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Chemistry Paper 2,Nov/Dec 2008  
Questions:   1 2 3 4 5 6   Main
General Comments

Question 3

(a)       Consider the following reaction scheme:






           (i)   What processes are represented by X and Y respectively?
          (ii)    State the reagent used in each of the processes.
         (iii)    Write an equation for the complete oxidation of CH3CH2OH
                                                                                                                                [6 marks]
(b)      Consider the following molecule:          
           (i)   Draw the structure of the molecule showing atoms and bonds.
           (ii)  Give IUPAC name of the molecule.
          (iii)  List the constituents in the molecule.
          (iv)  Give a test for the functional group in the molecule.                           [7 marks]

(c)      A hydrocarbon with a vapour density of 28 decolourized bromine water, but does
          not form a precipitate with ammoniacal copper (I) chloride.
          (i)     Deduce the molecular formula of the hydrocarbon.
          (ii)    Draw the possible structures of the hydrocarbon.
         (iii)    Name three of the structures drawn in 3(c)(ii) above.
                                   [ H   =   1,   C  =  12]                                                                                                                                   


This question was attempted by most candidates and the performance was fair.

In (a)(i), candidates correctly stated that X and Y represented dehydration and chlorination/addition reaction respectively.
In (a)(ii), candidates correctly stated the reagent used in each of the processes X and Y  as follows:

            X     -     (conc) H2SO4/alumina/CaO/H3PO4/   KH3SO4;
            Y     -      Cl2 or chlorine
In (a)(iii), candidates wrote the equation for complete oxidation of CH3CH2OH thus:

                      acidified K2Cr2O7/KMnO4                           
 CH3CH2OH  oo    CH3COOH

       OR                            K2Cr2O7/KMnO4                          
CH3CH2OH  +   2[O]  ll   CH3COOH  +   H2O

In (b)(i) most candidates correctly drew the structure of the molecule showing atoms and bonds thus:







In (b)(ii), only few candidates could correctly give the IUPAC name of the molecule as
2 – chloro – 3 – methylbutanoic acid.

In (b)(iii), candidates could not list the substituents in the molecule.  The substituents are chloro (Cl) and methyl (CH3).

In (b)(iv), candidates knew that when  NaHCO3/ Na2CO3 is added, there is effervescence or evolution of a colourless, odourless gas which turns lime water milky or on addition of ethanol and conc H2SO4  to the substance followed by warming, a sweet/fruity smell of alkanoate is formed or when magnesium ribbon is put in the substance, there is effervescence of a colourless, odourless gas which gives a pop sound with lighted taper.

In (c)(i), candidates correctly deduced that the hydrocarbon is either an alkene or a non – terminal alkyne. 

Hence they deduced the formula as follows:

R. M. M.            =       2  x   vapour density
                           =      2  x   28
                           =      56

The general formula of alkene or non-terminal alkyne I
                       CnH2n               or        CnH2n-2             
                  ∴ 14n   =   56      or    14n  -  2   =  56
                          n    =   4       or    n is not a whole number
Hence, molecular formula   =    C4H8.

In (c)(ii) and (iii), only very few candidates could correctly draw and name the possible structures of the hydrocarbon thus:






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