Chemistry Nov/Dec 2008

ANSWER

ANSWER

ANSWER

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      CH3COOH

       OR                            K2Cr2O7/KMnO4                          
CH3CH2OH  +   2[O]     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:

ANSWER

In(a)(i), candidates correctly stated Faraday’s first law of electrolysis that the mass of an element discharged during electrolysis is directly proportional to the quantity of electricity passed.

In (a)(ii), candidates could mention two factors that influence the preferential discharge of ions during electrolysis from among the following:

1. position of the ions in the electrochemical series
2. concentration of the ions in the electrolyte
3. nature of the electrodes.

In (a)(iii), candidates correctly calculated the time as follows:
Ag+(aq)  + e-     Ag (s)  
                        1 mole of electrons Ξ IF   =   96500C   
                       96500C  deposit 1 mole of Ag
                             Q  =   It
                              t   =   96500C
                                            6A
                                  = 16083.3s 

In (a)(iv), candidates correctly gave two applications of electrolysis from the following applications:
            –           extraction of metals

1. purification
2. preparation of some named compounds/elements.

In (b)(i), candidates knew that X is more electropositive.

In (b)(ii), candidates correctly gave the change in oxidation of the less electropositive species as + 2 to O and in (b)(iii) they correctly wrote the half cell equation thus:

I.          X(s)   →    +   X2+((aq)  +   2ҽ-
II.         X2+ (aq) +  2ҽ-   →     Y(s).

In (c)(i), only very few candidates could explain why copper plated iron pillar would corrode when the surface is scratched.  The expected answer was that iron is higher than copper in the electrochemical series hence, more reactive than copper.  When the surface is scratched, an electrolytic cell develops and the iron is oxidized by forming a brown deposit of  iron (III) compound.  In (c)(ii), most candidates correctly gave one advantage of lead acid battery cell over Leclanche dry cell.     

In (d)(i), most candidates correctly wrote an equation for the dissociation of each of  the acids thus:

In (d)(ii), candidates correctly gave the basicity of  H3PO4  and HCOOH as 3 and 1 respectively.   

In (d)(iii), most candidates gave a correct arrangement of the solution in order of increasing pH values as  NH4Cl, KNO3,  Na2CO3.  

ANSWER

In(a)(i), candidates correctly stated the procedures that would be carried out in the laboratory for each of I – III as follows:

            I.          Add washing soda (Na2CO3 10H2O)/pass it through an ion-exchange
                        resin add calculated amount of slaked lime (Ca(OH)2).
           II.          Distillation
          III.         Filtration (accept decantation)/centrifuging.

In (a) (ii), candidates stated correctly that alum is for coagulation of fine/colloidal particles and chlorine is for killing bacteria in a water treatment plant.
  
                                                                                                                                            

In(a)(iii), candidates correctly gave two advantages of soft water over hard water from among the following:

1. it does not waste soap/lathers readily with soap
2. it does not cause furring/scales/deposits in kettles and boilers
3. it is cheaper when used in dyeing and tanning industries.

In (b)(i), candidates gave the name of a metallic oxide which decreases in mass without dissolving in it when exposed to the atmosphere as calcium oxide/copper(II) oxide/magnesium oxide.

In (b)(ii), most of the candidates knew that the phenomenon is hygroscopy.

In (b)(iii), candidates could not write a balanced chemical equation for the reaction between the metallic oxide and dilute trioxonitrate (V) acid.  The expected answer was

                        CaO(s) +  2HNO3(aq),  →   Ca(NO3)2(aq) + H2O 
                                OR
                        CuO(s) +  2HNO3(aq),  →   Cu(NO3)2(aq) + H2O 

In (c)(i), candidates could not identify W, X, Y and Z.

The expected answers were:

W        –           oxygen/ O2(g)
X         –           flat bottom flask
Y          –           conc  H2SO(4)/fused   CaCl2/U-tube
Z          –           mercury

In (c)(ii), candidates knew that manganese (IV) oxide is acting as a catalyst.

In (c)(iii), candidates could not give a balanced chemical equation for the preparation of the gas W
                                    MnO2
   2H2O2(aq)      2H2O(l) +  O2(g)

In (c)(iv), candidates knew that W cannot be collected by the displacement of air because it has nearly the same density with air.                                                                                                                                  

In (d), most candidates did not know that nitrogen is diatomic hence lost marks for using N = 14 instead of N2  =  28.  The expected solution to the question is as follows:

Number of moles of nitrogen  =  mass                   =   2.8
                                                     molar mass               28
                                                                           =   0.10 moles

            1 mole of nitrogen  =   6. 02 x 1023  moles
            0.1 moles of nitrogen will contain 6.02 x1023  x  0.10
                                            =  6.02 x 1022   particles

ANSWER

n(a)(i), candidates correctly gave one example each of I – III from the list of examples thus: 
            I.          FeCl3 /KOH/CaCl2/NaOH/MgCl2/P2O5.
            II.         CuO/CoCl2/conc H2SO4/ silica gel/CuSO4.
            III.       Na2CO3 IOH2O/ Na2SO4  IOH2O /MgSO4   7H2O .

IUPAC names of any of the compound was accepted instead of the formula.

In (a)(ii), candidates gave three products of the destructive of coal from the following:

1. coke
2. ammoniacal liquor
3. coal tar
4. coal gas

In(a)(iii), candidates correctly gave two uses of graphite from among the following uses:

1. as lubricant
2. as electrodes
3. for making lead pencils
4. moderators in nuclear reactors

In(b)(i), candidates correctly wrote a balanced chemical equation for the complete reaction thus:

            H2SO4(aq)  +    2NaOH(aq) →   Na2SO4(aq) +  2H2O(l), 

In(b)(ii), candidates correctly stated that the reaction is neutralization.

In(b)(iii), majority of the candidates could correctly calculate the number of moles of sodium hydroxide required for a complete reaction as follows:

No. of moles of H2SO4   =   20 x 0.125
                                                1000
                                       =    0.0025 moles

1 mole of H2SO4  Ξ       2  moles of NaOH
                                      
∴   0.005 moles of  H2SO4  Ξ       2 x  0 . 0025   moles of NaOH
                                                              1
                                         =    0.005 moles of NaOH.

Other correct method were accepted.
                                                                                                                                           

In (b)(iv), only few candidates could correctly outline how a solid sample of the product of the reaction in (b)(iii) could be obtained.  The expected answer from candidates is as stated below:

1. put solution into evaporating dish
2. heat to concentrate/evaporate
3. obtain the solid/allow to cool and crystallize.

In (c)(i), most candidates could not draw resonance structure of benzene.  The correct structures expected from candidates are as follows:

In (c)(ii), only few candidates knew that benzene undergoes substitution and addition reactions.

In (c)(iii), most candidates gave chemical properties of benzene instead of physical properties:  The expected answers were as follows:

1. colourless liquid
2. less dense than water
3. insoluble in polar solvents/soluble in organic solvents
4. has boiling point of 80ºC.