Extent of Reaction - Chemical Equilibrium Video Lecture | Physical Chemistry

the extent of reaction is typically
denoted by the Greek letter Sai and it's
used to relate the products and
reactants of a particular reaction
together so for example if we have the
elementary reaction a plus two B is in
equilibrium with C or you can have a
forward or a reverse reaction an initial
look at the number of unknowns in this
expression may lead us to believe that
we don't know how many moles of a there
are we don't know how many moles of B
there are and we don't know that how
many moles of C there are but since
these three different compounds are
connected through the same chemical
reaction the ratios of a B and C are all
actually dependent on one another for
example we can't create more components
C without consuming a and B so we can't
independently vary the amount of our
species in a particular system for a
reaction the extent of reaction can be
viewed as the variable that connects
these unknowns if we have the extent of
reaction here and I'll never draw quite
as well as the Greek symbol can if this
is greater than zero this means that the
reaction moves from the left to the
right yes the extent of reaction is less
than zero we have the opposite occurring
now to further explore on this
mathematically we can write out the
extent of reaction is equal to the
number of moles of component I in the
system minus the number of moles
initially of component I in the system
divided by its stoichiometric ratio or
we can rearrange this expression to have
the number of moles of I
as a function of the extent of reaction
in this case here we can see that this
will have units of moles so in this
particular example if we have our
reaction here of a plus 2 B going to C
we can write out that at some
configuration of our system the number
of moles of a can be related to the
initial number of moles of a minus the
extent of reaction in this case here
this is because the stoichiometric
coefficient of a is equal to minus 1 as
a reaction moves from the left to the
right side it consumed one mole of a
likewise for B in the system
consuming 2 moles of B and for C we
would be generating 1 mole now if for
example we were to not know whether or
not the reaction was a plus 2 B going to
C or if the reaction was C going to a
plus 2b everything about this expression
would just switch you know the
stoichiometric coefficients would go
from positive negative but the math
overall would work out exactly the same
so you can have a positive extent of
reaction a negative extent of reaction
but the overall purpose behind it is
that it relates between the amount of
all the different species in a
particular reaction