Heep Woh College

Form 5 Chemistry Quiz 7

Chapter 24 Part II : Alkane and Alkene

Marking Scheme

 

 

 

 

 

 

 

 

 

 

When the pumice stone was heated as shown in the above diagram, some water droplets were collected on the sides of the heated glass tube and a gas Y was formed which was collected over water.

Gas Y reacted with the solution of bromine in tetrachloromethane to form a colourless liquid, Z. Chemical analysis showed that Y was a pure hydrocarbon.

 

(a)  Explain why the rocksil was not heated directly?                                                   [2]

 

Prevent too rapid / fast vaporization of compound Y                                            1

        which would pass over the catalyst without being cracked.                              1

 

(b)  What is the function of the heated pumice stone?                                              [1]

 

Catalyst / speed up the reaction                                                                                 1

 

(c)  Liquid X is an unsaturated hydrocarbon contains 6 carbon and 12 hydrogen atoms.

(i)   What is the meaning of “unsaturated”.                                                                   [1]

 

Double / triple bond exists                                                                                      1

 

(ii)   Write the structural formulae for any THREE possible isomers liquid X.      [3]

 

       CH₃CH₂CH₂CH₂CH=CH₂                   CH₃CH₂CH₂CH=CHCH₃

       CH₃CH₂CH=CHCH₂CH₃                CH₃CH₂CH₂C(CH₃)=CH₂;

       CH₃CH₂CH=CH(CH₃)₂                    CH₃CH=CH₂CH(CH₃)₂

Accept any possible answers                                  (Any THREE)                       3

 

(iii)   Use any ONE compound in (c) (ii), write an equation to show the formation of gas Y                                                                                                                    [1]

 

C₆H₁₂ ¾® 2CH₃CH=CH₂                                                                                          1

OR  C₆H₁₂ ¾® 3CH₂=CH₂                                                                                                (1)

 

 

(d)  Give IUPAC name and structural formula for liquid Z.                                          [2]

 

        CH₃CHBrCH₂Br                  1,2-dibromopropane                                                         1+1

OR  CH₂BrCH₂Br                         1,2-dibromoethane                                                               (1+1)

 

(e)  (i)    Why “over-water” is used to collect gas Y?                                                     [1]

 

               Gas Y is insoluble in water.                                                                                     1

 

        (ii)   Suggest another possible method to collect gas Y.                                             [1]

 

               Use syringe                                                                                                                  1

               (NOT accept downward delivery)

 

(f)  (i)   What would be observed when X was shaken with acidified potassium permanganate?                                                                                                   [1]

 

               Turn from purple to colourless / brown ppt.                                                       1

 

        (ii)   Give IUPAC name and structural formula for the product.                        [2]

 

Hexane-1,2-diol      CH₃CH₂CH₂CH₂CH(OH)CH₂OH                                         1

               Accept other possible answers.

 

(iii)  Give an equation for this reaction.                                                                      [1]

 

                CH₃CH₂CH₂CH₂CH=CH₂ + H₂O + [O] ® CH₃CH₂CH₂CH₂CH(OH)CH₂OH     1

               Accept other possible answers.

 

(g)  Before stopping the experiment, delivery tube should be removed from water first. Why?                                                                                                                           [2]

 

        Prevent sucking back of water                                                                                     1

        which may crack / break the hot reaction tube                                                     1

 

(h)  State ONE use for this experiment in industry and ONE use for gas Y.             [2]

 

To balance the supply and demand of different petroleum fractions / To produce more lower fractions.                                                                                        1

 

        Propene: make plastic / polypropene                                                                      1

 

 

 

 

The End