Molecularity of a Reaction Video Lecture | Chemistry for ACT

order versus molecularity in this video
we're going to walk through in define
order in terms of a particular reactant
and overall order we've sort of hinted
at this throughout the previous videos
now we're going to spend a bit of time
actually talking about it with a purpose
we're also going to define something
called molecularity which we haven't
discussed yet and then we're going to
explain the difference between order and
molecularity because they're not
particularly similar concepts but
they're easily confused if you aren't
very diligent about thinking about what
they mean so let's talk about order of
the reaction first so this is some
freezing on things that we have already
talked about so let's say that we have a
very generic reaction which I have here
and we have a very generic rate law to
go along with it which I have here if we
want to talk about the order of things
here's how the phrasing works for this
reaction we would say that it is X order
and a so X is the exponent on a so it's
X order in a if that was a to the second
power we would say it was second order
it was a to the third power we'd say it
was third order and then for B B is
raised to the Y power and so we say it
is y order and B so if Y was equal to
five we would say this fifth order in B
Y is equal to one we would say it is
first order in B if we want to talk
about order overall order we can do that
too you just add the exponents up and
that is what the overall order is so
let's do a few examples here to make it
a little bit more clear let's take a
random rate law so we have a raised to
the second power B raised to the fifth
power the way that we would phrase this
is that it is second order and a fifth
order in B and seventh order overall
let's look at another one
here we would look at this and say that
it is first-order an a because the
exponent is a 1 right if there's no
exponent listed it's implied a 1 we
would say it is second order in B and
that is the third order and see if we
add all of these up we end up with the
sixth order overall so this is comes off
the rate law and is usually
experimentally determined it's very
different than what molecularity is so
with molecularity this is a theoretical
concept that is based on collision
theory so think back to our very brief
introduction on collision theory and
this was the idea that molecules in
order to react must first collide so
this is the number of colliding entities
that would be required in a given step
notice I don't say a given reaction I
say a given step so we have multiple
step reactions which we've talked about
this would have a different molecularity
for each step so let's look at the
examples I have here we have three
options that we're going to list here
which is you know molecular by molecular
intermolecular so you know molecular is
if you have just one colliding entity or
in other words it just reacts on its own
by molecular is two colliding entities
intermolecular is three colliding
entities it's far less common so here I
have a two-step reaction it's
effectively a plus B goes to D when you
look at the overall reaction but we have
two different steps our first step has a
plus B going to C that has two entities
and so it is by molecular the second
step has that intermediate C reacting on
its own to form D and so it's just one
entity and therefore it is you know
molecular
now let's look at the same reaction and
compare the order with the molecularity
so remember our molecularity is
theoretical based on collision theory
our order is an experimental concept
based on the overall rate law and so if
I give you a rate law for this I say
this was experimentally determined to be
a squared B to the fifth it would be
seventh order okay so one last time wrap
up we have molecularity versus order
molecularity is a theoretical concept
based on collision theory while order is
an experimentally determined rate law
and rate laws are going to be for the
entire reaction while molecularity is
for each individual step