Factors Affecting Equilibrium Chapters 18 When a system is at equilibrium, it will stay that way...
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Transcript of Factors Affecting Equilibrium Chapters 18 When a system is at equilibrium, it will stay that way...
Factors Affecting Equilibrium
Chapters 18
When a system is at equilibrium, it will stay that way until something changes this condition.
Le Chatelier’s Principal
when a change (“stress”) is applied to a system at equilibrium, the system will shift its equilibrium position to counter act the effect of the disturbance.
Factors affecting equilibrium include changes in:
Concentrations of reactants or products
Temperature
Pressure (gases)
Changes in Concentration:Consider this reaction at equilibrium:
H2(g) + I2(g) 2HI(g)
What will happen to the equilibrium if we:add some H2?
Reaction shifts to the right
(forms more product)
Changes in Concentration:Consider this reaction at equilibrium:
H2(g) + I2(g) 2HI(g)
What will happen to the equilibrium if we:remove some H2?
Reaction shifts to the left
(forms more reactants)
Changes in Concentration:When a substance is added, the stress is relieved by shifting equilibrium in the direction that consumes some of the added substance.
When a substance is removed, the reaction that produces that substance occurs to a greater extent.
Changes in Temperature:Consider this reaction at equilibrium:
2SO2(g) + O2(g) 2SO3(g) + 198 kJ
What will happen to the equilibrium if we:increase the temperature?
Reaction shifts to the left
(forms more reactants)
Changes in Temperature:Consider this reaction at equilibrium:
2SO2(g) + O2(g) 2SO3(g) + 198 kJ
What will happen to the equilibrium if we:decrease the temperature?
Reaction shifts to the right
(forms more products)
Changes in Temperature:
Increasing the temperature always favors the reaction that consumes heat, and vice versa.
Changes in Pressure:Consider this reaction at equilibrium:
2NO2(g) N2O4(g)
What will happen to the equilibrium if we:increase the pressure?
Reaction shifts to the right
(forms more product)
Changes in Pressure:Consider this reaction at equilibrium:
2NO2(g) N2O4(g)
What will happen to the equilibrium if we:decrease the pressure?
Reaction shifts to the left
(forms more reactant)
Changes in Pressure:
Increasing the pressure favors the reaction that produces the fewer moles of gas, and vice-versa.
Example: consider the rxn at equilibrium: N2(g) + 3H2(g) 2NH3(g) + 94 kJ
How would the equilibrium be influenced by: Increasing the temp:Decreasing the temp: Increasing the pressure:Decreasing the pressure:Adding more H2:
Removing some NH3:Adding a catalyst:
rxn shifts to the left
rxn shifts to the right
rxn shifts to the left
rxn shifts to the right
rxn shifts to the right
rxn shifts to the right
no change in equilibrium position
Catalysts
A catalyst increases the rate at which equilibrium is reached, but it does not change the composition of the equilibrium mixture.
Action of a Catalyst
Activation Energy
Without a catalyst
Action of a Catalyst
Lower Activation Energy
With a catalyst: A catalyst lowers the activation energy.
Example: How will an increase in pressure affect the equilibrium in the following reactions:
4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)
RXN SHIFTS LEFT
2H2(g) + O2(g) 2H2O(g)
RXN SHIFTS RIGHT
Example: How will an increase in temperature affect the equilibrium in the following reactions:
2NO2(g) N2O4(g) + heatRXN SHIFTS LEFT
H2(g) + Cl2(g) 2HCl(g) + 92 KJRXN SHIFTS LEFT
H2(g) + I2(g) + 25 kJ 2HI(g)
RXN SHIFTS RIGHT
How does this affect K??Concentration stress– no change
Temperature stress– K will go up or down depending on the shift
Pressure stress-- K will go up or down depending on the shift
SHIFT TOWARDS PRODUCT– K gets BIGGER SHIFT TOWARD REACTANT– K get SMALLER
To close, let’s talk about Fritz Haber and the “process” that made him famous…
Let’s say you want to manufacture ammonia gas (NH3). What are the optimum conditions?N2 + 3H2 2 NH3 + Heat
We wish to favor the forward reaction, thus producing more NH3 gas.
For example
N2 + 3H2 2 NH3 + Heat
By cooling the reaction, the reactions counters by producing heat. It does this by shifting to the right, producing heat, and more NH3 gas.
For example
N2 + 3H2 2 NH3 + Heat
By increasing the pressure, the reaction tries to reduce the pressure. It does this by shifting to the side with the fewest moles of gas. This is the product side with 2 moles of gas. Thus the reaction shifts to the right reducing pressure, and producing more NH3 gas.
4 moles of gas 2 moles of gas
For example
N2 + 3H2 2 NH3 + Heat
By adding additional N2 and H2, the reactions tries to use them up. In doing so, the reaction shifts to the right.
By removing NH3 as soon as its formed, the reactions tries to produce more. Shifting the reaction to the right.
For example
N2 + 3H2 2 NH3 + Heat
So if you want to produce maximum ammonia gas you should
Cool the reaction
Conduct the reaction under high pressure
Add N2 and H2
Remove NH3
con
cen
trati
on
time
H2 added here NH3 removed here
equilibrium equilibrium equilibrium
Example: N2(g) + 3H2(g) 2NH3(g) ∆H=-93 kJ mol-1
NH3
N2
H2