Chapter 15 Chemical Equilibrium

1
Chapter 15Chemical Equilibrium
2
For the gas-phase reaction A B the
forward reaction rate is 3.0 ? 10?4 s?1 and the
reverse reaction rate is 1.5 ? 10?2 s?1. What is
the value of the equilibrium constant, Keq?

• 0.02
• 50
• 0.0004
• 2500

3
Correct Answer

• 0.02
• 50
• 0.0004
• 2500

4

• Homogenous
• Heterogeneous

5
Correct Answer

• Homogenous
• Heterogeneous

This is definitely a heterogeneous equilibrium
because the reactants and products exist in two
different phases, the solid and gaseous states.
6
Which of the following is the correct Keq for the
equilibrium equation below? 2 Ag(s) Zn2(aq)
2 Ag(aq) Zn(s)
7
Correct Answer
2AgZn

• Keq ___________
• Keq ___________
• Keq ___________

2AgZn2
Zn2

• 4. Keq _________
• 5. Keq _________

2Ag
Ag2Zn
Zn2
Ag2Zn2
Ag2
Ag2
Zn2
8
Correct Answer (cont.)
Remember the equilibrium constant expression is
given in terms of concentrations of products
(raised to the exponent of their coefficients)
divided by concentrations of reactants (raised to
the exponent of their coefficients). Pure
solids, liquids, and solvents are not included in
the expression.
9
For the following hypothetical reaction 2 Y(aq)
3 Z(aq) A(aq) Calculate Keq given the
following equilibrium concentrations A 0.50
M, Y 0.10 M, Z 1.0 M

• 2.5
• 5.0
• 0.40

• 0.025
• 50.

10
Correct Answer

• 2.5
• 5.0
• 0.40
• 0.025
• 50.

0.50
K





eq
3
2
1.0
0.10
11
For the following hypothetical reaction 2 X(g)
Y(g) A(g) Keq 250. At a point during
the reaction, the concentrations are A 1.0 M,
X 0.50 M, and Y 0.10 M. How will the
reaction proceed to achieve equilibrium?

• From left to right (toward products)
• From right to left (toward reactants)
• Already at equilibrium

12
Correct Answer

• From left to right (toward products)
• From right to left (toward reactants)
• Already at equilibrium

1.0

K




c
2
0.10
0.50
Because Kc lt Keq, the reaction will shift from
reactants toward products to achieve equilibrium.
13
For the following hypothetical reaction Y(g)
Z(g) 2 A(g) Keq 4.0 ? 10?2. Given that,
what is the value of Keq for the reaction 4 A(g)
2 Y(g) 2 Z(g)

• 25
• 5.0

• 0.016
• 6.3 ? 102

14
Correct Answer

• 25
• 5.0
• 0.016
• 6.3 ? 102

The second equation is twice the first and
reversed. Thus, Keq of the second equation is
related to the first as shown below
15
The following reaction is at equilibrium N2(g)
3 H2(g) 2 NH3(g) If we remove NH3(g), in what
direction will the reaction move to reestablish
equilibrium?

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

16
Correct Answer

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

If NH3 is decreased, to return to equilibrium,
more NH3 must be produced from the reactants.
Thus, the reaction shifts from the reactants to
the products.
17
The following reaction is at equilibrium N2(g)
3 H2(g) 2 NH3(g) If we increase the volume
while holding the temperature constant, in what
direction will the reaction move to reestablish
equilibrium?

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

18
Correct Answer

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

Increasing the volume causes a shift in the
equilibrium in the direction that produces more
gas molecules, in this case on the reactants side.
19
The following reaction is at equilibrium N2(g)
O2(g) 2 NO(g) If we increase the pressure,
in what direction will the reaction move to
reestablish equilibrium?

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

20
Correct Answer

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

The number of gas molecules on both sides of the
equation are equal thus, changing the pressure
will not change the equilibrium position.
21
The following reaction is at equilibrium N2(g)
O2(g) 2 NO(g) DH 180.8 kJ. In what
direction will the reaction move to reestablish
equilibrium if the temperature is decreased?

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

22
Correct Answer

• From left to right (toward products)
• From right to left (toward reactants)
• No change in equilibrium

As the temperature is lowered, equilibrium will
shift to the side of the equation that produces
heat. Thus, for an endothermic reaction, the
equilibrium shifts from right to left toward the
reactants.