5.3 Flashcards
1
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Le Chatelier’s Principle
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-pressure/volume
-concentration
-temperature
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• Changes in temperature
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pressure and concentration affect
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equilibrium (in different ways).
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•These factors will make the system undergo an equilibrium
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5
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shift (i.e. the movement of a system at equilibrium
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resulting
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in a change in the concentrations of reactants and products)
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7
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•The outcome is governed by LE CHATELIER’S PRINCIPLE:
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8
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•”..when a chemical system at equilibrium is disturbed by a
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change in a property
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the system adjusts in a way that
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OPPOSES the change.”
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• Any change to a chemical reaction at equilibrium causes the
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reaction to proceed in the direction that REDUCES the effect
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of the change.
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14
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Le Chatelier’s Principle & Concentration Changes
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Addition of a reactant
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or product causes the
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reaction to proceed in
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the direction that
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consumes the added
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substance.
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Removal of a reactant
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or product causes the
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reaction to proceed in
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the direction that
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produces the missing
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substance.
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Concentration (M)
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co
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3
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•2503(8)
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2
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Concentration Changes
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1
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Initial
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equilibrium
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(aq)
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H-
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NH3
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N2
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Nɔ added at
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+ HCO
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this time i
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Nz(0) + 3H2(a) 2NH3(a) Time
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•CO2 g) t 2H
0
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increase in acidity [H
0"] will cause increase in
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3 (ag)
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New
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equilibrium
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H2
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NH3
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N
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ONCSSM 2002
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- increase in SO
drives reaction to favour production
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of S0
and O
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Concentration Changes:
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Explanation Based on Kinetic Theory
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•reactant added = more frequent collisions =
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increase in rate of forward reaction
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•rate for reverse reaction does not increase
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immediately
therefore forward and reverse rates
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are not equal
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•rate of reverse reaction eventually increases as
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more product is formed
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Le Chatelier's Principle & Temperature Change
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•Heat evolves in an exothermic reaction and is absorbed in an
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endothermic reaction.
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• Changes in temperature have the same effect as changes in
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concentration
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Example:
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• If the mixture is heated
the equilibrium shifts toward NO.
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•If the mixture is cooled
the equilibrium shifts toward N
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• favours exothermic reaction
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NOTE: the value of the equilibrium constant changes
since an
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equilibrium constant is only constant at a specific
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temperature.
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Temperature Change:
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• favours endothermic reaction
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Explanation Based on Kinetic Theory
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Le Chatelier'
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a) Cooling an exothermic reaction
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Nz(g) + O
(o) t 180.5 kJ = 2NO
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uces the missing
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Concentrabor
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• Thse facxzwil make tre sste nd:rg3 En squlibeiun
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(A + B C+ energy)
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inges
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forward and reverse reaction rates are slower
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raverse rate decreases more (requires heat but less heat available)
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pre
more products and more energy will be formed
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the right (towards products)
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em tequlitbrhm
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rinciple & Concentratlon Changes
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afs
irtars rrartian ta fn
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endothermic reaction
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+ energy B+ C)
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ard rate decreases more (requires heat but less heat available)
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fore
more reactants and energy will be formed
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to the left (towards reactants)
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hermic and endothermic reactions (=
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respectively).
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Le Chatelier's Principle & Volume/Pressure Changes
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• only for gaseous equilibria.
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• decrease in volume (or an increase in pressure) causes
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reaction to proceed in the direction of fewer number of
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moles of gaseous reactant or product. These occupy a
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smaller volume
thus relieving the stress of the added
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pressure on the system.
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•an increase in volume or a decrease in pressure causes the
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reaction to proceed in the direction of greater number of
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moles of gaseous reactant or product.
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Example: N
O4(e2NO2 (e)
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increase in volume drives the reaction to the right
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decrease in volume drives the reaction to the left
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Volume/Pressure Changes:
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Explanation Based on Kinetic Theory
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decrease in volume = increase in # of collisions
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.. increase in forward and reverse reaction rates
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• rate of forward reaction increases more
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." more particles
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• leads to the production of more products
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Note: the opposite is true if volume increases
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250
( t Oz (a 250a (e)
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Changes that DO NOT affect position of
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equilibrium systems:
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Adding Catalysts
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increases the rate of reaction but does not affect the final
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citin of equilibrium.
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Concentrallian crasie heor
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B ot fonart reartion
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iediately
therefere lor vtard everse rate
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* rate of reverse reaction gventually ngreases zx.
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vation energy for both forward and re
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inert gas at constant volume
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9-10/12
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the pressure by adding another gas. If this gas is
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react with any of the gases (ie. inert) there will be no
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equilibrium
because it doesn't affect the
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ions.
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finert aas chanaes nrobability of effective collisions
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qually.
