1. Le Chatelier’s Principle: “If a system at equilibrium
is disturbed by a change
in temperature, pressure,
or concentration of a
component, the system
will shift its equilibrium
position to counteract the
effect of the disturbance.”
Henri Le Chatelier
(1850–1936)

2. (A) change in concentration
N2(g) H2(g) NH3(g)
Diagram shows what
happens when we add
H2 to an equilibrium
system… H2
shift  to est.
new eq.
NH3
Suppose we remove
NH3... N2
shift  to est. new eq.
In industrial settings, the
ammonia would be
removed from the system
by applying high pressure
and low temperatures to
induce liquefaction of
the ammonia.
orig.
eq.
system
establishing
new eq.
new
eq.
H2 added
at this
time

3. (B) change in pressure – for gaseous equilibrium
systems only
2 NO2(g) N2O4(g)
SHIFT
to est. new eq.
If we increase pressure, the
system “wants” to...
get smaller
SHIFT
to est. new eq.
If we decrease pressure, the
system “wants” to...
get bigger
For H2(g) + I2(g) HI(g),
pressure changes result in...
NO SHIFT

4. “Ne!” “Ne!” “Ne!” “Ne!” “Ne!”
More specifically, if the partial pressures
are affected, then there will be shifting.
Otherwise...no.
“Ne!”
“Ne!”
“Ne!”
e.g.,
2 SO2(g) + O2(g) SO3(g)
“Ne!”
If we add neon into
reaction vessel...
Partial pressures of
SO2, O2, and SO3
are unchanged.
“Ne!”
NO SHIFT
As long as temperature doesn’t change, K is...
NOT affected by P-V changes.

5. – + (C) changes in temperature These always result in...
shifts in eq. AND changes in K.
For exothermic reactions: –
R P + heat
(DH is ____)
-- as T increases...
shift , K
Don’t memorize…
THINK!
-- as T decreases...
shift , K
For endothermic reactions: +
heat + R P
(DH is ____)
-- as T increases...
shift , K
-- as T decreases...
shift , K

6. For PCl5(g) PCl3(g) + Cl2(g), DHo = 87.9 kJ.
Predict shifts for...
PCl5(g) kJ PCl3(g) + Cl2(g)
Rewrite this as…
SHIFT
(a) adding Cl2
SHIFT
(b) increasing temperature
SHIFT
(c) decreasing volume
SHIFT
(d) adding PCl5

7. Using standard molar enthalpies, predict the eq. shifts
and changes in K with changes in temperature for
2 POCl3(g) 2 PCl3(g) + O2(g)
DHfo (kJ/mol) – –
Strategy: 1) Use DHfos to estimate DHrxn.
2) Use Le Chatelier to predict shifts.
DHrxn = 2(–288.1) – 2(–542.2)
= kJ
(endothermic)
as T ...
shift , K
as T ...
shift , K

8. -- the forward and reverse reaction rates are both increased
Effect of a Catalyst
-- the forward and reverse reaction
rates are both increased
Catalytic converters in cars
help to quickly convert
nitrogen oxides into nitrogen
and oxygen and carbon
monoxide into carbon
dioxide. --
NO SHIFT --
Catalysts increase the rate
at which eq. is achieved,
but the final composition
of the system is unchanged,
as is K.