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10 Chemistry Problems on half-life and formation

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11. The half-life of a first-order reaction is 13 min. If the initial concentration of reactant is 0.085 M, it takes _____ min for it to decrease to 0.055 M.

a. 8.2
b. 11
c. 3.6
d. 0.048
e. 8.4

12. The mechanism for formation of the product X is:
A+B ® C+D (slow)
B+D ® X (fast)
The intermediate in the reaction is

a. A
b. B
c. C
d. D
e. X

13. The rate of disappearance of HBr in the gas phase reaction 2HBr(g) ® H2(g)+Br2(g) is 0.301 M s-1 at 150ºC. The rate of appearance of Br2 is _____M s-1.

a. 1.66
b. 0.151
c. 0.0906
d. 0.602
e. 0.549

14. A particular first-order reaction has a rate constant of 1.35x102s-1 at 25ºC. What is the magnitude of k at 95ºC if Ea=55.5 kJ/mol?

a. 9.60x103
b. 2.85x104
c. 576
d. 4.33x1087
e. 1.36x102

15. At equilibrium,

a. all chemical reactions have ceased
b. the rates of the forward and reverse reactions are equal
c. the rate constants of the forward and reverse reactions are equal
d. the value of the equilibrium constant is 1
e. the limiting reagent has been consumed

16. Which one of the following will change the value of an equilibrium constant?

a. changing the temperature
b. adding other substances that do not react with any of the species
involved in the equilibrium
c. varying the initial concentrations of reactants
d. varying the initial concentrations of products
e.changing the volume of the reaction vessel

17. The equilibrium constant for the gas-phase reaction 2NH3(g) <-> H2(g) = 3H2 (g) is Keq=230 at 300ºC. At equilibrium,

a. products predominate
b. reactants predominate
c. roughly equal amounts of products and reactants are present
d. only products are present
e. only reactants are present

18. The value of Keq for the following reaction is 0.25:
So2 (g) + NO2 (g) <->So3 (g) = NO(g)
The value of Keq at the same temperature for the reaction below is 2SO2(g) + 2NO (g) <-> 2SO3 (g) 2NO (g) _____

a. 0.50
b. 0.062
c. 0.12
d. 0.25
e. 16

19. The value of Keq for the equilibrium
H2 (g) = Is (g) <-> 2HI (g)
is 54.0 at 427ºC. What is the value of Keq for the equilibrium
HI (g) <-> 1/2H2 (g) = 1/2 I2 (g)

a. 27
b. 7.35
c. 0.136
d. 2.92x103
e. 3.43x10-4

20. Consider the following equilibrium:
2SO2 (g) O2 (g) <-> 2SO3 (g)
The equilibrium cannot be established when _____ is/are placed in a
1.0-L container.

a. 0.25 mol SO2(g) and 0.25 mol O2(g)
b. 0.75 mol SO2(g)
c. 0.25 mol SO2(g) and 0.25 mol SO3(g)
d. 0.50 mol O2(g) and 0.50 mol SO3(g)
e. 1.0 mol SO3(g)

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Solution Summary

10 Chemistry Problems on half-life and formation

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Exam Review 2 Questions

Would like to see answers worked out for exam review.

Question 1
P(C6H14)/t was found to be -6.2 x 10-3 atm/s for the following reaction:

C6H14(g) C6H6(g) + 4H2(g)

Determine P(H2)/t for this reaction at the same time.

6.2 x 10-3 atm/s
1.6 x 10-3 atm/s
2.5 x 10-2 atm/s
-1.6 x 10-3 atm/s
-2.5 x 10-2 atm/s


Question 2
The reaction A + 2B products was found to have the rate law: rate = k[A][B]2. While holding the concentration of A constant, the concentration of B was increased from x to 3x. Predict by what factor the rate of reaction will increase.



Question 3

Nitric oxide gas (NO) reacts with chlorine gas according to the equation

NO + ½Cl2 NOCl.
The following initial rates of reaction have been measured for the given reagent concentrations.

Expt. no. Rate (M/hr) NO (M) Cl2 (M)
1 1.19 0.50 0.50
2 4.79 1.00 0.50
3 9.59 1.00 1.00

Which of the following is the rate law (rate equation) for this reaction?

rate = k[NO]
rate = k[NO][Cl2]1/2
rate = k[NO][Cl2]
rate = k[NO]2[Cl2]
rate = k[NO]2[Cl2]2


Question 4
A first-order reaction has a rate constant of 3.0 x 10-3/s. The time required for the reaction to be 75 percent complete is ________.

95.8 s
201 s
231 s
462 s
41.7 s


Question 5
The units for a second-order rate constant are ________.

1 / M s
1 / M2 s


Question 6

The isomerization of cyclopropane to form propene


is a first-order reaction. At 760 K, 15 percent of a sample of cyclopropane changes to propene in 6.8 minutes. What is the half-life of cyclopropane at 760 K?

3.4 x 10-2 min
2.5 min
23 min
29 min
230 min


Question 7
For the chemical reaction system described by the diagram below, which statement is true?

The forward reaction is endothermic.
The activation energy for the forward reaction is greater than the activation energy for the reverse reaction.
At equilibrium, the activation energy for the forward reaction is equal to the activation energy for the reverse reaction.
The activation energy for the reverse reaction is greater than the activation energy for the forward reaction.
The reverse reaction is exothermic.


Question 8
When the concentrations of reactant molecules are increased, the rate of reaction increases. The best explanation is: As the reactant concentration increases, the ________.

average kinetic energy of molecules increases
frequency of molecular collisions increases
rate constant increases
activation energy increases
order of reaction increases


Question 9
Which is the correct equilibrium constant expression for the following reaction?

Fe2O3(s) + 3H2(g) 2Fe(s) + 3H2O(g)

Kc = [Fe2O3] [H2]3 / [Fe]2[H2O]3
Kc = [H2] / [H2O]
Kc = [H2O]3 / [H2]3
Kc = [Fe]2[H2O]3 / [Fe2O3] [H2]3
Kc = [Fe] [H2O] / [Fe2O3] [H2]


Question 10
Consider the two gaseous equilibria (K1 and K2):

SO2(g) + ½O2(g) SO3(g) K1

2SO3(g) 2SO2(g) + O2(g) K2

The values of the equilibrium constants K1 and K2 are related by ________.

K2 = (K1)2
(K2)2 = K1
K2 = 1/(K1)2
K2 = 1/K2
None of the above


Question 11
The following reactions occur at 500 K. Arrange them in order of increasing tendency to proceed to completion (i.e., least completion greatest completion).

2NOCl 2NO + Cl2 KP = 1.7 x 10-2
N2O4 2NO2 KP = 1.5 x 103
2SO3 2SO2 + O2 KP = 1.3 x 10-5
2NO2 2NO + O2 KP = 5.9 x 10-5
2 < 1 < 3 < 4
3 < 1 < 4 < 2
3 < 4 < 1 < 2
4 < 3 < 2 < 1
4 < 3 < 1 < 2


Question 12
When the following reaction is at equilibrium, which choice is always true?

2NOCl(g) 2NO(g) + Cl2(g)

[NO] [Cl2] = [NOCl]
[NO]2 [Cl2] = [NOCl]2
[NOCl] = [NO]
2[NO] = [Cl2]
[NO]2 [Cl2] = Kc[NOCl]2


Question 13
On analysis, an equilibrium mixture for the reaction

2H2S(g) 2H2(g) + S2(g)

was found to contain 1.0 mol H2S, 4.0 mol H2, and 0.80 mol S2 in a 4.0 L vessel. Calculate the equilibrium constant for this reaction.



Question 14
2.50 mol NOCl was placed in a 2.50 L reaction vessel at 400oC. After equilibrium was established, it was found that 28 percent of the NOCl had dissociated according to the equation:

2NOCl(g) 2NO(g) + Cl2(g)

Calculate the equilibrium constant, Kc, for the reaction.



Question 15
At 35oC, the equilibrium constant for the following reaction is Kc = 1.6 x 10-5.

2NOCl(g) 2NO(g) + Cl2(g)

An equilibrium mixture was found to have the following concentrations of Cl2 and NOCl:

[Cl2] = 1.2 x 10-2 M; [NOCl] = 2.8 x 10-1 M. Calculate the concentration of NO(g) at equilibrium.

1.0 x 10-4 M
1.0 x 10-2 M
2.8 x 10-1 M
2.4 x 10-2 M
1.6 x 10-3 M


Question 16
Consider the reaction

N2(g) + O2(g) 2NO(g)

Kc = 0.10 at 2000oC. Starting with initial concentrations of 0.040 M of N2 and 0.040 M of O2, calculate the equilibrium concentration of NO.

5.4 x 10-3 M
0.0096 M
0.011 M
0.080 M
0.10 M


Question 17
For the following reactions the equilibrium constants are defined.

A + 2B C K1
C D + E K2

Then for the reaction

A + 2B D + E Kc

the equilibrium constant must be equal to ________.

Kc = K1 + K2
Kc = K1/K2
Kc = K1 - K2
Kc = (K1)(K2)
Kc = K2/K1


Question 18
At 700 K, the reaction

2SO2(g) + O2(g) 2SO3(g)

has an equilibrium constant Kc = 4.3 x 106, and the following concentrations are present:

[SO2] = 0.10 M
[SO3] = 10 M
[O2] = 0.10 M

Is the mixture at equilibrium? If not at equilibrium, in which direction-left to right or right to left- will the reaction occur to reach equilibrium?

Yes, the mixture is at equilibrium.
No, left to right
No, right to left
There is not enough information to tell.


Question 19
For the following reaction at equilibrium, which choice gives a change that will shift the position of equilibrium to favor formation of more products?

2NOBr(g) 2NO(g) + Br2(g) Horxn = 30 kJ

Increase the total pressure by decreasing the volume.
Add more NO.
Remove Br2.
Lower the temperature.
Remove NOBr selectively.


Question 20
The following reaction is endothermic:

2SO3(g) 2SO2(g) + O2(g)

What will happen if the temperature is increased?

More SO3 will be produced.
Kc will decrease.
No change will occur in Kc.
Kc will increase.
The pressure will decrease.

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