Transcript Notes

Equilibrium Systems and
Stress
Calculating Equilibrium Constants
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Equilibrium Constant (Keq) is the ratio
of the molar concentrations of products
to reactants when at equilibrium.
Keq = [products]
[reactants]
[ ] = concentration - molarity (M)
Example Problem #1
Find the equilibrium constant for the following reaction
if the concentrations at 25°C are [HI]=0.00998M,
[H2]=0.000867M, and [I2]=0.00264M
H2 + I2
2HI
Keq = [products]
[reactants]
Keq=
[0.00998M]2
= 43.5
[0.000867M][0.00264M]
Le Chatlelier’s Principle
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Stress: something that causes a
change in a system at equilibrium
However, the system will adjust to this
new stress and come back into
equilibrium again
This is LE CHATLELIER’S
PRINCIPLE
Le Chatelier’s Principle
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Chemical equilibria responds to three
kinds of stress:
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Changes in the concentration of
reactants or products
Changes in temperature
Changes in pressure
Le Chatelier’s Principle
1. Changes in concentration of
reactants and products
 This is a model of the system at
equilibrium.
Changes in concentration of
reactants and products

If more reactants are added to the
system, the equilibrium changes. The
system will respond by making more
product.
Stress is being added to the system here
Changes in concentration of
reactants and products
1 CO(g) + 2H2(g) ↔
System at equilibrium:
CH3OH(g)
System with added reactant CO:
To reach equilibrium again, the system will make more
products:
Changes in concentration of
reactants and products
1 CO(g) + 2H2(g) ↔ CH3OH(g)
System at equilibrium:
System with added product (CH3OH)
To reach equilibrium, the system will make more reactants
The reaction will shift to the left (reverse reaction)
Effects of Temperature
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Temperature effects equilibrium the
same way as concentration changes
Remember…

Exothermic Reactions – reactions that
produce (or give off) heat
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Heat is a product
Endothermic Reactions – reactions that
absorb (or use) heat

Heat is a reactant
Effects of Temperature
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For exothermic forward reactions:
 Increasing the temperature of an
equilibrium system usually leads to
a shift in favor of the reactants
For endothermic forward reactions:
 Increasing the temperature usually
leads to a shift in favor of the
products
Effects of Temperature
N2(g) + O2(g) ↔ 2NO(g)
System at Equilibrium
System with added heat on the product side
To reach equilibrium, the system will make more reactants
The reaction favors the formation of reactants (reverse rxn)
Effects of Pressure
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For solutions, pressure has almost no
effect on the equilibrium
Gases are greatly effected by changes
in pressure
An increase in pressure causes the
system to shift its equilibrium position
to REDUCE THE PRESSURE
To reduce pressure, the system needs to
reduce the number of gas particles
Effect of Pressure
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Rule: At constant temperature,
increasing the pressure on a gas
causes the system to shift in the direction
that reduces the number of moles of
gas
Effects of Pressure

Example:
N2O4(g) ↔
2NO2(g)
How many moles of gas are on the reactant side?_____
How many moles of gas are on the product side?_____
Which side has the greater amount of moles?________
If adding pressure to this system causes the system to
respond by reducing the number of moles of gas,
which way will the reaction go?
Real Life Applications
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The “Bends”
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Nitrogen and other gases are dissolved in
our blood
nitrogen(g) ↔ nitrogen(dissolved)
As the diver comes up from the high
pressures of the ocean, the equilibrium shifts
to the left
If they come up too fast, nitrogen bubbles
form in the blood and other body fluids
cause severe abdominal pain and maybe
death
Chemical Equilibrium
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Chemical Equilibrium
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When the rates of the forward and
reverse reactions are equal in a chemical
reaction
The concentration (amount) of
reactants and products of the reaction
remains the same
Ex:
2CO(g) + O2(g) ↔ 2O2(g)
Chemical Equilibrium
Le Chatelier’s Principle
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This is LE CHATELIER’S PRINCIPLE

When a system at equilibrium is
disturbed, the system adjusts in a way
to reduce the change