Klotz & Treichel

Chapter 15: Principles of Reactivity: Chemical Kinetics-page 741: 4, 6, 9, 18, 19, 21, 24, 26, 28, 30, 34, 46, 47, 48, 49, 52, 53, 54, 55, 56, 57, 67, 74, 80

Answers at the bottom

15.4. Using the rate equation Rate = k[A]2[B], define the order of the reaction with respect to A and B and the total order of the reaction.


15.6. A reaction has the experimental rate equation Rate = k[A]2[B]. If the concentration of A is doubled, and the concentration of B is halved, what happens to the reaction rate?


15.9. If you plot 1/[reactant] versus time and observe a straight line, what is the order of the reaction? If Ln[reactant] is plotted versus time, and a straight line of negative slope is observed, what is the order of the reaction?

15.18. Give the relative rates of disappearance of reactants and formation of products for each of the following reactions:

  1. 2O3(g) 3O2(g)
  2. 2HOF(g) 2HF(g) + O2(g)
  3. 2NO(g) + Br2(g) 2BrNO(g)

15.19. Consider the reaction

N2(g) + 3H2(g) -> 2NH3(g)

At the instant N2 is reacting at a rate of 0.25 mol/L-min, at what rates are H2 disappearing and NH3 forming?

15.21. A compound called phenyl acetate reacts with water according to the equation

and the data in the table are collected at 5oC.

Time (min) [phenyl acetate](mol/L)
0 0.55
0.25 0.42
0.50 0.31
0.75 0.23
1.00 0.17
1.25 0.12
1.50 0.085

  1. Plot these data, and describe the shape of the curve observed. Compare this with Figure 15.2 in the text.
  2. Calculate the rate of change of [phenyl acetate] during the period of 0.20 min to 0.40 min and then during the time period 1.2 min to 1.4 min. Compare the values and tell why the one is smaller than the other.
  3. Wat is the rate of change of [phenol] during the time period 1.00 min to 1.25 min?
  4. What is the instantaneous rate at 15s? At 35s?

24. Data are given in the table at 660 K for the reaction

2 NO(g) + O2(g) 2 NO2(g)

Reactant Concentration (mol/L)

[NO] [O2} Rate of Disappearance
of NO (mol/L - s)

0.020 0.010 1.0 x 10-4
0.040 0.010 4.0 x 10-4
0.020 0.040 4.0 x 10-4

(a) Write the rate equation for the reaction.
(b) Calculate the rate constant.
(c) Calculate the rate (expressed in moles perliter x seconds) at the instant when [NO] = 0.045 M and [O2] = 0.025 M.
(d) At the instant when O2 is reacting at the rate 5.0 x 10-4 mol/L - s, at what rate is NO reacting and NO2 forming?



26. The decomposition of N2O5 in nitric acid is a first-order reaction. It takes 4.26 min at 55ºC to decrease 2.56 mg of N2O5 to 2.50 mg. Find k in minutes-1 and seconds-1.


28. The transformation of cyclopropane to propene was scribed in Example 15.3. If the initial concentration of cyclepropane is 0.050 M, how many hours must elapse for concentration to drop to 0.025 M? (The first-order rate constant is 5.4 x 10-2 h-1.)


30. The rate constant for the decomposition of nitrogen dioxide

NO2(g) NO(g) + 1/2 O2(g)

with a laser beam is 3.40 L/mol - min. Find the time in seconds needed to decrease the concentration of NO2 from 2.00 mol/L to 1.50 mol/L.


34. The decomposition of phosphine, PH3, proceeds according to the equation

4 Ph3(g) P4(g) + 6 H2(g)

It is found that the reaction has the rate equation Rate k[PH3]. If the half-life is 37.9 s, how much time is required for three fourths of the Ph3 to decompose?



46. For the hypothetical reaction A + B C + D, the action energy is 32 kJ/mol. For the reverse reaction (C + A + B), the activation energy is 58 kJ/mol. Is the reaction A + B + D exothermic or endothermic?


47. The reaction of H2 molecules with F atoms has an activation energy of 8 kJ/mol and an enthalpy change of -133 kJ.

H2(g) + F(g) HF(g) + H(g)

Draw a diagram like Figure 15.14 for this process.

click for a larger image


48. Calculate the activation energy (Ea) for the reaction

N2O5(g) 2 NO2(g) + 1/2 O2(g)

from the observed rate constants: k at 25ºC = 3.46 x 10-5 s-1 and k at 55ºC = 1.5 x 10-3s-1.


49. If the rate constant for a reaction triples in value when the temperature rises from 300.K, what is the activation energy of the reaction?


52. Each of the following equations represents an elementary step. Write the rate equation for each elementary step and give the molecularity of the step.

(a) NO(g) + NO3(g) 2 NO2(g)
(b) Cl(g) + H2(g) HCl(g) + H(g)
(c) (CH3)3CBr(aq) (CH3)3C+(aq) + Br-(aq)


53. The reaction between chloroform (CHCl3) and chlorine gas proceeds in a series of three elementary steps.

Step 1 Fast, reversible Cl2(g) 2 Cl(g)

Step 2 Slow CHCl3(g) + Cl(g) CCl3(g) + HCl(g)

Step 3 Fast CCl3(g) + Cl(g) CCl4(g)

Overall CHCL3(g) + Cl2(g) CCl4(g) + HCl(g)


54. The ozone, O3, in the earth's upper atmosphere decomposes according to the equation 2 O3(g) 3 O2(g). A mechanism of the reaction is thought to proceed through initial fast, reversible step and then a slow second step.

Step 1 Fast, reversible O3(g) O2(g) + O(g)

Step 2 Slow O3(g) + O(g) 2 O2(g)

(a) Which of the steps is the rate-determining step?
(b) Write the rate equation for the rate-determining step.
(c) What is the molecularity of each step?

55. Iodide ion is oxidized in acid solution by hydrogen peroxide.

H2O2(aq) + 2H+(aq) + 2 I-(aq) I2(aq) + 2 H2C

A proposed mechanism is

Step 1 H2O2(aq) + I-(aq) H2O(l) + OI-

Step 2 H+(aq) + OI-(aq) HOI

Step 3 HOI(aq) + H+(aq) + I-(aq) 12(aq) H2O

(a) Show that the three elementary steps add up to give overall, stoichiometric
Equation.
(b) What is the molecularity of each step?
(c) For this mechanism to be consistent with kinetic data what must be the
experimental rate equation?
(d) Identify any intermediates in the elementary steps in reaction.


56. Which of the following statements is (are) true?


(a)The concentration of a homogeneous catalyst may appear in the rate equation.
(b)A catalyst is always consumed in the reaction.
(c)A catalyst must always be in the same phase as the reactants.
(d)A catalyst can change the course of a reaction and a different products
to be produced.
(e)A catalyst can cause a change in which elementary step rate-determining
for a particular reaction.


57. What is the effect of a catalyst on
(a) The activation energy
(b) The enthalpy of the reactants and products?


66. A catalyst can cause a reaction to proceed along a different pathway with a lower activation energy. If the catalyst lowers the activation energy by 5 kJ/mol, how much faster is the reaction, if both occur at 298 K (and the frequency factor A does not change)?


15.74 An enthalpy diagram is given here for the absorption and dissociation of O2 on a platinum surface.

  1. What is for
    1. O2(g) O2(absorbed)
    2. O2(absorbed) 2O (absorbed)
    3. O2(g) 2O (absorbed)
  2. What is Ea for O2 (absorbed) 2O (absorbed)?



80. The radioactove noble gas radon has been the focus of much attention lately because it can be found in homes . Radon-222 (222Rn) has a half-life of 3.82 days. Assume radon gas is in the basement of a home (with the dimensions 12m by 7.0 m by 3.0 m) and that the gas has a partial pressure of 1.0 x 10-6 mm Hg.
(a) How many atoms of 222Rn are in a liter of air in the basement?
(b) If the radon gas is not replenished in the basement, how many atoms of 222Rn
Remain per liter of air after 30 days?


answers

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