lechatelier’s principle: pressure & temperature
DESCRIPTION
LeChatelier’s Principle: Pressure & Temperature. LeChatelier’s Principle At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction. Systems under stress shift to relieve the stress and restore equilibrium . Causes of stress = change of: Concentration - PowerPoint PPT PresentationTRANSCRIPT
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LeChatelier’s Principle: Pressure & Temperature
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LeChatelier’s Principle• At equilibrium, the rate of the forward
reaction is equal to the rate of the reverse reaction.
• Systems under stress shift to relieve the stress and restore equilibrium.
• Causes of stress = change of:• Concentration• Temperature• Pressure (gases only)
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PressureGases onlyIncrease pressure = Decrease volume
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Pressure and Equilibrium:1. Increase in pressure favors the direction
that produces a smaller number of moles of gas (less volume).
A + B AB• Reactants = 1 mol A + 1 mol B• Products = 1 mol AB• 2 moles of gas react to form 1 mole of gas• Increasing pressure will drive reaction to the
right, favoring product formation because it has the smaller # of moles.
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Animation
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2. Decrease in pressure favors the direction that produces a larger number of moles (larger volume).
N2 + 3H2 2NH3
• Reactants = 4 moles• Products = 2 moles
• Decrease in pressure will favor the reverse reaction since there are 4 moles of reactants and 2 moles of products.
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Decreased Pressure
Equilibrium shifts left.Reactants are favored.
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3. Change in pressure has no effect on equilibrium if there are an equal number of moles of reactants and products.
Reactants = I mol H2 + 1 mol I2 = 2 molProducts = 2 mol HI
Since both the reactants and products have the same volume, no shift in reaction will occur if the pressure is changed. *Same # of particles on both sides.
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4. Change in pressure does not change the equilibrium constant – KEQ
The position of equilibrium may be changed by pressure, but the equilibrium constant will not be changed.
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Temperature and Equilibrium• Increasing the temperature causes more
frequent, and more intense collisions between particles, causing faster reaction rates.
• Increasing temperature favors the endothermic reaction.
**Change in temperature does change the equilibrium constant (KEQ)
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Forward Reaction = ExothermicSmall Activation Energy
Reverse Reaction = EndothermicLarger Activation Energy
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H2 + Br2 2HBr2 + heat
Endothermic (absorbs heat)
Increase temperature, endothermic reaction increases, reactant concentration increases.
Exothermic (gives off heat)
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Increase temperature:
CO + 3H2 CH4 + H2O + 206.5KJ
• Reaction will shift to the left• Concentrations of CO and H2 increase
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Increase temperature:
N2O2 + heat 2NO
• Reaction will shift to the right• Concentration of NO will increase
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Haber ProcessN2 + 3H2 2NH3 + 92KJ
How can more NH3 be produced?1. Increase concentration of N2 or H2
2. Remove NH3 from container3. Increase pressure4. Temperature – high enough to speed up
reaction while keeping KEQ high5. Add a catalyst to reach equilibrium quickly
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Learning Check (all gases)CO + Cl2 COCl2 + heat
1. Add Cl22. Remove CO3. Add COCl24. Increase pressure5. Increase temperature
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Learning Check (all gases)H2O + CO H2 + CO2
1. Add H2
2. Remove CO3. Add H2O
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Learning Check (all gases)N2O2 + heat 2NO2
1. Increase pressure2. Increase temperature