Download T8 Equilibrium

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© 2009, Prentice-Hall, Inc.
The Concept of Equilibrium
Chemical equilibrium occurs when a
reaction and its reverse reaction proceed at
the same rate.
© 2009, Prentice-Hall, Inc.
The Concept of Equilibrium
• As
a
system
approaches equilibrium,
both the forward and
reverse reactions are
occurring.
• At
equilibrium,
the
forward and reverse
reactions
are
proceeding at the same
rate.
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A System at Equilibrium
Once equilibrium is
achieved, the
amount of each
reactant and
product remains
constant.
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The Equilibrium Constant
Since, in a system at equilibrium, both the
forward and reverse reactions are being
carried out:
N2O4 (g)
2 NO2 (g)
The ratio of the rate constants is a constant at
that temperature, and the expression becomes
kf
Keq =
kr
[NO2]2
=
[N2O4]
© 2009, Prentice-Hall, Inc.
Summary
1. At equilibrium, the concentrations of
reactants and products no longer change
with time.
2. For equilibrium to occur, neither reactants
nor products can escape from the system.
3. At equilibrium a particular ratio
concentration terms equals a constant.
of
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The Equilibrium Constant
• Consider the generalized reaction
aA + bB
cC + dD
• The equilibrium expression for this
reaction would be
[C]c[D]d
Kc =
[A]a[B]b
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Sample Exercise
Write the equilibrium expression for Keq
for these three reactions:
a) 2 O3(g) D 3 O2(g)
b) 2 NO(g) + Cl2(g) D 2 NOCl(g)
c) Ag+(aq) + 2 NH3(g) D Ag(NH3)2+(aq)
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What Does the Value of K Mean?
• If K>>1, the reaction is
product-favored;
product predominates
at equilibrium.
• If K<<1, the reaction is
reactant-favored;
reactant predominates
at equilibrium.
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Heterogeneous Equilibria
• Equilibria in which all reactants and
products are present in the same phase
are called homogeneous equilibria.
• Equilibria in which one or more
reactants or products are present in a
different phase are called
heterogeneous equilibria.
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Le Châtelier’s Principle
“If a system at equilibrium is disturbed
by a change in temperature, pressure,
or the concentration of one of the
components, the system will shift its
equilibrium position so as to counteract
the effect of the disturbance.”
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What Happens When More of a
Reactant Is Added to a System?
Effects of Pressure Changes
Le Châtelier’s principle predicts that if
pressure is increased:
• The system shifts to remove gases and
decrease pressure.
• An increase in pressure favors the direction
that has fewer moles of gas.
In a reaction with the same number of moles of gas in
the products and reactants, changing the pressure has
no effect on the equilibrium.
© 2009, Prentice-Hall, Inc.
The Effect of Changes in Temperature
Co(H2O)62+(aq) + 4Cl–(aq)+ heat D CoCl42–(aq) + 6H2O(l) DH > 0
Pale pink
Deep blue
When the solution is heated it turns blue, indicating that
the equilibrium has shifted to form more
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Catalysts
Catalysts increase
the rate of both the
forward and reverse
reactions.
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Exercise 9 Using Le Châtelier’s Principle to Predict Shifts in Equilibrium
Consider the equilibrium
In which direction will the equilibrium shift when (a) N2O4 is added, (b)
NO2 is removed, (c) the pressure is increased by addition of N2(g), (d) the
volume is increased, (e) the temperature is decreased?
Solution
a) Equilbrium shifts to the right
b) Shifting to the side that produces more NO2 (right)
c) Total pressure is increased but N2 is not involved in the
reaction. Therefore equilibrium does not change
d) System will shift in the direction that occupies a larger volume
(more gas molecules), thus, shifting to the right.
e) Shifting in the direction that produces heat (left)