THE WEST AFRICAN EXAMINATION COUNCIL

West African Senior School Certificate Examination

MODEL ANSWERS


November 2005

Subject: Chemistry

Section I (Theory)


Question

1 (a) Elements X and Y are represented as 4020X and 168Y respectively:

  • the electronic configuration using the s,p,d, notation;
  • the number of unpaired electrons present in the atom;
  • the orbital in which the valence electrons are found;
  • the group and period to which the element belongs.
  • Chlorine can be represented as 3517Cl and 3717Cl
    • What name is given to these two types of chlorine atoms;
    • Explain why the two atoms are chemically active;
    • Name another element that exhibits the same property

(c) (i) Distinguish between nuclear fission and nuclear fusion;

(ii) Balance the following equations:

I. 23892U ------------> 23490Th + Y

II. 214?Pb ---------> 21483Bi + 0-1Z III. ?7N + 10n -------> C + 11H





(d) (i) Give the reason why electrovalent liquids have higher boiling points than covalent liquids;

(ii) Calculate the number of molecules in 3.2g of oxygen gas. [O=16; Avogadro's

constant = 6.02 x 1023 ]

Solution

1. (a) (i) I. 1s2 2s2 2p6 3s2 3p6 4s2

1s2 2s2 2p4

II. 4020X -nil unpaired electron; 168Y - 2 unpaired electrons;

III. 4020X -s-orbital; 168Y - p-orbital;

IV -Group II because there are two valence electrons and 4th period since the

fourth energy level is being filled.

(ii) Electrovalent (Ionic) bonding;

X would transfer the two valence electrons to attain the octet configuration of Ar,

while Y receives the two electrons to become iso-electric with Ne.



(b) (i) Isotopy;

(ii) They share the same atomic number; the chemical property is dictated to a large

extent by the number of electrons in the outer shell and not the atomic mass;

(iii) Carbon (146C and 126C) or Bromine (8035Br and 7935Br) or Uranium (23592U

and 23892U) -----[any would score]

(c) (i) Nuclear fission is the process by which a heavy nucleus is split into two smaller

nuclei of roughly equal mass and charge with the release of energy and radiation

23592U + 10n --------------> 14156Ba + .9236Kr + 3 10n

Nuclear fusion, on the other hand, is the union of two light nuclei to form a

heavier nucleus with the liberation of large amount of energy and radiation.

31H + 21H ---------> 42He + 10n



(ii) I. 23892U ------------> 23490Th + 42Y

II. 21482Pb ---------> 21483Bi + 0-1Z

III. 147N + 10n -------> 146C + 11H





(d) (i) Electrovalent liquids have higher boiling points than covalent liquids because of the strong electrovalent bonds existing between the ions.

(ii) O = 16

O2 = 32

32g -------------------1 mole of O2

3.2g ----------------------3.2/ 32 ---------0.1 mole

Number of molecules = 0.1 x 6.02 x 1023

= 0.602 x 1023

= 6.02 x 1022

[To be concluded]

Question:

2 (a) (i) State two natural phenomena which can be explained by the kinetic theory of matter.

(ii) Consider the following equilibrium reaction:

Fe3+(aq) + SCN- <------------> [Fe (SCN)]2+

yellow   colourless     Blood-red colour

State with reason what would be observed when the following substances are

added separately to the reaction represented by the equation above:

I. Few crystals of KSCN

II. Few crystals of Fe (NO3)3

(b) (i) State Gay-Lussac's law of combining volumes

(ii) Calculate the volume of CO and O2 which will combine to give 4 dm3 of CO2 at constant temperature and pressure.

(c) Consider the reaction represented by the following equation:

2H2O2 (g)--------------> 2H2O(g) + O2 (g)

(i) State what would be observed if MnO2 is added to H2O2(aq) in small quantity and

then in excess;

(ii) Explain how MnO2 affects the rate of decomposition of H2O2.

(d) (i) Draw the diagram for the electroplating of an iron spoon with silver indicating the

anode, cathode, and electrolyte used;

(ii) What is an electrochemical cell?

Consider the following half-cell reactions:

Fe2+(aq) + 2e- ----------->Fe(s)

Cu2+(aq) +2e- -------------> Cu(s)

Write the cell reaction.

Solution

2 (a) (i) Diffusion & Boiling

Evaporation, osmosis, Brownian motion and Diffusion will also score.

(ii) The colour of blood-red will predominate because equilibrium position shifts to

the right with the addition of KSCN;

Blood-red colour will also be observed for the same reason (stated above)

  • Gay Lussac's law of combining volume states that when gases combine, they do so in volume which bear simple ratio to one another and to the product (if gaseous), provided temperature and pressure remain constant.

(ii) 2CO + O2 -------------> 2CO2

2vol 1vol 2vol

4dm3 : 2dm3 : 4dm3

4dm3 of CO would combine with 2dm3 of O2 to give 4dm3 of CO2



  • When MnO2 is added to H2O2 in small quantity, the decomposition to oxygen is increased. The MnO2 acts as a positive catalyst. Excess quantity of the catalyst (that is MnO2) is not an advantage since it is only needed in small amounts.

MnO2 affects the rate of decomposition by lowering the activation energy of the

reaction. [Activation energy is the minimum energy that is required for a reaction

to take place or the energy that must be surmounted before a reaction can occur]



(d)

(i)

electroplating

(ii) Electrochemical cell is a device that converts chemical energy to electrical

energy.

Fe + Cu2+ ------------------>Fe2+ +Cu

Fe------------->Fe2+ + 2e-

Cu2+ +2e- ------------> Cu

© 2005All rights reserved. The questions are provided courtesy of WAEC. The model answers are supplied by Kingsley Idiagbor and so any mistake should not be traced to WAEC..







Back | Home  |Top of Page  |