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Honors Chemistry
Name ________________________________
Chemical Equilibrium Questions and Problems
Date _____/_____/_____
1.
Period _____
Write the expressions for the equilibrium constant Kc and indicate if the reactions are homogeneous
equilibriums or heterogeneous equilibriums for the following reactions:
a.
4 NH3(g) + 7 O2(g)
4 NO2(g) + 6 H2O()
Kc = [NO2]4 / [NH3]4 [O2]7
b.
HCN (aq) + H2O()
H3O+1(aq) + CN-1(aq)
Kc = [H3O+1] [CN-1] / [HCN] [H2O]
c.
PCl5(g) +
PCl3(g) + Cl2(g)
Kc = [PCl3] [Cl2] / [PCl5]
d.
CaCO3(s)
CaO(s) + CO2(g)
Kc = [CO2(g)]
e.
3 O2(g)
2 O3(g)
Kc = [O3(g)]2 / [O2(g)]3
f.
2 H2O()
H3O+(aq) + OH-1(aq)
Kc = [H3O+1(aq)] [OH-1(aq)]
g.
3 Zn(s) + 2 Fe+3(aq)
2 Fe(s) + 3 Zn+2(aq)
Kc = [Zn+2(aq)]3 / [Fe+3(aq)]2
2.
Write the equilibrium constant expressions for the following reactions. How are they related to one
another?
a.
2 N2O(g) + 3 O2(g)
4 NO2(g)
3
b.
N2O(g) + /2 O2(g)
2 NO2(g)
c.
4 NO2(g)
2 N2O(g) + 3 O2(g)
a & b have the same equilibrium expression:
Kc =
[NO2(g) ]4
[N2O(g) ]2 [O2(g) ]3
c has the equilibrium expression that the inverse of the above expression as it is the reverse of
reactions a & b, or
Kc =
[N2O(g) ]2 [O2(g) ]3
[NO2(g) ]4
3.
Calculate the value of the equilibrium constant for the following system, given the data shown:
H2(g) + CO2(g)
H2O(g) + CO(g)
Concentrations at equilibrium:
[H2]
=
1.5 mol Liter-1
[CO2]
=
2.5 mol Liter-1
Kc = 0.4
[H2O]
=
0.5 mol Liter-1
[CO]
=
3.0 mol Liter-1
4.
The following reaction is exothermic: Ti(s) + 2 Cl2(g)
TiCl4(g) + energy(exothermic)
List all the ways the yield of the product TiCl4 could be increased.
1) Lower the temperature
2) Increase the pressure
3) Increase the concentration of chlorine gas added to the reaction vessel
5. What three things must be taken into account when determining if a reaction has enough energy
to overcome the activation energy (Ea)?
6. What does a catalyst do to a reaction? Explain.
Something that is added to a reaction to speed up the reaction – it lowers the activation energy
(energy necessary for reactants to become products), therefore helping the reaction reach a
state of equilibrium sooner – the addition of a catalyst does NOT change the equilibrium
constant.
7. What does it mean when an equilibrium system is described as a dynamic system? Explain.
Dynamic is a term that refers to a process that never stops – the forward process & reverse
processes occur at the same rate giving the appearance that the process has stopped, but that
is NOT the case.
8. Draw a diagram for activation energy that indicates that the reaction is endothermic and one that
indicates that the reaction is exothermic. Label the x-axis, y-axis, reactants, products, and what
amount of activation energy is needed for each reaction.
9. What does the Ksp value indicate about an ionic solid?
Ksp is a number that indicates the solubility of an ionic solid that has low to no solubility. It is a
product of the ion concentrations in solution. The larger the Ksp, the more ions in solution, the
greater the solubility of the salt.
10. For the reaction
2 N2(g) + O2(g)
2 N2O(g)
it is found that, at equilibrium at a particular temperature, the concentrations of reactants and
products are: [N2(g)] = 1.41 x 10-4 M; [O2(g)] = 1.25 x 10-5 M; [N2O(g)] = 1.01 x 10-2 M. Calculate the
value of the equilibrium constant K for the reaction at this temperature.
Kc = 4.10 x 108
11. For the reaction 2 H2O(g)
2 H2(g) + O2(g)
K = 2.4 x 10-3 at a given temperature. At equilibrium it is found that [H2O] = 1.1 x 10-1 M and
[H2] = 1.9 x 10-2 M. What is the concentration of O2(g) under these conditions?
Kc = 8.0 x 10-2
12. Approximately 1.5 x 10-3 g of Iron II hydroxide, Fe(OH)2(s), dissolves per liter of water at 18 oC.
Calculate the Ksp for Fe(OH)2(s) at this temperature.
1.5 x 10-3 g Fe(OH)2(s) x __1 mol___ = 1.6 x 10-5 mol/L
89.8 g
Fe(OH)2(s)  Fe+2(aq)
1.6 x 10-5
1.6 x 10-5
+
2OH-(aq)
2(1.6 x 10-5)
Keq = Ksp = [Fe+2(aq)][OH-(aq)]2 = (1.6 x 10-5)(3.2 x 10-5)2 = 1.6 x 10-14
13. Magnesium fluoride dissolves in water to the extent of 8.0 x 10-2 g/L at 25 oC. Calculate the
solubility of MgF2(s) in moles per liter, and calculate the Ksp for MgF2(s) at 25 oC.
8.0 x 10-2 g MgF2(s) x __1 mol___ = 1.3 x 10-3 mol/L
62.1 g
MgF2(s) 
8.0 x 10-2
Mg+2(aq)
8.0 x 10-2
+
2F-(aq)
2(8.0 x 10-2 )
Keq = Ksp = [Mg+2(aq)][F-(aq)]2 = (8.0 x 10-2)(1.6 x 10-1)2 = 2.0 x 10-3
14. For the reaction N2(g) + O2(g)
2 NO(g)
the equilibrium constant K has the value 1.71 x 10-3 at a particular temperature. If the
concentrations of both N2(g) and O2(g) are 0.0342 M in an equilibrium mixture at this temperature,
what is the concentration of NO(g) under these conditions?
K = [NO]2 or
[N2][O2]
1.71 x 10-3 =
(X)2
(0.0342)(0.0342)
or X = 1.41 x 10-3
15. Consider the equilibrium PCl3(g) + Cl2(g)
PCl5(g).
How would the following changes affect the partial pressures (concentration) of each gas at
equilibrium?
PCl3(g) + Cl2(g)
PCl5(g)
a) addition of PCl3
_-__
__↓_
__↑_
b) removal of Cl2
_↑__
_-__
_↓__
c) removal of PCl5
_↓__
_↓__
_-__
d) decrease in the volume of the container
_↓__
_↓__
_↑__
e) addition of He without change in volume
_↓__
_↓__
_↑__
16. Indicate how each of the following changes affects the amount of each gas in the system below,
for which Hreaction = +9.9 kcal. (Endothermic)
H2(g) + CO2(g)
H2O(g) + CO(g)
a) addition of CO2
_↓_
_-_
_↑_
_↑_
b) addition of H2O
_↑_
_↑_
_-_
_↓_
c) addition of a catalyst
No change to anything
d) increase in temperature
_↓_
_↓_
_↑_
_↑_
e) decrease in the volume of the container
No change – equal moles of gases
17. Consider the equilibrium: 2N2O(g) + O2(g)
4NO(g)
How will the amount of chemicals at equilibrium be affected by
2 N2O(g) + O2(g)
4 NO(g)
a) adding N2O
_-_
_↓_
_↑_
b) removing O2
_↑_
_-__
_↓_
c) increasing the volume of the container
_↓_
_↓_
_↑_
d) adding a catalyst
no change – equilibrium reached sooner
18. For the reaction, 4 NH3(g) + 3 O2(g)
2 N2(g) + 6 H2O(l)
How will the concentration of each chemical be affected by
a)
b)
c)
d)
adding O2 to the system
adding N2 to the system
removing H2O from the system
decreasing the volume of the container
There will be one question like number 19 on the quiz.
4 NH3(g) + 3 O2(g)
___
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2 N2(g) + 6 H2O(l)
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19. When ammonia is dissolved in water, the following equilibrium is established. If the equilibrium constant is
1.8 x 10-5, calculate the hydroxide ion concentration in the solution if 0.100 mole of ammonia is dissolved
in sufficient water to make 500 mL of solution.
NH3(aq) + H2O(l)
NH4+1(aq) + OH-1(aq)