14 - 1 Chemical Equilibrium Chemical Equilibrium Introduction to Chemical Introduction to Chemical Equilibrium Equilibrium Equilibrium Constants and Equilibrium Constants and Expressions Expressions Calculations Involving Calculations Involving Equilibrium Constants Equilibrium Constants Using Le Châtelier’s Principle Using Le Châtelier’s Principle Some Important Equilibria Some Important Equilibria
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14 - 1 Chemical Equilibrium Introduction to Chemical Equilibrium Equilibrium Constants and Expressions Calculations Involving Equilibrium Constants Using.
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14 - 1
Chemical EquilibriumChemical Equilibrium
Introduction to Chemical Introduction to Chemical EquilibriumEquilibrium
Equilibrium Constants and Equilibrium Constants and ExpressionsExpressions
Hetrogeneous equilibriaHetrogeneous equilibriaWe don’t include the pure solids and liquids because their concentrations do not vary. Their values end up being included in the K value.
CO2
CaO &CaCO3
As long as the temperature is constant and some solid is present, the amount of solid present has no effect on the equilibrium.
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Writing an equilibrium expressionWriting an equilibrium expression
• Write a balanced equation for the equilibrium.
• Put the products in the numerator and the reactants in the denominator.
• Omit pure solids and liquids from the expression
• Omit solvents if your solutes are dilute (<0.1M).
• The exponent of each concentration should be the same as the coefficient for the species in the equation.
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Writing an equilibrium expressionWriting an equilibrium expression
Example.Example.What would be the equilibrium expression for the following?
(NH4)2CO3 (s) 2 NH3 (g) + CO2 (g) + H2O (g)
Kc = [NH3]2 [CO2] [H2O]
(NH4)2CO3 is a pure solid so is not included Kc.
We use [NH3]2 because the coefficient for NH3(g) in the equation is 2.
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Equilibrium and rate of reactionEquilibrium and rate of reaction
Chemical reactions tend to go to equilibrium providing that reaction takes place at a significant rate.
There is no relationship between the magnitude of the equilibrium constant and the rate of a reaction.
Example.Example. 2H2 (g) + O2 (g) 2H2O (g)
Kc = 2.9 x 1031 =
However, the reaction will take years to reach equilibrium at room temperature.
If you know the initial concentrations of all of the reactants, you only need to measure the concentration of a single species at equilibrium to determine the Kc value.
If the stoichiometry and Kc for a reaction is known, calculating the equilibrium concentrations of all species is possible.
• Commonly, the initial concentrations are known.
• One of the concentrations is expressed as the variable x.
• All others are then expressed in terms of x.
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Equilibrium calculation exampleEquilibrium calculation example
A sample of COCl2 is allowed to decompose. The value of Kc for the equilibrium
COCl2 (g) CO (g) + Cl2 (g)
is 2.2 x 10-10 at 100 oC.
If the initial concentration of COCl2 is 0.095M, what will be the equilibrium concentrations for each of the species involved?
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Equilibrium calculation exampleEquilibrium calculation example
COCl2 (g) CO (g)Cl2 (g)
Initial conc., M 0.095 0.000 0.000
Change in conc. - X + X + Xdue to reaction
EquilibriumConcentration, M (0.095 -X) X X
Kc = =[ CO ] [ Cl2 ]
[ COCl2 ]X2
(0.095 - X)
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Equilibrium calculation exampleEquilibrium calculation example
X2
(0.095 - X)Kc = 2.2 x 10-10 =
Rearrangement gives
X2 + 2.2 x 10-10 X - 2.09 x 10-11 = 0
This is a quadratic equation. Fortunately, thereis a straightforward equation for their solution
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Quadratic equationsQuadratic equations
An equation of the form
a X2 + b X + c = 0
Can be solved by using the following
x =
Only the positive root is meaningful in equilibrium problems.
-b + b2 - 4ac
2a
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Equilibrium calculation exampleEquilibrium calculation example
-b + b2 - 4ac
2a
X2 + 2.2 x 102.2 x 10-10-10 X - 2.09 x 102.09 x 10-11-11 = 0a b b c c
X =
X = - 2.2 x 10-10 + [(2.2 x 10-10)2 - (4)(1)(- 2.09 x 10-11)]1/2
2
X = 9.1 x 10-6 M
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Equilibrium calculation exampleEquilibrium calculation example
Now that we know X, we can solve for the concentration of all of the species.
COCl2 = 0.095 - X = 0.095 M
CO = X = 9.1 x 10-6 M
Cl2 = X = 9.1 x 10-6 M
In this case, the change in the concentration of is COCl2 negligible.
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Summary of method of calculating Summary of method of calculating equilibrium concentrationsequilibrium concentrations
• Write an equation for the equilibrium.
• Write an equilibrium constant expression.
• Express all unknown concentrations in terms of a single variable such as x.
• Substitute the equilibrium concentrations in terms of the single variable in the equilibrium constant expression.
• Solve for x.
• Use the value of x to calculate equilibrium concentrations.
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Predicting shifts in equilibriaPredicting shifts in equilibria
Le Châtelier’s PrincipleLe Châtelier’s PrincipleWhen subjected to a stress, a system in equilibrium will act to relieve the stress.
The position of a chemical equilibrium will shift in a direction to relieve a stress
aA + bB cC + dD
ExampleExampleAdding A or B or removing C or D will shift equilibrium to the right.
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Predicting shifts in equilibriaPredicting shifts in equilibria
Equilibrium concentrations are based on:–The specific equilibrium–The starting concentrations–Other factors such as:
•Temperature•Pressure•Reaction specific conditions
Altering conditions will stress a system, resulting in an equilibrium shift.
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Changes in concentrationChanges in concentration
Changes in concentration do not change the value of the equilibrium constant at constant temperature.
When a material is added to a system in equilibrium, the equilibrium will shift away from that side of the equation.
When a material is removed from a system in equilibrium, the equilibrium will shift towards that sid of the equation.
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Changes in concentrationChanges in concentration
HI
H2
I2
Log
con
cen
trati
on
Time
Example.Example. I2 is added to an equilibrium mixture. The system will adjust all of the concentrations to reestablish a new equilibrium with the same Kc.
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Changes in concentrationChanges in concentration
HI
H2
I2
Log
con
cen
trati
on
Time
Example.Example. Some H2 is removed. Again, the system adjusts all of the concentrations to reestablish a new equilibrium with the same Kc.
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Changes in pressureChanges in pressure
Changing the pressure does not change the value of the equilibrium constant at constant temperature.
• Solids and liquids are not effected by pressure changes.
• Changing pressure by introducing an inert gas will not shift an equilibrium.
• Pressure changes only effect gases that are a portion of an equilibrium.
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Changes in pressureChanges in pressure
In general, increasing the pressure by decreasing volume shifts equilibria towards the side that has the smaller number of moles of gas.
H2 (g) + I2 (g) 2HI (g)
N2O2 (g) 2NO2 (g)
Unaffected by pressureUnaffected by pressure
Increased pressure, shift to leftIncreased pressure, shift to left
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Changes in temperatureChanges in temperature
Changes in temperature usually change the value of the equilibrium constant.
• Kc can either increase or decrease with increasing temperature.
• The direction and degree of change is dependent on the specific reaction.