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Chemical Equilibrium

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Title: Chemical Equilibrium

1
Chemical Equilibrium
2
Equilibrium

A state where the reactants and products remain
constant over time.
For some reactions, the equilibrium position
favors the products and the reaction appears to
have gone to completion (amount of reactants is
negligible)-Equilibrium lies to the right (in
direction of the products)
Example 2 H2 O2 ? 2 H2O
Other reactions only occur to a small extent with
the product virtually undetectable-Equilibrium
lies to the left (in direction of the reactants)
Example 2 CaO ? 2Ca O2

3
Equilibrium is not static

Because the concentrations do not change, it
appears that the reaction has stopped.
Instead, equilibrium is highly dynamic with the
reactions continuing to occur in both directions
at the same rate.

4
As the concentration of the reactants decreases,
the forward reaction slows down. As the
concentration of the products increases, the rate
of the reverse reaction increases Eventually,
the rates become equal.
products
reactants
5
Equilibrium Constants (Application from the lab)

The equilibrium constant is a ratio of the amount
of product to the amount of reactant.
At a given temperature
1) K always has the same value regardless of
the starting concentrations
2) the equilibrium concentrations will not
always be the same
(The set of equilibrium concentrations is called
the equilibrium position.)
There is only one equilibrium constant at a
particular temperature, but there is an infinite
number of equilibrium positions.
The specific equilibrium position depends on the
starting concentrations, but the equilibrium
constant does not.

6
The Extent of a Reaction

The tendency for a reaction to occur is indicated
by the magnitude of the equilibrium constant.
A value of K much larger than 1 means that at
equilibrium the reaction will consist mostly of
product-the equilibrium lies to the right. (The
reaction goes to completion)
A value of K much smaller than 1 means that at
equilibrium the reaction will consist mostly of
reactants-the equilibrium lies to the left. (The
reaction does not occur to any given extent)
The size of K and the time required to reach
equilibrium are not directly related.

7
Equilibrium Constant

Through experimentation and observation, the Law
of Mass Action was proposed.
The law suggests that for a reaction of the
following type
jA kB ? lC mD
the following equilibrium expression is used
to represent the reaction.
K Cl Dm
Aj Bk

K is the equilibrium constant represent the
concentrations at equilibrium Coefficients become
the exponents
8
Le Chateliers Principle

Le Chateliers Principle states that if a change
is imposed on a system at equilibrium, the
position of equilibrium will shift in a direction
that tends to reduce that change.

9
Effect of a Change in Concentration

If a component is added to a reaction system at
equilibrium (at constant T and P), the
equilibrium position will shift in the direction
that lowers the concentration of that component
(away from).
If a component is removed from the system, the
system will shift in the direction that increases
the concentration of that component (towards)

10
Copy the following equation

As4O6(s) 6C(s) lt -- gt As4(g) 6CO (g)

11
In which direction will the equilibrium position
shift if CO is added?

Left
Right
No shift will occur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
12
In which direction will the equilibrium position
shift if C is removed?

Left
Right
No shift will occur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
13
In which direction will the equilibrium position
shift if CO is removed?

Left
Right
No shift will occur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
14
Effect of a Change in Pressure

Adding an inert gas has no effect on the
equilibrium position (has no effect on the
concentrations or partial pressures of the
reactants or products).
Increasing the volume of the container decreases
the pressure. The system responds by increasing
the pressure through the production of more
gaseous molecules (a shift toward the side with
the greatest number of gas molecules).
Decreasing the volume of the container results in
the opposite occurrence.

15
What shift in the equilibrium position will occur
if the volume is reduced for the following
processP4(s) 6Cl2(g) lt -- gt 4PCl3(l)

Left
Right
No shift will occur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
16
What shift in the equilibrium position will occur
if the volume is reduced for the following
processPCl3 (g) Cl2 (g) lt -- gt PCl5 (g)

Left
Right
No shift will occur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
17
What shift in the equilibrium position will occur
if the volume is reduced for the following
processPCl3(g) 3NH3(g) lt -- gt P(NH2)3(g)
3HCl(g)

Left
Right
No shift will occur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
18
Effect of a Change in Temperature

Changing the temperature changes the value of K.
In exothermic reactions (heat is given off and is
therefore written as a product), the equilibrium
position will shift to the left if the
temperature is increased and to the right if the
temperature is decreased.
For endothermic reactions, the opposite will
occur.

19
For the following reaction, predict how the value
of K changes as the temperature is
increased.N2(g) O2(g) lt -- gt 2NO(g) ?H 181
kJ

Increases
Decreases
Remains the same

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
20
For the following reaction, predict how the value
of K changes as the temperature in
increased.2SO2(g) O2(g) lt -- gt 2SO3(g) ?H
-198 kJ