gas H2 reacting with chlorine
gas cl2 to make hydrogen chloride gas so
let's say that we have three moles of H2
and 2 moles of cl2
they are not present in their
stoichiometric amounts and so we need to
figure out which reagent would be
limiting which one would determine the
amount of product that we could form so
our approach should be to determine how
much each of these reagents would
produce in terms of our product and then
the one that gives less would be our
limiting reagent so let's work out this
example so I'm going to calculate based
upon the amount of H2 how much product
hydrogen chloride gas we would form so
our problem told me that we have three
moles of H2
and I'm going to use the reaction
Stoichiometry to determine how much
hydrogen hydrogen chloride gas we would
make so here I use my stoichiometric
Factor where I'm going to put one mole
of H2 in the denominator and 2 moles of
HCL in the numerator
so this calculates that I'm going to
generate six moles
of hydrogen chloride gas HCL
now if I consider how much cl2 I have
and so in a similar way I'm going to use
my stoichiometric factor to determine
how much hydrogen chloride gas would be
produced if we used up all of the
chlorine gas so from the balanced
Stoichiometry I'm putting one mole of
cl2 in the denominator and two moles of
HCL in the numerator
and doing this calculation I get four
moles of HCL
so because 4 moles is less than six this
tells me that the cl2 is my limiting reagent
reagent
so the chlorine gas will be completely
consumed in the process of this reaction
this also means that the H2 is our
so when the reaction is finished there's
going to be some hydrogen gas that is
actually going to be left over we can
calculate how much that would be right
well if we know that we're going to
generate four moles
of hydrogen chloride gas
based upon the fact that the chlorine
gas is limiting
we can calculate how much hydrogen gas
would be needed in order to prepare
these four moles
so for every two moles
of HCL based on the reaction
Stoichiometry I will need one mole
of H2
so this math tells me that only two
now we know that we started with three
moles so three moles minus two moles
means that I will have one mole of H2
in this example we have that silicon
nitride as a very hard temperature
resistant ceramic used as a component of
turbine blades in jet engines it's
prepared according to the following
equation where we have three moles of silicon
silicon
reacting with two moles of nitrogen to
generate one mole of silicon nitride
if we have two grams of silicon and 1.5
grams of nitrogen which is our limiting reactant
reactant
in the same way I'm going to consider
how much I have of each of these
reactants and how much product they
would form the one that forms less
product is going to be my limiting
reagent so I'm going to start with silicon
silicon
in my problem I'm told that I have 2.00
grams of silicon
now my stoichiometric factor is in moles
so my first step should be to convert
this from grams of silicon to moles of
silicon I'm going to want grams of
silicon to cancel so I'll put that in my denominator
and I'm going to use the fact that the
taking that product my grams of silicon
should cancel and now I have moles of
silicon and I can use my stoichiometric
Factor my equation tells me that for
every three moles
of silicon I'm going to generate one
and so doing this math should tell me
how much silicon nitride will be
produced if all of the Silicon is used
up and this number comes out to .0237
moles of silicon nitride
now let's do the same thing for the
the problem tells me that I have 1.50
in the same way as our how we approach
silicon we want to First convert this to
moles of nitrogen so I'm going to want moles
moles
of nitrogen to be here in my numerator
and in the bottom I'll have grams of
nitrogen so this would be
28.0 grams for every one mole of
the grams of nitrogen will cancel and so
now I'm ready to use my stoichiometric Factor
Factor
I see that for every two moles of
nitrogen this is going to generate one
mole of silicon nitride based upon the
balanced Stoichiometry of my chemical reaction
reaction
and so this will cancel moles of
nitrogen and my answer will be in moles
of silicon nitride the number here is .0268
moles of silicon nitride
we see that the reactant that gives us
less product is going to be our limiting reagent
reagent
so the Silicon
Click on any text or timestamp to jump to that moment in the video
Share:
Most transcripts ready in under 5 seconds
One-Click Copy125+ LanguagesSearch ContentJump to Timestamps
Paste YouTube URL
Enter any YouTube video link to get the full transcript
Transcript Extraction Form
Most transcripts ready in under 5 seconds
Get Our Chrome Extension
Get transcripts instantly without leaving YouTube. Install our Chrome extension for one-click access to any video's transcript directly on the watch page.
