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Unit 17
Daily 2
Reaction Equilibrium
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What is Le Chatelier’s Principle?
Key Question:
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1. Explore factors affecting equilibrium2. Predict direction of equilibrium shift3. Predict resulting changes in concentration4. Make equilibrium serve your own evil
purposes
Today’s Goals:
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• Equilibrium is a RATE balance.• Anything that can affect the rate of the
forward or reverse reaction can throw equilibrium off.
• What are the factors that affect reaction rate?1) Concentration2) Pressure or Volume (gases)3) Temperature4) Surface Area5) Catalysts/Inhibitors
Factors Affecting Equilibrium
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“If a system at equilibrium is subjected to an external stress, the equilibrium will shift to minimize the effects of that stress”
The Steps of Le Chatelier’s Principle:1) Stress: Adding or removing a reactant, adding or
removing a product, changing the pressure, volume, or temperature.
2) Shift: The equilibrium will shift away from what is added and towards what is removed.
3) Change in Concentration: The concentration of what is being shifted towards increases, the concentration of what is being shifted away from decreases.
Le Chatelier’s Principle
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Fe3+(aq) + SCN-
(aq) FeSCN2+(aq)
(colorless) (red)
If more Fe3+ is added to the system, what will happen?equilibrium shifts to the rightequilibrium shifts to the product side the forward reaction is favored (forward shift)
How are concentrations affected? [Fe3+] increases, [SCN-] decreases, [FeSCN2+] increases
How is Keq affected?Keq does not change when changes in concentration
cause a shift in equilibrium
Changes in Concentration
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Fe3+(aq) + SCN-
(aq) FeSCN2+(aq)
(colorless) (red)
If more FeSCN2+ is added to the system, what will happen?equilibrium shifts to the leftequilibrium shifts to the reactant sidethe reverse reaction is favored (reverse shift)
How are concentrations affected?[Fe3+] increases, [SCN-] increases, [FeSCN2+]
increases
Changes in Concentration
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Fe3+(aq) + SCN-
(aq) FeSCN2+(aq)
(colorless) (red)
If SCN- is removed from the system (by adding AgNO3 so that AgSCN(s) precipitate forms), what will happen?equilibrium shifts to the leftequilibrium shifts to the reactant sidethe reverse reaction is favored (reverse shift)
How are concentrations affected?[Fe3+] increases, [SCN-] increases (but also decreases
since it was removed), [FeSCN2+] decreases
Changes in Concentration
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Only affect an equilibrium system if gases are present
What is the relationship between moles of gas and volume/pressure?P = (nRT)/(V)
If pressure is increased (or volume decreased):equilibrium will shift towards the side with
FEWER moles of gas to bring the pressure back down
If pressure is decreased (or volume increased):equilibrium will shift towards the side with
MORE moles of gas to bring the pressure back up
Changes in Volume & Pressure
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N2(g) + 3H2(g) 2NH3(g)
(4 moles) (2 moles)If pressure is increased (volume decreased) on
the system, what will happen?equilibrium shifts to the rightequilibrium shifts to the product sidethe forward reaction is favored (forward shift)
How are concentrations affected?[N2] decreases, [H2] decreases, [NH3] increases
Changes in Volume & Pressure
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Notes:If the number of moles of gas are equal on
both sides, then the equilibrium can’t shiftH2(g) + I2(g) 2HI(g)
Make sure to only count the moles of GAS when considering pressure effects on equilibrium
Changes in Volume & Pressure
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If temperature is increased in the system, what will happen?equilibrium will shift in the endothermic
directionthe endothermic direction is away from the
side with heat
If temperature is decreased in the system, what will happen?equilibrium will shift in the exothermic
directionthe exothermic direction is towards the side
with heat
Changes in Temperature
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For Those Who Prefer Visuals…
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See Saw
H2O H OH
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Add More Hydrogen
H2O
HOH
H
Which way does the see saw have to shift to reach equilibrium again?
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Add More Hydrogen
H2O
HOH
H
Which way does the see saw have to shift to reach equilibrium again?
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Add More Hydrogen
H2O H OH
H
More H2O is produced due to the shift in equilibrium
SHIFT LEFT
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Synthesis of Ammonia
N2 H2 NH3
N2 (g) + 3H2 (g) 2NH3 (g)
H2 H2
NH3
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Increase Pressure
N2H2
NH3
SHIFT will occur to side with the least number of MOLESN2 (g) + 3H2 (g) 2NH3 (g)
H2H2 NH3
SHIFT RIGHT
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Decreasing Volume(Think about how P and V relate with Boyle’s Law)
N2 H2 NH3
N2 (g) + 3H2 (g) 2NH3 (g)
H2 H2
NH3
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Decrease Volume = Increase Pressure
N2H2
NH3
SHIFT will occur to side with the least number of MOLESN2 (g) + 3H2 (g) 2NH3 (g)
H2H2 NH3
SHIFT RIGHT
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Change in TemperatureDepends if reaction is Endothermic or
Exothermic.
Endothermic: Heat on LEFTExothermic: Heat on RIGHT
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Exothermic
Increase Temperature… which way will the see saw tip?
A BDC
HEAT
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Exothermic
Which way will the reaction shift?
AB
DC
HEAT
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Exothermic
Reaction will SHIFT LEFT
AB
DC
HEAT
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Exothermic
DecreaseTemperature… which way will the see saw tip?
A BDC
HEAT
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Exothermic
Which way will the reaction shift?
A
B
DC
HEAT
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Exothermic
Reaction will SHIFT Right
A
B
DC
HEAT
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Endothermic
Increase Temperature… which way will the see saw tip?
A BDC
HEAT
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Endothermic
Which way will the reaction SHIFT?
A B
DC
HEAT
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Endothermic
Reaction will SHIFT Right
A B
DC
HEAT
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Endothermic
Decrease Temperature… which way will the see saw tip?
A BDC
HEAT
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Endothermic
Which way will the reaction SHIFT?
A B
DC
HEAT
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Endothermic
Reaction will SHIFT Left
A B
DC
HEAT
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2NO2(g) N2O4(g) + energy
(Dark Brown) (Clear)
Demo: Changes in Temperature