professor dave here, let's talk about limiting reagents
when we react two substances
it's essentially impossible to have them present in precisely the correct amounts
for both of them to react completely. one of them will be the reagent in
excess and one of them will be the limiting reagent
the limiting reagent is the one that runs out first
which makes reacting further impossible. the other reagent or reagents
will then be said to be in excess because there will be some left over
in order to tell which is the limiting reagent we have to see which quantity
will limit the reaction stoichiometrically and it won't automatically be the
substance that is present in the lesser amount. for example
if we are making bologna sandwiches we might use the following equation:
the balanced equation shows that we need one baloney
and two bread to make one sandwich but say we have seven baloney
and 10 bread, how many sandwiches can we make?
7 baloney could potentially make 7 sandwiches
but 10 bread can only make five because we need two
per sandwich so even though there's more bread than Bologna
it is bread that is the limiting reagent in this
example. the bread will react fully and leave an excess
of two Bologna so baloney is in access
let's look at a real reaction. urea is produced this way
if we start with 10 grams of each reactant which one limits the reaction?
well the first thing we have to do is convert to moles
because mass doesn't tell us anything we need numbers
so we convert both to moles using their respective molar masses
we can now look at these numbers a few different ways. one way is to see how
many moles of urea
each has the potential to produce. use the stoichiometric ratios to see
that CO2 can't produce as much so that must be the limiting reagent
you could also see how much of the other reactant each could react with
this many moles of ammonia could react with this many moles
of CO2 which we don't have
this many moles of CO2 could react with this many moles of
ammonia which we do have
so CO2 will react completely and limit the reaction
once we identify the limiting reagent we just use the data for that substance
to calculate how much product we will get since that substance
will get used up
if we used the other one we would get the wrong answer
because not all of the reagent in excess will go towards generating product
another real-life limitation of chemistry is that the amount of product
we calculate that we should get
is actually just the theoretical yield. this is the amount of product we can
expect
if every molecule of reactant converts flawlessly into product
in reality this does not occur and the actual yield
will always be some fraction of the theoretical yield which we call
the percent yield. percent yield is actual yield
over theoretical yield times a hundred.
if you expected 10 grams of product from your stoichiometric calculations
but after doing the experiment only collected 8.4 grams of product
your reaction had an eighty-four percent yield.
percent yield is an important measure of the efficiency of a chemical reaction
or technique and is very useful in planning syntheses
organic compounds. let's check comprehension
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