To get the class handouts for the Kinetics chapter, click here.
Here are some typical exam questions concerning kinetics.
| 1. Which of the following will not
affect the rate of a particular chemical reaction carried out at a constant temperature? |
| a. the magnitude of delta H |
d. the presence of a catalyst |
| b. the size of reactant particles e. the
temperature |
|
| c. the concentration of the reactants |
|
| 2. The oxidation of ammonia
produces nitrogen and water via the reaction 4 NH3 + 3 O2 ------->2 N2 + 6 H2O
If the rate of formation of N2 is 2.0 M/sec,
then the rate at which... |
| a. ...water is formed is 2.0 M/sec |
d. ...ammonia reacts is 4.0
M/sec |
| b. ...oxygen reacts is 1.5 M/sec |
e. ...ammonia reacts is 0.50 M/sec |
| c. ...water is formed is 0.67 M/sec |
|
| 3. For the reaction between NO
and Cl2 to produce NOCl, 2
NO(g) + Cl2(g) -------> 2 NOCl(g)
it is found that tripling the concentration of both
reactants increases the initial rate of reaction by a factor of 27. But, if only the
concentration of chlorine is tripled, the initial rate increases by a factor of 3. The
order of this reaction with respect to NO is: |
| a. 0 |
d. ½ |
| b. 1 |
e. 2 |
| c. 3 |
|
| 4. For the reaction of the
ammonium ion with nitrous acid, the net reaction is NH4+ + HNO2 -----> N2
+ 2 H2O + H+
the rate law for the formation of H+ is: |
| a. Rate = k [NH4+][HNO2] |
d. Rate = k[H+] |
| b. Rate = k{[NH4+][HNO2]}
/ {[H2][H2O]2[H+]} |
e. insufficient data given
to answer this question |
| c. Rate = k[HNO2] / [H+] |
|
| 5. If a reaction is first-order
with a specific rate constant of 5.48 x 10-2 min-1 at 25C, how long
will it take for three-fourths of the reactant to be consumed? |
| a. 6.3 min |
d. 12.6 min |
| b. 36.5 min |
e. 50.6 min |
| c. 25.3 min |
|
| 6. The main reason for the
increase in reaction rate with temperature is that |
| a. heat acts as a catalyst |
d. the concentration of high-energy molecules
increases with temperature |
| b. the activation energy decreases rapidly
with increasing temperature |
e. two of the above are
correct. |
| c. there is a dramatic increase in the number
of collisions at higher temperatures |
|
| 7. The rate constant for a
second-order reaction is 0.0025 M-1sec-1. If you have an initial
concentration of 0.888M reactant, how much time is required for the passage of two
half-lives? |
| a. 900 sec |
d. 1350 sec |
| b. 554 sec |
e. 1110 sec |
| c. 1800 sec |
|
| 8. A kinetic study was performed
on a reaction, and the second-order rate constants were measured at two different
temperatures. What is the activation energy for the reaction?
R = 8.314 J/mol-K temperature:
27oC
57oC
k =
0.0025 M-1min-1 0.0085 M-1min-1
|
| a. 33.5 kJ |
d. 7.6 kJ |
| b. 22.4 kJ |
e. 52.4 kJ |
| c. 46.6 kJ |
|
| 9. Which of the following statements
concerning the rates of chemical reactions is false? |
| a. The reason most chemical reactions slow
down as time passes is because the concentration of reactants decrease. |
| b. A catalyst increases
the rate of a chemical reaction by becoming intimately involved with the reaction
mechanism causing an increase in the overall activation energy. |
| c. The mechanism of a chemical reaction is
the series of single-event steps taken as reactants are converted to products. |
| d. If the concentration of a reactant is
doubled, the reaction rate may not increase at all. |
| e. In the collision of two reactant molecule,
the most important thing that must take place is the correct molecular orientation in the
collision, no matter how much energy is involved in the collision. |
| 10. Consider the equation for the
oxidation of oxalic acid by permanganate ion in acid solution: 6 H+(aq) + 5 H2C2O4
+ 2 MnO4- -----> 10 CO2 + 2 Mn2+ + 8
H2O
The rate of this reaction was measured at 25C in terms of
the disappearance of MnO4- and was found to be 7.5 x 10-3
M/sec. What would the rate be in terms of the disappearance of oxalic acid, H2C2O4
at the same temperature? |
| a. 7.5 x 10-3 M/sec |
d. 2.3 x 10-4 M/sec |
| b. 1.9 x 10-2
M/sec |
e. 3.8 x 10-4 M/sec |
| c. 3.0 x 10-3 M/sec |
|
| 11. The decomposition of N2O5
is first order with k = 0.0045 sec-1. How much time is required for an initial
concentration of 1.50 M N2O5 to fall to exactly one-tenth of its
initial molarity? |
| a. 770 sec |
d. 308 sec |
| b. 222 sec |
e. 3 half lives |
| c. 512 sec |
|
| 12. Which of the following would
probably not have much affect on the rate of a chemical reaction? |
| a. increasing the temperature of the reaction |
d. adding an appropriate catalyst to the
reaction mixture |
| b. increasing the amount of
a solid reactant in the reaction mixture |
e. grinding up a solid reactant to a fine
powder |
| c. increasing the concentration of a reactant |
|
| 13. The rate constant for a
second-order reaction was measured to be 1.55 x 10-3 M-1sec-1.
If you start with 5.35 mole of a gaseous reactant in a 3.50 L vessel, what concentration
would remain after 30.0 minutes? |
| a. 0.382 M |
d. 1.43 M |
| b. 0.094 M |
e. 0.290 M |
| c. 1.46 M |
|
| 14. Radioactive decay is a
first-order process. The half-life of the radioactive isotope technetium 99m, Tc-99m,
is 6.0 hours. It is widely used in medical imaging. If a patient receives a dose of
Tc-99m at 8:00 am on Monday, what fraction of the sample remains radioactive by 8:00 am on
Tuesday, that is, after 24.0 hours? |
| a. one-half of it |
d. one-sixteenth of it |
| b. one-fourth of it |
e. more information is needed to answer this
question |
| c. one-eight of it |
|
Here are a few matching questions concerning solutions and
kinetics.
| 15. In the blank please place the
letter corresponding to the item on the right that best fits the statement. |
| _____ a first-order rate constant |
a. changes only with temperature |
| _____ the rate law of a third-order reaction |
b. mole fraction |
| _____ biological catalysts |
c. concentration |
| _____ for all first-order reactions, the
half-life is only a function of _?_ |
d. delta Tbp |
| _____ the initial rate of a reaction is the
rate at _?_ |
e. rate = k[A]2[B] |
| _____ the units of rate are shown in _?_ |
f. t½ |
| _____ the best colligative property to measure
the molecular weight of a polymer or protein is _?_ |
g. answer not given |
| _____ in most cases, as time passes the rate
of a chemical reaction _?_ |
h. substrates |
| _____ in any reaction the time required for
the concentration of a reactant to fall from to 0.693 M to 0.346 M is the _?_ |
i. enzymes |
| _____ cannot be accurately measured for salts
using colligative properties |
j. decreases |
|
k. osmotic pressure |
|
l. 0.071 s-1 |
|
m. 0.693 M/sec |
|
n. reactivity |
|
o. rate=k[A]3[B]1 |
|
p. time = 0 |
|
q. 2.10 M1sec1 |
|
r. molecular weight |
|
s. dialysis |
Here are some short answer questions, graphing problems,
etc.
16. The following reaction was studied at 904oC:
2 NO(g) + 2 H2(g)
-------> N2(g) + 2 H2O(g)
expt. |
[NO]o |
[H2]o |
delta [N2]/delta t (initial) |
1 |
0.42 |
0.12 |
0.136 mol/L-s |
2 |
0.21 |
0.12 |
0.034 mol/L-s |
3 |
0.21 |
0.24 |
0.068 mol/L-s |
4 |
0.10 |
0.49 |
0.034 mol/L-s |
(a) Determine the experimental rate law from these data and write it in the blank to the
right.
the rate law is: Rate
= k[NO]2[H2]
(b) Calculate the value of k for this reaction at 904C.
k = 6.4 M-2sec-1
17. The following data were recorded in a study of the
decomposition of ammonia gas at 2000 K.
NH3(g) -----> NH2(g)
+ H(g)
| time (hours) |
[NH3]t |
| 0 hour |
8.00 x 10-7 M |
| 25 hours |
6.75 x 10-7 M |
| 50 hours |
5.84 x 10-7 M |
| 75 hours |
5.15 x 10-7 M |
| 100 hours |
4.61 x 10-7 M |
(a) Prepare the appropriate plot or plots which will
clearly show whether this decomposition is first-order in ammonia or is second-order in
ammonia.
To see if it is first order, plot ln[NH3]t
versus time. If a straight line plot results, then the reaction is first order in ammonia.
If it is not a straight line, then plot 1/[ammonia]t versus time. If this is a
straight line plot, then the reaction is second order in ammonia.
As it turns our, the order of the reaction is: second order
(b) Once you have determined the order of reaction,
determine the numerical value of the rate constant for the decomposition at 2000 K.
k = the slope of the straight line
On the exam you must show all work clearly if you desire
partial credit and correct units must accompany answers.
3. The following data were gathered in the study of the
decomposition of an organic compound at 250oC. Show work on the pack side of
this page. The equation is:
compound -----> products
| time(sec) |
[compound]t |
| 0 |
0.100 |
| 100 |
0.0924 |
| 200 |
0.0855 |
| 300 |
0.0790 |
| 400 |
0.0731 |
| 500 |
0.0675 |
a. What is the order of the reaction: (Prove your answer
with the appropriate graph. )
Doing both a first and second order
plot, you will see the reaction is 1st Order in the compound.
b. From the graph, determine the value of the rate
constant, k, for the decomposition.
Prepare a plot of ln[compound]
versus time. A straight-line plot will be obtained. The specific first-order rate constant
is the slope time minus one and will have units of per second.
c. Calculate the magnitude of the first half-life, t½.
For first order reactions, the half-life
equals 0.693/k
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