2.3     Alkanes

Section A

 

For each of the questions only one of the lettered responses (A-D) is correct.

 

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

 

1.     One of the terminal steps in the photochemical chlorination of methane is:

       A  Cl2                                 2Cl•

       B  CH3•  + CH3•                 CH3 CH3

       C       CH4  +  Cl-                     CH3Cl  +  H+

       D  CH4  +  Cl•                     CH3  +  HCl

 

2.       Which one of the following represents a propagation step in the chlorination of methane?

       A       CH4  +  Cl2                          CH3Cl  +  HCl

              B       CH4  +  Cl•                         CH3•  +  HCl

       C   Cl2                                       2Cl•

              D  CH3• + Cl•                          CH3Cl

 

3.       Which one of the following species is not formed in the photochemical reaction between chlorine and methane?

              A H•

              B   Cl•

              C   HCl

              D CH3CH3

 

4.       Which one of the following is formed in the initiation step in the mechanism of the chlorination of methane?

          A chlorine atoms

          B   hydrogen atoms

          C   hydrogen chloride

          D methyl radicals

 

 

5.    10 cm3 of an equimolar mixture of hydrogen and methane were combusted by exploding with 25 cm3 of pure oxygen.  All volume measurements were made at 20oC and 1 atmosphere pressure.

 

The volume of the mixture (in cm3 ) after the explosion was:

          A 12.5

B 15

C 17.5

D 20

Section B

 

Answer all questions in the spaces provided.

 

1.       Methane reacts with chlorine in the presence of ultraviolet light, the reaction occurring in a series of steps.

 

(a)     The initiation step involves the action of ultraviolet light on chlorine gas.

 

Write an equation and explain what happens to the chlorine.

 

equation                                                                                                                     [1]

      

       explanation                                                                                                              

 

                                                                                                                                     [2]

 

(b)  Write two equations for the second step in this mechanism.

 

1.                                                                                                                                    [1]

 

2.                                                                                                                                   [1]

 

 

(c)     Suggest why traces of ethane are found in the product mixture.

 

                                                                                                                                         

 

                                                                                                                                         

 

                                                                                                                                         

 

2. (a)     Kerosene, which contains a mixture of C11 and C12 hydrocarbons, is obtained from crude oil by fractional distillation.  Explain the term fractional distillation.

 

                                                                                                                                         

 

                                                                                                                                         

 

                                                                                                                                 [2]

 

 

(b)  The hydrocarbons from kerosene can be cracked to form other useful products.

 

(i)     Explain the term cracking.

 

                                                                                                                                         

 

                                                                                                                                     [1]

 

 

(ii) Using the molecule C11H24, write an equation to illustrate cracking.

 

                                                                                                                                     [2]

 

(c)     Another fraction contains several isomers of C7H16.  Draw structures and give systematic names for any two branched chain isomers of C7H16.

 

Structure (i)

 

 

 

 

 

 

Name                                                                                     

 

 

Structure (ii)

 

 

 

 

 

Name                                                                                                                       [4]

 

 

(d)  Hexane is exploded in the internal combustion engine according to the equation

 

2C6H14 + 19O2                                     12CO2 + 14H2O

 

 

 

 

 

  Calculate the molar enthalpy of combustion of C6H14 using the

          bond enthalpies given.

 

                                                                  kJ mol-1

                                                C-C            346

                                                C-H            413

                                                0-H            453

                                                0=0            490

                                                C=0            740

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

enthalpy of combustion                                                               kJ mol-1      [4]