UNIT 12 REVIEW
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Write the equilibrium expression for the
following reaction:
4 HCl(g) + O2(g)   2 Cl2(g) + 2 H2O(g)
ANSWER: Keq = [Cl2]2 [H2O]2
[HCl]4 [O2]
Write the equilibrium expression for
the following reaction:
SO3(g) + H2O(l)   H2SO4(l)
Answer:
Keq =
1
[SO3]
Write the equilibrium expression
for the following reaction:
Cu(s) + 2 AgNO3(aq)   Cu(NO3)2(aq) + 2 Ag (s)
Answer:
Keq = [Cu(NO3)2]
[AgNO3]2
• CH4(g) + 2 O2(g)   CO2(g) + 2 H2O(g)
• Calculate [O2] if:
[CH4] = 0.0150 M, Keq = 3,620
[CO2] = 0.0824 M, [H2O]= 0.0603 M
• Are the reactants or products favored at
equilibrium?
Answer:
Keq = [CO2] [H2O]2
[CH4] [O2]2
3,620 = (0.0824)(0.0603)2
(54.3)x2 = 0.0002996
x2 = 0.0000552
(0.0150)(x)2
54.3
54.3
[O2] = 0.00235 M
Products are favored because Keq is greater than one.
Write the equilibrium expression
& calculate [CH3OH] for this
reaction:
CO(g) + 2H2(g)  CH3OH(g)
[CO]=0.025M, [H2]=0.080M,
Keq = 394
Answers:
Keq = [CH3OH]
[CO] [H2]2
[CH3OH]: 394 =
x
= 0.063 M
(0.025)(0.080)2
• Calculate Keq:
• PCl5(g)   PCl3(g) + Cl2(g)
• [PCl5] = 1.00M, [PCl3]=3.16x10-2M,
[Cl2]=7.18x10-3M
• Are the reactants or products favored at
equilibrium?
• If you change the temperature, would the Keq be
the same?
Answers: Keq = [PCl3][Cl2] = (3.16x10-2)(7.18x10-3) = 2.27x10-4
[PCl5]
(1.00)
Reactants are favored because Keq is less than 1.
No, because temperature affects concentration.
• What 2 types of changes cause a reaction
to go to completion?
• If a reaction does not go to completion,
what may happen to some of the
products?
Answers:
•Forms a Gas (g) or a Precipitate ( )
•Can reform reactants (Reverse Reaction)
• A reaction that does not go to completion
is called a __________________ reaction.
• What part of the equation shows if a
reaction goes to completion or not?
Answer:
• Reversible
• The Arrow
(double = reversible, single = completion)
• Why does the word “dynamic” describe
equilibrium?
• Define reversible reaction.
• Define completion reaction.
Answers:
Dynamic describes equilibrium because the reaction does not
stop. The forward and reverse reactions continue at equal
rates.
A reversible reaction can form products and then reform
reactants.
A completion reaction can only form products and forms a gas
or a precipitate.
• When a system is at ________________________ , it will stay that
way until something changes.
• State Le Chatelier’s Principle.
• List three examples of stresses that can change a system at
equilibrium.
• 1)
• 2)
• 3)
Answers:
• Equilibrium
• When a reaction at equilibrium is disturbed, it will shift to eliminate the stress.
• Changing temperature, concentration, or pressure/volume (only for gases)
• You can tell if a reaction is endothermic or exothermic by
looking at the equation. If heat is on the reactant side, the
reaction is ________________. If heat is on the product side,
the reaction is ________________.
• Determine if these reactions are endothermic or exothermic.
a) 2NO2 ↔ N2O4 + 58.8 J
b) 2AgCl + energy ↔ 2Ag + Cl2
• What direction will the system shift if the temperature in “a” is
raised?
• How could you make reaction “b” produce more products?
Add energy or remove energy?
Answers: endothermic, exothermic
a) exothermic b) endothermic
•Shift left
•Add energy
• When an equilibrium mixture is
subjected to a decreased
pressure, it will shift to the side
with ____________________.
Answer: More Moles
Decrease Pressure = Increase Volume =
Shift to Side with MOST Moles
2 NO2(g)   N2O4(g) + energy
According to Le Chatelier’s Principle, what direction will
the system above shift if the following stresses are applied?
A. Lower Temperature
B. Increase N2O4(g)
C. Decrease NO2(g)
D. Increase Pressure
E. Increase Volume
Answer:
A. Right (to replace heat lost on right side)
B. Left (to decrease [N2O4]
C. Left (to replace [NO2])
D. Right (Increase P = Decrease V so shift to side with less moles
E. Left (Increase V so shift to side with more moles)
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What does Ksp stand for?
What does it describe?
Which lead compound below is the least soluble?
Ksp = PbBr2 = 6.3 x 10-6
Ksp = PbCl2 = 1.7 x 10-5
Ksp = PbCrO4 = 1.8 x 10-14
Answers:
•Solubility Product Constant
•Ksp describes the solubility (ability to dissolve) of
slightly soluble substances.
• PbCrO4 because it has the smallest Ksp