7.1 c) LE CHATELIER’S PRINCIPLE
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swf
*Le Chatelier's Principle: If a "____________" is applied to a system at equilibrium, the equilibrium condition
is _________; a net reaction (“shift”) occurs in that direction which tends to ___________ the "stress," and a
___________ equilibrium is obtained.
Shift" to left or right
" “non-equilibrium” state "
   new equilibrium
initial equilibrium state "Stress
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“shift to right” = ______________ reaction predominates (i.e. is temporarily faster than the _____________
reaction) until equilibrium is re-established (system counteracts the “stress” by increasing the concentration of
_____________)
“shift to left” = _____________ reaction predominates (i.e. is temporarily faster than the ________________
reaction) until equilibrium is re-established (system counteracts the “stress” by increasing the concentration of
____________)
*Variables Affecting Chemical Equilibria
Stress
Δ Concentration
Δ Temperature
Type of Change
Increase
System Response…according to Le Chatelier’s Principle
Shifts to ____________ some of the added reactant or product
Decrease
Shifts to ____________ some of the removed reactant or
product
Increase
Shifts to ___________ some of the added __________energy
(i.e. shifts in the __________________direction)
Decrease
Shifts to _____________ some of the removed thermal energy
(i.e. shifts in the ________________ direction)
Δ Pressure
(Recall: Boyle’s
Law
↑P = ↓V and vice
versa)
Increase
(↓ in volume)
Shifts toward the side with the ____________ total amount of
gaseous entities
(i.e. the side with _____________ total moles of gas)
Decrease
(↑ in volume)
Shifts toward the side with the ____________ total amount of
gaseous entities
(i.e. the side with ___________ total moles of gas)
Variables that do not affect chemical equilibria:
 the presence of ________________
 the presence of ________ gases (e.g.
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GRAPHICAL REPRESENTATIONS :
1. Δ Concentration
*Addition of CO2 = shift ___________
*Why does [CO] ↑ 2 times as much as [O2] ?
2. Δ Temperature
3. Δ Pressure
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4. Effect of Catalysts on the Position of Equilibrium
*Catalysts _____ _______ alter the final position of equilibrium. (i.e. the ______________________ of reactants and
products at equilibrium are unaffected by the addition of catalysts.)
*Catalysts do, however, ____________ the time required to reach the equilibrium position. That is, equilibrium is
established much more ___________________ in the presence of a catalyst.
*The forward and reverse reaction rates are increased _________________.
*PRACTICE ACTIVITY  Le Chatelier’s Principle
1. Concentration Effects
*Increasing the concentration of a reactant causes a shift to the __________.
*Decreasing the concentration of a reactant causes a shift to the _________.
*Increasing the concentration of a product causes a shift to the __________.
*Decreasing the concentration of a product causes a shift to the __________.
*The shift continues until a______ equilibrium is established.
 In which direction does the equilibrium shift as a result of the change to each homogeneous equilibrium system?
(a) 2Cl2(g) + O2(g)
2Cl2O(g)
*Adding Cl2(g)
= shift to the ____________
*Removing Cl2O(g) = shift to the ____________
*Adding Cl2O(g) = shift to the ____________
(b) 2NO2(g)
N2(g) + 2O2(g)
*Removing N2(g) = shift to the ___________
*Removing NO2(g) = shift to the ___________
*Adding O2(g)
= shift to the ___________
2. Temperature Effects
*Increasing the temperature of an equilibrium system causes a shift in the ____________
direction.
*Decreasing the temperature of an equilibrium system causes a shift in the ___________
direction.
 For each of the following reversible reactions, determine whether the forward reaction is favoured by high
temperatures or low temperatures.
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(a) N2O4(g)
2NO2(g)
∆H = +59 kJ
or: N2O4(g) + 59 kJ
2NO2(g)
*Forward = ____________, therefore ________ temperature will favour forward reaction.
(b) 2ICl(g)
or:
I2(g) + Cl2(g)
∆H = -35 kJ
*Forward = ___________, therefore _________ temperature will favour forward reaction.
3. Volume/Pressure Effects
*Decreasing the volume (______________ the pressure) of an equilibrium system favours the reaction that produces the
__________ total amount of gaseous entities.
*Increasing the volume (______________ the pressure) of an equilibrium system favours the reaction that produces the
__________ total amount of gaseous entities.
 In each of the following equilibrium systems, the volume of the reaction vessel is decreased (pressure is _________).
What is the effect (if any) of the position of equilibrium? (i.e. What is the direction of the equ’m shift?)
(a) 4HCl(g) + O2(g )
2Cl2(g) + 2H2O(g)
*Gaseous entities left side = ______
*Gaseous entities right side = ______
*Therefore, ↑ in pressure = shift to ___________.
(b) 2H2S(g) + CH4(g)
4H2(g) + CS2(g)
*Gaseous entities left side = ______
*Gaseous entities right side = ______
*Therefore, ↑ in pressure = shift to ___________.
(c) N2(g) + O2(g)
2NO(g)
*Gaseous entities left side = ______
*Gaseous entities right side = ______
*Therefore, ↑ in pressure = ____ shift.
Extension:
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