Transcript Document

Chapter 13
Chemical Equilibrium
Section 13.6
Solving Equilibrium Problems
Solving Equilibrium Problems
1) Write the balanced equation for the reaction.
2) Write the equilibrium expression using the law of mass
action.
3) List the initial concentrations.
4) Calculate Q, and determine the direction of the shift to
equilibrium.
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Section 13.6
Solving Equilibrium Problems
Solving Equilibrium Problems
5) Define the change needed to reach equilibrium, and
define the equilibrium concentrations by applying the
change to the initial concentrations.
6) Substitute the equilibrium concentrations into the
equilibrium expression, and solve for the unknown.
7) Check your calculated equilibrium concentrations by
making sure they give the correct value of K.
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Section 13.6
Solving Equilibrium Problems
EXERCISE!
Consider the reaction represented by the equation:
Fe3+(aq) + SCN-(aq)
FeSCN2+(aq)
Fe3+
Trial #1 9.00 M
Trial #2 3.00 M
Trial #3 2.00 M
SCN5.00 M
2.00 M
9.00 M
FeSCN2+
1.00 M
5.00 M
6.00 M
Find the equilibrium concentrations for all species.
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Section 13.6
Solving Equilibrium Problems
EXERCISE!
Answer
Trial #1: [Fe3+] = 6.00 M; [SCN-] = 2.00 M; [FeSCN2+] = 4.00 M
Trial #2: [Fe3+] = 4.00 M; [SCN-] = 3.00 M; [FeSCN2+] = 4.00 M
Trial #3: [Fe3+] = 2.00 M; [SCN-] = 9.00 M; [FeSCN2+] = 6.00 M
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Section 13.6
Solving Equilibrium Problems
CONCEPT CHECK!
A 2.0 mol sample of ammonia is introduced into a
1.00 L container. At a certain temperature, the ammonia
partially dissociates according to the equation:
NH3(g)
N2(g) + H2(g)
At equilibrium 1.00 mol of ammonia remains.
Calculate the value for K.
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Section 13.6
Solving Equilibrium Problems
CONCEPT CHECK!
A 1.00 mol sample of N2O4(g) is placed in a 10.0 L vessel and
allowed to reach equilibrium according to the equation:
N2O4(g)
2NO2(g)
K = 4.00 × 10-4
Calculate the equilibrium concentrations of: N2O4(g) and
NO2(g).
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Section 13.7
Le Châtelier’s Principle
 If a change is imposed on a system at equilibrium, the
position of the equilibrium will shift in a direction that
tends to reduce that change.
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Section 13.7
Le Châtelier’s Principle
Effects of Changes on the System
1. Concentration: The system will shift away from the
added component. If a component is removed, the
opposite effect occurs.
2. Temperature: K will change depending upon the
temperature (endothermic – energy is a reactant;
exothermic – energy is a product).
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Section 13.7
Le Châtelier’s Principle
Effects of Changes on the System
3. Pressure:
a) The system will shift away from the added gaseous
component. If a component is removed, the opposite
effect occurs.
b) Addition of inert gas does not affect the equilibrium
position.
c) Decreasing the volume shifts the equilibrium toward
the side with fewer moles of gas.
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Section 13.7
Le Châtelier’s Principle
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Section 13.7
Le Châtelier’s Principle
Equilibrium Decomposition of N2O4
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