<

1. The half-life of l4C is 5570 years.
i) What is the first-order Rate constant, k.
a) 1.24x10-4y-1 b) 6x10-12s-1 c) 5.40x10-5y-1 d) 8040y e) 18500y

ii) How many radioactive disintegrations will occur per minute in a one-gram sample of pure 14C?
a) 4x1013 b) 2.00x10-8 c) 1.02x1013 d) 5.35x1018 e) 5
iii) A fresh sample of wood shows 15.3 disintegrations per gram of carbon. 1.50x10-10%of its carbon content is therefore 14C. A sample of wood from an Egyptian mummy case gives 9.4 decay counts per minute per gram of carbon. How old is the mummy case?
a) 3910y b) 190 000y c) 483y d) 5630y e) 2830y

2. For the reaction 2NO + H2 ---> N2O + H2O it was found in a series of experiments that doubling the initial concentration of NO made the initial reaction rate four times as fast, whereas doubling the initial H2 concentration only made the initial reaction twice as rapid. What is the rate law for this reaction?
Rate=:

a) k[NO]3 b) k[NO]2[H2] c) k[NO][H2]2 d) k[NO][H2] e) k[NO][H2]1/2

3. The method of studying the order of a reaction as outlined in the preceding problem is known as the initial-rate method. The reaction S2O82- + 2I- --> 2SO42- + I2 in aqueous solution was studied by the initial-rate method, with the results for three trial runs given below.

 

What is the rate expression for this reaction and what is the overall order (n) of the reaction?
a) k[S2O82-][I-]
n=1
b) k[S2O82-]
n=1 		c) k[S2O82-]2[I-]
n=2 		d) k[S2O82-]
n=1 		e) k[S2O82-][I-]
n=2

4. In the gas-phase reaction 2NO + Cl2 --> 2NOCl, the results given below were obtained in three runs of an initial-rate experiment.

 
What is the correct rate expression and order (n) for the reaction?
a) rate=k[Cl2][NO]
n=2 		b) rate=k[Cl2]2[NO]
n=3		 c) rate=k[NO]2
n=2		 d) rate=k[Cl2]
n=1 		e) rate=k[Cl2][NO]2
n=3
5. Tert-butyl bromide reacts with water to form tert-butyl alcohol and HBr by the overall reaction
(CH3)3CBr + H2O --> (CH3)3COH + HBr.
i) What would be the rate expression if the reaction were a simple bimolecular collision? Rate =
a) k[H2O]2 b) k[(CH3)3CBr] c) k[(CH3)3CBr][H2O] d) k[(CH3)3CBr]2 e) k[H2O]
ii) What would the rate expression be if the mechanism were
(CH3)3CBr --> (CH3)3C+ + Br- (slow)
(CH3)3C+ + H2O --> (CH3)3COH + H+ (fast)
a) rate=
k[(CH3)3CBr]		 b) rate=
k[(CH3)3CBr]1/2 		c) rate=
k[(CH3)3CBr]2 		d) rate=
k[(CH3)3COH] 		e) rate=
k[(CH3)3CBr][H2O]
iii) Could you distinguish between the two mechanisms in (i) & (ii) if the reaction takes place in dilute aqueous solution?
a) Yes-in excess H2O the rate law in (ii) becomes dependent on [H2O] b) Yes-in excess H2O the second step in the mechanism in (ii) becomes rate determining c) No-both rate laws adopt the same rate law and have the same value for the rate constant. d) Yes-rate law means [H2O] makes a difference even in excess. e) No-the rate law in (i) is pseudo first order in excess[H2O]
6. The following data were measured for the decomposition of ammonia, 2NH3 --> N2 + 3H2
Time (sec) 0 1 2
[NH3] (mole liter-1) 2.000 1.993 1.987

Plot the logarithms of the concentrations against time and show that this is a first-order process.
i) What is the rate constant?
a) 3.25x10-5s-1 b) 3.25x10-3s c) 308s-1 d) 3.25x10-3s-1 e) 308s

ii) What is the half-life for ammonia decomposition?
a) 4.69s b) 92.6s c) 4.69x10-3s d) 213s e) 0.108s

 
7. The reaction SO2Cl2 --> SO2 + Cl2 is a first-order process with a rate constant of k = 2.2x10-5 sec-1 at 320�C. What fraction of the initial SO2Cl2 will have decomposed after heating at 320�C for 90 minutes?
a) 0.112 b) 0.239 c) 0.761 d) 0.888 e) 0.684
8. For the decomposition of N205 in carbon tetrachloride, a plot of ln [N205] against time gives a straight line. The rate constant is k = 6.2 x 10-4 sec at 45�C. If one begins with 1 mole of N205 in a 1-liter flask, how long will it take for 20% of the N205 to decompose and what is the half-life (t1/2)of this decomposition?
a) 1118s b) 540s c) 360s d) 784s e) 296s

9. The following data give the temperature dependence of the rate constant for the decomposition of N205:

 

Make an Arrhenius plot. From the graph what is the energy of activation for the reaction?
a) 370kcalmol-1 b) 490calmol-1 c) 23.8kcalmol-1 d) 12.0kcalmol-1 e) 99.4kcalmol-1
10. For a reaction with an activation energy of 4.0 kcal mole-1 calculate the fraction of molecules with energy Ea, or greater, f = e- Ea/RT, for 0�K, 100�K, 1000�K, 10,000�K, and 100,000�K?. Now plot these values on a linear graph of f versus T.
What does this graph tell you about the influence of temperature on reaction rate?
a) There is an exponential relationship between the two. The maximum rate is approached as the temperature increases (and f approaches unity) b) As the temperature increases the reaction rate falls exponentially. c) As the temperature increases the reaction rate increases linearly. d) Temperature and reaction rate are not related. e) As the temperature increases, so does the rate. The reaction rate reaches a maximum at a threshold temperature and then falls rapidly beyond this temperature.
11. The rate constant for the reaction H2 +I2 --> 2HI was measured at several temperatures, with the following results:
 

What is the energy of activation for the reaction?
a) 37.3kcalmol-1 b) 25.7kcalmol-1 c) 18.8kcalmol-1 d) 187kcalmol-1 e) 15.0 kcalmol-1
12. Using the Ea for the reaction H2 +I2 --> 2HI calculated in the previous problem, and standard heat of formation data from the Appendix what is the activation energy for the reverse reaction, 2HI --> H2 +I2?
a) 49.7kcalmol-1 b) 39.8kcalmol-1 c) 52.2kcalmol-1 d) 28.6kcalmol-1 e) 22.4kcalmol-1
13. It is often said that, near room temperature, the rate of a reaction will double if the temperature is increased by 10�C. What is the activation energy, Ea, of a reaction whose rate exactly doubles between 27�C and 37�C?
a) 55 600calmol-1 b) 1.80calmol-1 c) 128calmol-1 d) 12 800calmol-1 e) 7.81x10-6calmol-1
14. What is the activation energy, Ea, for a reaction whose rate is tripled by a temperature increase from 20�C to 30�C?
a) 8 420calmol-1 b) 19 400calmol-1 c) 81 100calmol-1 d) 26 700calmol-1 e) 35 200calmol-1
15. For the decomposition of CH3I at 285�K, Ea, is 43 kcal mole-1.
i) What fraction of the molecules have this energy or greater at 285�K?
a) 1.31x10-8 b) 0.927 c) 1.05x10-33 d) 0.982 e) 1.17x10-76
ii) What is the percentage increase in the fraction of molecules with energy greater than Ea when the temperature is raised to 300�K. (Assume that Ea is not a function of temperature - which is nearly so but not quite).
a) 351 b) 4 980 c) 470 d) 4 355 e) 6 360
16. The rate constant for the decomposition of N205 in carbon tetrachloride is 6.2 x 10 4 sec-1 at 45�C. In light of this information what is the rate constant at 200�C, if the activation energy is 24.7 kcal mole-1?
a) 227s-1 b) 5.87x1089s-1 c) 517s-1 d) 4.41x10-3s e) 439s-1