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Reaction Equilibrium. Unit 17 Daily 2. Key Question:. What is Le Chatelier’s Principle?. Today’s Goals:. Explore factors affecting equilibrium Predict direction of equilibrium shift Predict resulting changes in concentration Make equilibrium serve your own evil purposes. - PowerPoint PPT Presentation
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Unit 17Daily 2
Reaction Equilibrium
What is Le Chatelier’s Principle?
Key Question:
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:
• 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
“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
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
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
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
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
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
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
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 heatIf 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
For Those Who Prefer Visuals…
See Saw
H2O H OH
Add More Hydrogen
H2O
H OH
H
Which way does the see saw have to shift to reach equilibrium again?
Add More Hydrogen
H2O
H OH
H
Which way does the see saw have to shift to reach equilibrium again?
Add More Hydrogen
H2O H OH
H
More H2O is produced due to the shift in equilibrium
SHIFT LEFT
Synthesis of Ammonia
N2 H2 NH3
N2 (g) + 3H2 (g) 2NH3 (g)
H2 H2
NH3
Increase Pressure
N2H2
NH3
SHIFT will occur to side with the least number of MOLESN2 (g) + 3H2 (g) 2NH3 (g)
H2 H2 NH3SHIFT RIGHT
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
Decrease Volume = Increase Pressure
N2H2
NH3
SHIFT will occur to side with the least number of MOLESN2 (g) + 3H2 (g) 2NH3 (g)
H2 H2 NH3SHIFT RIGHT
Change in TemperatureDepends if reaction is Endothermic or
Exothermic.
Endothermic: Heat on LEFTExothermic: Heat on RIGHT
Exothermic
Increase Temperature… which way will the see saw tip?
A B DC
HEAT
Exothermic
Which way will the reaction shift?
AB
DC
HEAT
Exothermic
Reaction will SHIFT LEFT
AB
DC
HEAT
Exothermic
DecreaseTemperature… which way will the see saw tip?
A B DC
HEAT
Exothermic
Which way will the reaction shift?
AB
DC
HEAT
Exothermic
Reaction will SHIFT Right
AB
DC
HEAT
Endothermic
Increase Temperature… which way will the see saw tip?
A B DC
HEAT
Endothermic
Which way will the reaction SHIFT?
A B
DC
HEAT
Endothermic
Reaction will SHIFT Right
A B
DC
HEAT
Endothermic
Decrease Temperature… which way will the see saw tip?
A B DC
HEAT
Endothermic
Which way will the reaction SHIFT?
A B
DC
HEAT
Endothermic
Reaction will SHIFT Left
A B
DC
HEAT
2NO2(g) N2O4(g) + energy (Dark Brown) (Clear)
Demo: Changes in Temperature