Le Chatelier’s principle and more
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Transcript of Le Chatelier’s principle and more
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Le Chatelier’s principle and more...7.2.3-7.2.5
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But first, a review of Topic 7.1
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Feature of Equilibrium State ExplanationEquilibrium is dynamic The reaction has not stopped
but the forward and reverse reaction are still occurring
Equilibrium is achieved in a closed system
A closed system prevents exchange of matter with the surroundings
The concentrations of reactants and products remain constant
They are being produced and degraded at an equal rate
The macroscopic properties do not change
Color and density are properties that depend upon concentrations
Equilibrium can be reached from either direction
The same equilibrium mixture will result under the same conditions, regardless of the starting point.
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Catalyst
• the same process still has to happen, catalysts just help out by lowering the activation energy
• increase the RATE of a reaction….and therefore the decrease the time in which equilibrium is reached – they speed up the forward and reverse reactions
equally• therefore decreases the time required for the system to
achieve equilibrium• less time equals $$$ when making chemicals
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Kc = [C]c[D]d
[A]a[B]b
aA + bB cC + dD
• Write the equilibrium expression for the following reaction:
• 4NH3(g) + 7O2(g) 4NO2(g) + 6H2O(g)
[NO2]4[H2O]6
[NH3]4 [O2]7Kc =
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Le Chatellier’s principle and more...
– Nice video- 20 minutes– Another good one- 15 minutes– states when a system in chemical equilibrium is
disturbed by a change, the system shifts in a way that tends to counteract this change of variable
– a change imposed on an equilibrium system is called a stress• a stress usually involves a change in the
temperature, pressure, or concentration• the equilibrium always responds in such a way so
as to counteract the stress
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Stress 1. Temperature change
• this is the ONLY STRESS THAT WOULD ACTUALLY CHANGE KC
• increasing temperature– favors the “cold side”/endothermic/the reaction that
needs heat– adding heat favors the reaction to the left since it needs
+92 kJ– Kc decreases
DH = + 92 kJ N2 (g) + 3H2 (g) 2NH3 (g) DH = - 92 kJ
Haber Process again
McGraw Hill Flash animation
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• decreasing temperature– favors the “hot side”/exothermic• the reaction is already giving off heat it
doesn’t need so cooling down is good• Kc increases
DH = + 92 kJ N2 (g) + 3H2 (g) 2NH3 (g) DH = - 92 kJ
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Stress 2. Pressure change
• an increase in pressure causes the equilibrium to shift in the direction that has the fewer number of moles– results in a decrease in N2 and H2 and an increase in NH3
• an decrease in pressure causes the equilibrium to shift in the direction that has the most number of moles– results in a an increase in N2 and H2 and an decrease in NH3
• does NOT affect the equilibrium constant Kc
N2 (g) + 3H2 (g) 2NH3 (g) DH = - 92 kJ
Haber Process again
McGraw Hill Flash animation
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Stress 3. Concentration change
• the equilibrium responds in such a way so as to diminish the increase or equalize the ratio– increasing concentration of reactants shifts the reaction
to the right (forward, more product)– increasing concentration of products shifts the reaction
to the left (reverse, more reactants)• does NOT affect the equilibrium constant Kc
N2 (g) + 3H2 (g) 2NH3 (g) DH = - 92 kJ
Haber Process again
Kc =[NH3]2
[N2] [H2]3
McGraw Hill Flash animation
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Practice Problem• Predict the effect of the following changes on the
reaction in which SO3 decomposes to form SO2 and O2. 2 SO3(g) 2 SO2 (g) + O2 (g) Ho = 197.78 kJ
• increasing the temperature of the reaction – shifts right
• increasing the pressure on the reaction– shifts left
• adding more O2 when the reaction is at equilibrium – shifts left
• removing O2 from the system when the reaction is at equilibrium– shifts right
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Le Chatelier’s Principle – Summary
Change Effect on Equilibrium Change in Kc?
Increase concentration
Shifts to opposite side No
Decrease concentration
Shifts to same side No
Increase pressure Shifts to side with least moles of gas
No
Decrease pressure Shifts to side with most moles of gas
No
Increase temperature
Shifts in endothermic direction
Yes
Decrease temperature
Shifts in exothermic direction
Yes
Add a catalyst No change No
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Le Châtelier’s Principle
• Changes in Concentration
Change Shifts the Equilibrium
Increase concentration of product(s) left
Decrease concentration of product(s) right
Decrease concentration of reactant(s)
Increase concentration of reactant(s) right
left
aA + bB cC + dD
AddAddRemove Remove
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If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.
Le Châtelier’s Principle
• Changes in Concentration
N2 (g) + 3H2 (g) 2NH3 (g)
AddNH3
14.5
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Le Châtelier’s Principle
• Changes in Volume and Pressure(Only a factor with gases)
A (g) + B (g) C (g)
Change Shifts the Equilibrium
Increase pressure Side with fewest moles of gas
Decrease pressure Side with most moles of gas
Decrease volume
Increase volume Side with most moles of gas
Side with fewest moles of gas
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• For each of the following reactions, predict how the equilibrium will shift as the temperature is increase
N2(g) + O2(g) 2NO(g) ∆H = +kJ mol-1
2SO2(g) + O2(g) 2SO3(g) ∆H = -kJ mol-1
Right
left
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Effect of: Position of Equilibrium
Value of Kc
Concentration Changes No changePressure Changes if
reaction involves a change in the number of gas molecules
No change
Temperature Changes Changes (depends on if exothermic or endothermic)
Catalyst No change No change