Question
Why equilibrium reactions can never have a 100% yield?
In my bio.org.chem lab we did an experiment to make aspirin. One of the question is to explain why the equilibrium reaction can never have a 100% yield, in detail.
Answers
we are used to seeing
mA + nB ---> xC + yD
where m moles of A added to n moles of B react to become x moles of C and y moles of D
we commonly make the assumption that the reaction will 'run to completion' (100% yield)
for many reactions this is a fair assumption but not exactly true
however these many reactions run 'nearly' to completion (well over 99% yield)
the actual result is better expressed as
mA + nB <---> xC + yD
where the two way arrow indicates a reversible reaction
the balance between reactants and products depends upon the energy change for each direction
nature tends toward the lower energy state
another take on this is to say that as the amounts of C and D build up, the likelihood of them colliding in such a way as to reform A and B increases
this is summarized in the equilibrium expression
. . . . . [C]^x * [D]^y
Keq =----------------------
. . . . . [A]^m * [B]^n
and if Keq is very large the result will be almost all product
but a smaller Keq results in a significant amount of reactants still being present at equilibrium
some reactions have a super small Keq and you have almost all reactants left
your aspirin reaction has a fairly small (or not too big) Keq
so it comes to equilibrium at a point less than 100% yield
by the way Keq is empirically derived although some theoretical calculations may come close
Keq is dependent on conditions, also
one of the tricks a chemist might use is to manipulate conditions to maximize desired results
another trick is to continuously remove a product while continuing to add reactants
then it gets real messy when a set of reactants can produce more than one set of products
ouch!
happens a lot in organic chemistry
A + B <---->C +D
K = [C][D]/[A][B]
if K aint infinity then rnx is in equilbrium and you will have some A and B
Because the products react in the opposite direction constantly to form the reactants (reverse reaction) just as the forward reaction is going on