waecE-LEARNING
Chemistry Paper 2 (Essay) ,May/June 2012  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments
Weakness/Remedies
Strength













































































Question 4

  1. (a)   (i)   What is a structural isomer?
           (ii)    Write all the structural isomeric alkanols with the molecular formula C4H10O.
          (iii)    Which of the isomers from 4(a)(ii) above does not react easily on heating
                    with acidified K2Cr2O7?                                                                                    

    (b)  Chlorine reacted with excess pentane in the presence of light.  Chloropentane and a gas
            which fumes on contact with air were produced.

    1.   Write an equation for the reaction.
    2.   Draw the structure of the major product.
    3.   What is the role of light in the reaction?
    4.   If a mixture  of pentane and the major product is heated, which compound would

      distil off first? Give a reason for your answer.

    1.   Write the formula of the main product that would have been formed

      If but -1- ene (C4H8) has been used instead of pentane.                                       [7 marks]

    (c)     Give the name and structural formula of the product which would be formed by hydration
              of each of the following compounds:

    1.  CH3CH(CH3)CH = CH2;
    2. CH2 = CHCOOH.                                                                                                            [4 marks]

    (d)    (i)  Write the structure of the amino acid, CH3CH(NH2)COOH in:
                   I. acidic medium;
                  II. alkaline medium.

        (ii)         On analysis, an ammonium salt of an alkanoic acid gave 60.5% carbon and 6.5%
                     hydrogen.  If 0.309 g of the salt yielded 0.0313 g of nitrogen, determine the empirical
                     formula of the salt.
                         [H = 1.00; C = 12.0; N= 14.0; O=16.0]                                                                        [7 marks]

 

_____________________________________________________________________________________________________
OBSERVATION

         This question was not well attempted by the candidates and the performance was poor.  The
    candidates had difficulty in drawing the structures of the organic compound.
    Also most candidates had low performance because they do not properly understand the
    rubrics of 4(d).

   The expected answers were:

  (a)      (i)         Structural isomers are compounds with the same molecular formula but different
                        arrangement  of constituent atoms/or different structures.                                                                                                                               
           
            (ii)        CH3 CH2 CH2 CH2  OH,          CH3 CHCH2 OH
                                                                                                  
                        CH3 CHCH2 CH3                              CH3
                                  OH                                            (CH3) 3- C- OH
                                                                                                              
            (iii)       (CH3)3 C-OH  
                                                                                                                                            
(b)        (i)         C5 H12 + Cl2 light  C5H11 Cl + HCl    
                                             
            (ii)                          H      H    H     H      H

                                 H      C       C    C     C       C   Cl     


                                         H       H     H    H      H

            (iii)       Light acts as catalyst.  
            (iv)       Pentane C5H12 would come out first.   
Pentane has a lower molar mass (72) as compared to chloropentane
 (C5H11 Cl) (86.5). 
Hence weaker inter molecular forces/van der Waals’ forces.
            (v)        C4 H6 Cl2                                                                                                                             

(c)        (i)                     H        H       H     H   
                         
  H          C        C        C      C       H            
                                    H         
                                                          OH   H
                                     H       C     H
                                                
                                               H

                         3-methyl butan–2–ol/ 3- methyl-2-butanol  

            (ii)        CH3 -  CH -  COOH
                                             
                                    OH
                                        
                         2–hydroxypropanonic acid    

                                                                                                                                            
(d)       (i)         I     CH3 -   CH  COOH      
                                          
                                        + NH3
                                            
                       II    CH3  - CH –COO-

                                          NH2                      
            (ii)        % N  =   0. 0313   x 100
                                      0. 309
                                                      =   10. 1 %     

                        %  O  =  100 – (60.5 + 6.5  + 10.1)                           
                                 =   22. 9 

                        C            H          N        O
                       
                        60.5        6.5      10.1      22.9          
                            12         1         14       16
                        5.04        6.5      0.72      1 43
                        5.04        6.5      0.72      1.43
                        0.72        0.72    0.72      0.72
                        7             9.027  1           1.99

                        Empirical formula
                                                            C7 H9 NO2    

                       
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