Reversible Reactions and Le Chatelier's principle - Chemical Equilibrium Video Lecture | Chemistry for GRE Paper II

professor Dave here let's talk about
Le Chatelier's principle.
Le Chatelier's principle says that if you
induce a stress on a system at
equilibrium the equilibrium will shift
so as to relieve that stress. so let's
see some examples of a stress we could
put on a system at equilibrium. first we
could modify the concentration of any of
the compounds. let's say we add some of
this reactant, that will unbalance the
equilibrium and the forward reaction
will speed up to use up some of the
additional reactants and turn them into
products and restore equilibrium. the
equilibrium is said to have shifted
right. if we add more products it would
shift left. likewise if we selectively
remove one of the components the
equilibrium will shift to produce more
of that species to restore balance. that
is one type of stress we could put on a
system. another stress would be to change
the temperature. to see how this would
affect an equilibrium we have to see
whether a given reaction is exothermic
or endothermic. this tells us whether a
reaction absorbs or releases energy and
is signified by delta H
the change in enthalpy. if delta H is
negative the reaction is exothermic and
releases energy so we could think of
heat energy as a product of this
reaction. if instead it's positive that
means it's endothermic and energy must
be absorbed for the reaction to go. so we
could think of heat energy as a reactant
in that scenario. once we translate
thermochemical data in this way we can
treat the word heat as just another
substance involved in the reaction
higher temperature means more heat so
things will shift to the other side to
use up some excess heat and relieve the
stress. cooling it down would have the
opposite effect. the third stress we can
examine is changing the volume or
pressure. let's say the equilibrium
involves gases in a balloon
what if we decrease the volume? if we
remember Boyle's law we know that the
pressure will go up since the smaller
the volume the more the particles will
hit the sides and exert pressure. if
there is a discrepancy in the number of
moles of particles on either side of the
equilibrium it will shift towards the
side with fewer particles so as to
alleviate some of the additional
pressure. in this equilibrium between a
diatomic molecule and two monoatomic
species the right side has twice as many
particles so shifting left means atoms
fusing together resulting in fewer
particles which lowers the pressure on
the container. if we increase the volume
thereby lowering the pressure the
equilibrium would shift towards the side
with more particles in order to regain some of the lost pressure. let's check comprehension.
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