Northrup's Chem 112 Section TTU General Chemistry

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Chem 1120 - Chapter 14: Chemical Equilibrium
Practice Quiz 2

1. Identify the INCORRECT statement below regarding chemical equilibrium:

a) All chemical reactions are, in principle, reversible.
b) Equilibrium is achieved when the forward reaction rate equals the reverse reaction rate.
c) Equilibrium is achieved when the concentration of species become constant.
d) Equilibrium is achieved when the reaction quotient Q equals the equilibrium constant.
e) Equilibrium is achieved when the reactant and product concentrations become equal.

2. Calculator problem - Nitrogen reacts with hydrogen to form ammonia:

N2(g) + 3 H2(g) <==> 2 NH3(g)

At a certain temperature the equilibrium constant is Kc = 2.0. An equilibrium mixture is found to contain concentrations of N2 equal to 0.15 M and H2 equal to 0.30 M. What is the concentration of NH3 in this mixture?

a) 2.7 x 10-2 M
b) 0.16 M
c) 8.1 x 10-3 M
d) 9.0 x 10-2 M

3. Write the appropriate equilibrium constant expression Kc for the following reaction:

2 CO(g) + O2(g) <==> 2 CO2(g)

4. Suppose we rewrote the balanced reaction in Problem #3 as:

What would be the equilibrium constant for this reaction?

a) same, Kc = Kc

b) Kc = sqrt( Kc )

c) Kc = Kc/2

d) Kc = Kc2

e) Kc = 1/ Kc

5. When Kc >> 1 for a chemical reaction:

a) the equilibrium would be achieved rapidly.
b) the equilibrium would be achieved slowly.
c) reactants would be much more stable than products.
d) product concentrations would be much greater than reactant concentrations at equilibrium.
e) reactant concentrations would be much greater than product concentrations at equilibrium.

6. In the rearrangement reaction:

cyclopropane <==> propene

the equilibrium constant at a certain temperature is Kc = 3.0. A reaction vessel initial having a cyclopropane concentration [cyclopropane]0 = 2.0 M and no propene is allowed to come to equilibrium. What will be the concentration of propene at equilibrium?

a) 0.0 M
b) 0.66 M
c) 1.5 M
d) 2.0 M

For Problems #7 - 11

A proposed mechanism for the decomposition of ozone is:

7. Which step is rate-determining?

a) step (1)
b) step (2)

8. Which species is (are) an unstable intermediate?

a) O2 and O
b) O2
c) O
d) Ar
e) Ar and O

9. Which species is a catalyst?

a) O2
b) O3
c) Ar
d) Ar and O3

10. Which is the best overall rate law consistent with this mechanism?

a) k[O3][O]
b) k[O3]2
c) k[O3]
d) k[O3]2/[O2]

11. The properly written overall balanced equation is:

a) 2 O3 --> 3 O2
b) O3 --> 2 O2
c) 2 O3 + O --> 3 O2 + O
d) 3 O2 --> 2 O3

12. Identify the INCORRECT statement below:

a) All chemical reactions are, in principle, reversible, i.e., they can go in both directions.
b) Chemical equilibrium exists when the two opposite reactions occur simultaneously at the same rate.
c) Concentrations of reactants and products become equal at equilibrium.
d) At equilibrium the reaction quotient Q becomes constant.
e) Increasing the temperature increases K of an endothermic reaction.

13. For the reaction:

2 H2(g) + O2(g) <-----> 2 H2O(g)

the concentrations in the reaction vessel at a given time are [H2] = 3.0 M, [O2] = 2.0 M and [H2O] = 0.01 M. What is the value of the reaction quotient Q expressed in terms of concentrations?

a) 6.0 x 102
b) 1.8 x 105
c) 1.66 x 10-3
d) 5.5 x 10-6

14. In a given mixture of N2, Cl2 and NCl3 gases, the following partial pressures are found:

PN2 = 0.17 atm, PCl2 = 0.43 atm, PNCl3 = 7.0 atm

If the equilibrium constant for the reaction N2 + 3 Cl2 <- ----> 2 NCl3 is Kp = 1.0 x 103, what is expected to happen to this mixture?

a) It is not in equilibrium and will proceed in the forward direction.
b) It is not in equilibrium and will proceed in the reverse direction.
c) The mixture is in equilibrium and will not change.

15. Which of the following changes can affect the value of the equilibrium constant?

a) introducing a catalyst
b) changing the concentrations of species
c) changing the temperature inside the reaction vessel
d) changing the initial concentrations of species
e) changing the pressure inside the reaction vessel

16. A collection of gases N2(g), Cl2(g) and NCl3(g) are in equilibrium in a reaction vessel.

N2(g) + 3 Cl2(g) <-----> 2 NCl3(g)

Suddenly the vessel size is compressed to half its volume. What will happen?

a) The system will no longer be in equilibrium and more product will form as equilibrium is restored.
b) The system will no longer be in equilibrium and more reactants will form as equilibrium is restored.
c) Nothing will change. The system is at equilibrium.
d) The equilibrium constant will get larger.
e) The equilibrium constant will get smaller.

17. In the gas phase reaction:

PCl3(g) + Cl2(g) <-----> PCl5(g)
Kc = 1.9

the system is in equilibrium with concentrations [PCl3] = 0.5 M and [Cl2] = 0.5 M. What is the PCl5 concentration?

a) 0.95 M
b) 0.500 M
c) 1.9 M
d) 2.1 M
e) 0.475 M

18. A simple reaction is the cis-trans isomerization of an alkene

cis-alkene <-----> trans-alkene

with an equilibrium constant Kc = 0.50. Suppose the initial concentrations of the cis form is 1.0 M and the trans is 0.0 M. What will the concentration of the trans form be when the system reaches equilibrium?

a) 0.50 M
b) 0.33 M
c) 1.0 M
d) 0.25 M
e) 1.5 M

19. For the reaction:

H2 + Cl2 <-----> 2 HCl

If 5.0 mol of H2 and 3 mol of Cl2 are placed in a 2 liter flask and allowed to reach equilibrium and at equilibrium there are 4.5 mol of H2 found, what is the Kc of the reaction?

a) 1.35
b) 0.088
c) 0.022
d) 0.066
e) 0.122

KEY

1)e 2)d 3)d 4)b 5)d 6)c 7)b 8)c 9)c 10)d 11)a 12)c 13)d 14)b 15)c 16)a 17)e 18)b 19)b
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