Rate Laws of Reactions Worksheet
1. For a particular reaction at constant temperature, 

A(g) + 2 B(g) ---> products 

           initial    initial    initial 
            [A]       [B]      rate
           1.00     1.00    1.00
           2.00     4.00    8.00
           3.00     9.00  27.00
           4.00     2.00     ? 

What is the value of "?" in this table? 

2. What is a rate law? What is the proportionality constant called? 
3.  What is meant by the order of a reaction? 
4. The rate law for the reaction 2 NO + O2 ----> 2 NO2 is rate= k[NO]2[O2]. At 25oC, k=7.1 X 109 L mol-2s-1. What is the rate of reaction when [NO] = 0.0010 mol/L and [O2]=0.034 mol/L? 
5. The initial rate of the reaction: 

BrO3-(aq) + 5 Br-(aq) + 8 H+(aq) ----> 3 Br2(l) + H2O(l) 

Has been measured at the reactant concentrations shown (in mol/L): 

Experiment [BrO3-] [Br-]     [H+] Initial rate (mol/Ls) 
       1           0.10    0.10    0.10     8.0 x 10-4
       2           0.20    0.10    0.10     1.6 x 10-3
       3           0.10    0.20    0.10     1.6 x 10-3
       4           0.10    0.10    0.20     3.2 x 10-3

According to these results what would be the initial rate (in mol/Ls) if all three concentrations are: 

[BrO3-]=[Br-]=[H+]=0.20 mol/L? 

6. The reaction of iodide ion with hypochlorite ion, OCl- (which is found in liquid bleach), follows the equation 
OCl- + I- ----> OI- + Cl-
It is a rapid reaction that gives the following rate data. 

Initial Concentrations      Rate of Formation
  [OCl-]            [I-]           (mol L-1 s-1)
                                        (mol/L) of Cl-
1.7 X 10-3   1.7 X 10-3         1.75 X 104
3.4 X 10-3   1.7 X 10-3       3.50 X 104
1.7 X 10-3   3.4 X 10-3       3.50 X 104

What is the rate law for the reaction. Determine the value of the rate constant. 

7. The decomposition of ozone, O3, is believed to occur by the two-step mechanism 

O3 -----> O2 + O (slow) 
O + O3 -----> 2 O2 (fast)
2 O3 -----> 3 O2 (net reaction) 

If this is the mechanism, what is the reaction's rate law? 
 

8. Why does the rate of the photochemical reaction of bromine with hydrogen decreases as the concentration of an inert gas (eg. Kr) increases? 
9.  Write expressions for the rate of formation of the product(s) in each of the following. Indicate the units of the rate constant. 
a) H2 + Cl2 ---> 2 HCl 
b) 2 O2 + S2 ---> 2 SO2
c) 3 O ----> O3
d) 2 HI + Cl2 ----> 2 HCl + I2
e) C6H6 + Cl2 ---> C6H5Cl + HCl 
10.  How can we double the rate of reaction of H2 with I2 without changing the temperature? 
11. What is a homogeneous catalyst? How does it function, in general terms?
12. What is a heterogeneous catalyst? How does it function? 
13. Consider the decomposition of dinitrogen pentoxide:   2 N2O5 (g) ----> 4 NO2(g) + O2(g)
Given that the initial concentration of N2O5 is 2a mol/L, which line in the graph shows the concentration of O2(g) as a function of time? 

a) Line A, which starts at 4a and ends near zero. 
b) Line B, which starts at zero and ends near 4a. 
c) Line C, which starts at zero and ends near 2a. 
d) Line D, which starts at zero and ends near a. 
e) Line E, which starts at 2a and ends near zero. 
14. Given the reaction: A + B ---> C + D 
The reaction will most likely occur at the greatest rate if A and B represent 
a. nonpolar molecular compounds in the solid phase 
b. ionic compounds in the solid phase 
c. solutions of nonpolar molecular compounds 
d. solutions of ionic compounds 
15. The value for the energy of activation of the foward reaction is represented by which letter in the diagram below? 

a) A 
b) B 
c) C 
d) D 
e) E 
16. Consider the following reactions: 
       I         Ag+(aq) + I-(aq) ------> AgI(s) 
      II        4 Fe(s) + 3 O2(g) ------> 2 Fe2O3(s)
Which one of the following statements best described the relative rates of the two reactions? 
a. II is faster than I 
b. I and II are both slow 
c. I and II are both fast 
d. I is faster than II 
17. Persons who have been submerged in very cold water and who are believed to have drowned sometimes can be revived. On the other hand, persons who have been submerged in warmer water for the same length of time have died. Explain this in terms of factors that affect the rates of chemical reactions.
18. Which of these reactions are likely to proceed rapidly once they have begun? Explain. 
a) H2(g) + Cl2(g) ---> 2 HCl(g) 
b) C6H12O5(aq) + 6 O2(g) ---> 6 CO2(g) + 6 H2O(l) 
c) Cu2+(aq) + S2-(aq) ---> CuS(s) 
d) A lump of iron in melted sulphur 
e) Powdered iron in melted sulphur 
19. a) What is meant by the order of a reaction? 
b) Can the order be determined from the equation for the overall reaction?
c) If the sum of the coefficients of the reactants in the equation equals the total order of a reaction, can it be assumed that the equation represents an elementary process? 
20.  The following data were collected for the reaction SO2Cl2 --> SO2 + Cl2 at a certain temperature. 
[SO2Cl2](mol/L) Time(s) 
  0.100                 0
  0.082              100
  0.067              200
  0.055              300
  0.045              400
  0.037              500
  0.030              600
  0.025              700
  0.020              800
Make a graph of concentration versus time and determine the rate of the reaction at t=200 seconds and t=600 seconds. 
Using the graph that you have prepared, determine the time required for the SO2Cl2 concentration to drop from 0.100 mol/L to 0.050 mol/L. How long does it take for the concentration to drop from 0.050 mol/L to 0.025 mol/L? What is the order of this reaction? (hint: How is the half-life related to concentration?) 
21. The rate of the chemical reaction between substances A and B is found to follow the equation     rate= k[A]2[B], 
where k is a constant. If the concentration of A is halved, what should be done to the concentration of B to make the reaction go to 75% of its former rate? 
a. The concentration of B should be kept constant 
b. The concentration of B should be doubled 
c. The concentration of B should be tripled 
d. The concentration of B should be halved 
e. The concentration of B should be multiplied by 4/3. 
22. What is the initiating reaction for photochemical smog? 
a. oxidation of NO to NO2
b. reaction of NO with hydrocarbons 
c. photo-dissociation of NO2
d. reaction of NO2 with hydrocarbons 
e. decomposition of N2
23. If doubling the pressure doubles the number of collisions in a sample of gas, one expects the number of productive collisions leading to a chemical reaction to be: 
a. unchanged. 
b. halved. 
c. doubled. 
d. more than halved. 
e. more than doubled. 
24. The reaction of hydrogen and bromine appears to follow the mechanism
             hv
Br2 -------> 2 Br.
Br. + H2 -------> HBr H.
H. + Br2 -------> HBr + Br.
2 Br. -------> Br.2
(a) Identify the initiation step in the mechanism. 
(b) Identify any propagation steps. 
(c) Identify the termination step. 

The mechanism also contains the reaction 
H. + HBr -----> H2 + Br.
How does this reaction affect the rate of production of HBr? 

25. In the upper atmosphere, a layer of ozone, O3, shields the earth from harmful ultraviolet radiation. The ozone is generated by the reactions 
O2 + hv ----> 2 O 
O + O2 ----> O3
Release of chlorofluorocarbon aerosol propellants and refrigerants such as Freon 12 (CCl2F2) into the atmosphere threatens to at least partially destroy the ozone shield by the reaction 
CCl2F2 -----> CClF2 + Cl 
Cl + O3 ----> ClO + O2 (1) 
ClO + O ----> Cl + O2 (2) 
Explain how reactions 1 and 2 constitute a chain reaction that can destroy huge numbers of O3 molecules and O atoms that might otherwise react to replenish the ozone. 

          Answers