Home Technical Mathematics Languages Science Social Science Art Literature Arabic Islamic Studies C.R.K HistoryMusicVisual Art Clothing/Textile Home Management Shorthand
 Chemistry Paper 2,Nov/Dec 2008
 Questions: 1 2 3 4 5 6 Main
Weakness/Remedies
Strength

Question 4

(a)     (i)    State Faraday’s  first law of electrolysis.
(ii)    Mention two factors that influence the preferential discharge of ions
during electrolysis.
(iii)    Calculate the time it will take to deposit 1 mole of silver, if a current of 6A
is passed through a solution of silver trioxonitrate (V).
[IF   =    96500C]
(iv)   Give two applications of electrolysis.                                         [11 marks]

(b)      Consider the following redox reaction:

X(s) +  Y2+(aq)    →     X2+(aq)  +   Y(s).

(i)      Which of X and Y is the more electropositive?
(ii)     Give the change in oxidation number of the less electropositive species.
(iii)     Write half cell equation for the
I.   oxidation reaction,
II.   reduction reaction respectively.                                            [4marks]

(c)        (i)      Explain why copper plated iron pillar would corrode when the surface is
scratched.
(ii)      Mention one advantage of lead acid battery cell over Leclanche dry cell.
[4 marks]

(d)       (i)       Write an equation to show the dissociation of each of the following acids:
I.     CH2COOH;
II.     H2CO3 .

(ii)       Give the basicity of each of the following acids:
I.      H3PO4 ;
II.     HCOOH.

(iii)        Arrange the following solutions in order of increasing pH values:
KNO3(aq),   NH4C1(aq),   Na2CO3(aq) .                           [6 marks]

_____________________________________________________________________________________________________
OBSERVATION

This question was attempted by many candidates and the performance was good.
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.