10 Chemistry Problems on half-life and formation

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
below?
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)