5.3 Flashcards
Le Chatelier’s Principle
-pressure/volume
-concentration
-temperature
• Changes in temperature
pressure and concentration affect
equilibrium (in different ways).
•These factors will make the system undergo an equilibrium
shift (i.e. the movement of a system at equilibrium
resulting
in a change in the concentrations of reactants and products)
•The outcome is governed by LE CHATELIER’S PRINCIPLE:
•”..when a chemical system at equilibrium is disturbed by a
change in a property
the system adjusts in a way that
OPPOSES the change.”
• Any change to a chemical reaction at equilibrium causes the
reaction to proceed in the direction that REDUCES the effect
of the change.
Le Chatelier’s Principle & Concentration Changes
Addition of a reactant
or product causes the
reaction to proceed in
the direction that
consumes the added
substance.
Removal of a reactant
or product causes the
reaction to proceed in
the direction that
produces the missing
substance.
Concentration (M)
co
3
•2503(8)
2
Concentration Changes
1
Initial
equilibrium
(aq)
H-
NH3
N2
Nɔ added at
+ HCO
this time i
Nz(0) + 3H2(a) 2NH3(a) Time
•CO2 g) t 2H
0
increase in acidity [H
0”] will cause increase in
3 (ag)
New
equilibrium
H2
NH3
N
ONCSSM 2002
- increase in SO
drives reaction to favour production
of S0
and O
Concentration Changes:
Explanation Based on Kinetic Theory
•reactant added = more frequent collisions =
increase in rate of forward reaction
•rate for reverse reaction does not increase
immediately
therefore forward and reverse rates
are not equal
•rate of reverse reaction eventually increases as
more product is formed
Le Chatelier’s Principle & Temperature Change
•Heat evolves in an exothermic reaction and is absorbed in an
endothermic reaction.
• Changes in temperature have the same effect as changes in
concentration
Example:
• If the mixture is heated
the equilibrium shifts toward NO.
•If the mixture is cooled
the equilibrium shifts toward N
• favours exothermic reaction
NOTE: the value of the equilibrium constant changes
since an
equilibrium constant is only constant at a specific
temperature.
Temperature Change:
• favours endothermic reaction
Explanation Based on Kinetic Theory
Le Chatelier’
a) Cooling an exothermic reaction
Nz(g) + O
(o) t 180.5 kJ = 2NO
uces the missing
Concentrabor
• Thse facxzwil make tre sste nd:rg3 En squlibeiun
(A + B C+ energy)
inges
forward and reverse reaction rates are slower
raverse rate decreases more (requires heat but less heat available)
pre
more products and more energy will be formed
the right (towards products)
em tequlitbrhm
rinciple & Concentratlon Changes
afs
irtars rrartian ta fn
endothermic reaction
+ energy B+ C)
ard rate decreases more (requires heat but less heat available)
fore
more reactants and energy will be formed
to the left (towards reactants)
hermic and endothermic reactions (=
respectively).
Le Chatelier’s Principle & Volume/Pressure Changes
• only for gaseous equilibria.
• decrease in volume (or an increase in pressure) causes
reaction to proceed in the direction of fewer number of
moles of gaseous reactant or product. These occupy a
smaller volume
thus relieving the stress of the added
pressure on the system.
•an increase in volume or a decrease in pressure causes the
reaction to proceed in the direction of greater number of
moles of gaseous reactant or product.
Example: N
O4(e2NO2 (e)
increase in volume drives the reaction to the right
decrease in volume drives the reaction to the left
Volume/Pressure Changes:
Explanation Based on Kinetic Theory
decrease in volume = increase in # of collisions
.. increase in forward and reverse reaction rates
• rate of forward reaction increases more
.” more particles
• leads to the production of more products
Note: the opposite is true if volume increases
250
( t Oz (a 250a (e)
Changes that DO NOT affect position of
equilibrium systems:
Adding Catalysts
increases the rate of reaction but does not affect the final
citin of equilibrium.
Concentrallian crasie heor
B ot fonart reartion
iediately
therefere lor vtard everse rate
- rate of reverse reaction gventually ngreases zx.
vation energy for both forward and re
inert gas at constant volume
9-10/12
the pressure by adding another gas. If this gas is
react with any of the gases (ie. inert) there will be no
equilibrium
because it doesn’t affect the
ions.
finert aas chanaes nrobability of effective collisions
qually.
