Problems du Jour
Write equilibrium-constant expressions for the following
reactions:
3NO(g) = N2O(g) + NO2(g)
CH4(g) + 2H2S(g) = CS2(g) + 4H2(g)
HF(aq) = H+(aq) + F-(aq)
Kp for the reaction
2Cl2(g) + 2H2O(g) = 4HCl(g) + O2(g)
is 0.0752 at 480 oC.
What is the value of Kp for:
(a) 4HCl(g) + O2(g) = 2Cl2(g) + 2H2O(g)
(b) Cl2(g) + H2O(g) = 2HCl(g) +
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1
O2(g)
2
Heterogeneous equilibria
Thus far, the reactions we've studied have involved
substances all in the same phase, e.g.,
N2(g) + 3H2(g) = 2NH3(g)
This is an example of homogeneous equilibrium
What if the substances in equilibrium are in different
phases, e.g.,
PbCl2(s) = Pb2+(aq) + Cl2(aq)
This is an example of heterogeneous equilibrium
Write the expression for Kc: What about the concentration
of PbCl2(s)?
The effective concentration of a pure solid/liquid is
approximately constant, regardless of how much of the
pure solid/liquid is present
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Incorporate [pure solid] into Kc to obtain
Kc = [Pb2+][Cl-]2
Note, however, that pure solids & liquids must be present
for such equilibrium to be established!
e.g., write equilibrium constant expressions for the
following:
NiCO4(g) = Ni(s) + 4CO(g)
2Ag(s) + Zn2+(aq) = 2Ag+(aq) + Zn(s)
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Evaluation of equilibrium constants
Want to evaluate K from a knowledge of:
equilibrium concentration of reactants &
products
OR
initial & equilibrium concentration of at least one
substance
In general:
tabulate known initial concentrations
tabulate known equilibrium concentrations
tabulate concentration changes, i.e., differences between
equilibrium/initial concentrations
use reaction stoichiometry to tabulate concentrations
changes for other species
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E.g., at 500 K the following equilibrium is established:
2NO(g) + Cl2(g) = 2NOCl(g)
An equilibrium mixture of the three gases is found to have
partial pressures of 0.095 atm, 0.171 atm, and 0.28 atm for
NO, Cl2, and NOCl. Calculate Kp for this reaction at 500 K.
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E.g., a mixture of 0.100 mol NO, 0.050 mol H2, and 0.100
mol H2O is placed in a 1.00-L flask at 300 K. The following
equilibrium is established:
2NO(g) + 2H2(g) = N2(g) + 2H2O(g)
At equilibrium, [NO] = 0.062 M. Find the equilibrium
concentrations of H2, N2, and H2O. Calculate Kc.
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In the above problem, we were given initial and one
equilibrium concentration, and we found Kc....
Try this one:
At 2000oC, Kp for the reaction
2NO(g) = N2(g) + O2(g)
is 2.4x103. If the initial partial pressure of NO is 37.3 atm,
what are the equilibrium partial pressures of NO, N2, and
O2?
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Other Applications of Equilibrium Constants
Recall:
K>>1: reaction goes to right (products favored)
K<<1: reaction goes to left (reactants favored)
Use Kp to predict direction in which a reaction mixture will
move to reach equilibrium
e.g., for the Haber process
N2(g) + 3H2(g) = 2NH3(g)
Kp=4.51x10-5 at 450oC. Suppose that 495 atm H2, 35 atm N2,
and 105 atm NH3 are placed in a container at 450oC.
Is the mixture at equilibrium? If not, indicate the direction
(towards products or reactants) in which the system must
shift to reach equilibrium.
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Substitute the concentrations given into the expression
for the equilibrium constant;
The result is the reaction quotient, Q, e.g.,
compare Q and K:
if Q > K, reaction moves 
if Q = K, reaction at equilibrium
if Q < K, reaction moves 
Remember; Q=K ONLY at equilibrium!
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Factors affecting chemical equilibrium
What factors can shift a chemical equilibrium?
Add/remove product/reactant
Change pressure
Change temperature
How does a system at equilibrium respond to these
changes?
Le Châtelier's Principle:
If a system at equilibrium is disturbed by a change in
pressure, temperature, 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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Cases:
1. Change in reactant/product concentration
Recall that equilibrium is dynamic
How does an equilibrium system respond
to addition of reactant/product?
Reaction shifts to reestablish
equilibrium by consuming
part of the added substance
How does an equilibrium system respond
to removal of reactant/product?
Why is this so?
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2. Effects of pressure/volume changes
If a system is at equilibrium and the total pressure is
increased by a decrease in volume, the system will
respond by a shift in equilibrium in the direction which will
decrease the pressure
Since P  n, equilibrium shifts to reduce
the number of moles of gas
conversely: decrease P by increasing V
causes a shift towards more gas
molecules
e.g., for the Haber process
N2(g) + 3H2(g) = 2NH3(g)
which way does the equilibrium shift if:
N2(g) is added to an equilibrium mixture?
NH3(g) is removed?
the volume is decreased?
pressure is decreased?
some Ar is added to the equil. mixture?
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3. Effect of temperature changes
Note that changes in concentration or P shift position of
equilibrium without changing the value of K
However, K is temperature-dependent; how does
equilibrium respond to changes in T?
treat heat as a chemical entity, i.e.,
endothermic reaction: heat is a reactant
exothermic reaction: heat is a product
equilibrium shifts in the direction that absorbs heat:
endothermic: add heat, shift 
exothermic: add heat, shift 
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Problems du Jour
For the following reaction, Ho = 2816 kJ:
6CO2(g) + 6H2O(l) = C6H12O6(s) + 6O2(g)
How is the equilibrium yield of C6H12O6 affected by:
Increasing PCO2
Increasing temperature
Removing CO2
Decreasing the total pressure
Removing part of the C6H12O6
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Problem du Jour
At 80oC, Kc = 1.2 x 10-4 for the equilibrium
NH4HS(s) = NH3(g) + H2S(g)
Calculate the equilibrium concentrations of NH3 and H2S if
a sample of solid NH4HS is placed in a closed vessel at
80oC and allowed to decompose until equilibrium is
established.
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Problem du Jour
For the equilibrium
Br2(g) + Cl2(g) =2BrCl(g)
At 400 K, Kc = 7.0. If 0.25 mol Br2 and 0.25 mol Cl2 are
introduced in to a 1.0 L container at 400 K, what will be the
equilibrium concentrations of Br2, Cl2, and BrCl?
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