Chemistry Paper 2 (Essay) ,May/June 2012

Questions:		1	2	3	4	5	6	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 3

(a)    (i)    Define nuclear fission
        (ii)    A certain natural decay series starts with and ends with .
                 Each step involves the loss of an alpha or a beta particle. Using the given information,
                deduce how many alpha and beta particles were emitted.                                       [5marks]

(b)    Consider the equilibrium reaction represented by the following equation:
         A2(g) + 3B2(g)                    2AB3(g);    H   = + kJmol-1
         Explain briefly the effect of each of the following changes on the equilibrium composition:

increase in concentration of B;
decrease in pressure of the system;
addition of catalyst. [5marks]

Compound

NaF

NaBr

NaI

Lattice energy/kJmol-1

890

719

670

Explain briefly the trend. [3 marks]

(d) State the property exhibited by nitrogen (IV) oxide in each of the following reactions:

(i) 4Cu + 2NO2 4CuO + N2;
(ii) H2O+ 2NO2 HNO3 + HNO2. [3marks]

(e) Iron is manufactured in a blast furnace using iron ore (Fe2O3), coke and limestone.
Write the equation for the reaction(s) at the:

top of the furnace;
middle of the furnace;
bottom of the furnace. [5 marks]

(f) (i) Name two products of destructive distillation of coal.
(ii) Give one use of each product in 3(f)(i). [4 marks]

_____________________________________________________________________________________________________

OBSERVATION

3(a)(i) and (ii): This question was attempted by majority of the candidates and the performance was poor. In this section most candidates were unable to correctly define nuclear fission. Most candidates omitted the word “heavy” nucleus and ‘large’ energy in the definition. Most candidates could not write the balanced nuclear equation but mention the numbers of alpha and beta particles emitted.

In b(i), (ii) and (III) majority of the candidates were not able to respond appropriately to questions on equilibrium. Most candidates lost marks by not further stating the effect of each increase, decrease and addition of catalyst.

In (c) majority of the candidates had no answer to this. In (d)(i) and (ii) most candidates got (i) correctly but lost mark in (ii) because they only mention oxidizing agents.
In (e) most candidates lost marks because they could not identify the equation at the top, middle and bottom of the furnace.

In (f) most candidates correctly name two product of the destructive distillation of coal and gave one use of each product.

The expected answers were:

(a) (i) Is the splitting up of heavy nucleus into smaller nuclei accompanied by
the release of large amount of energy

(ii) Let x and y represent number of alpha and beta particles emitted respectively.

                          U Th + x He + y e

                   By inspection
u Th + 2 He + 2 e


                      \    a particles         = 2
                                Beta particles    = 2

(b)       (i)         Equilibrium position shifts to the right and more of AB3 is formed/
                        decrease in concentration of A2.
(ii)        Equilibrium shift to the left and more of A2 and B2 formed/ decrease in                         concentrate of AB3
            (iii)       No effect. Catalyst affects both forward backward reactions equally.

(c)         For a constant cation Na+, lattice energy decreases with increase in size of the anion/ decrease in electronegativity of anion.
             F-,Br-, I- increasing order of size. Hence the decreasing order of lattice energy.
                ®

(d)       (i)         NO2 is an oxidizing agent.
           (ii)         NO2 is acting as an oxidizing as well as reducing agents or acid anhydride.

(e)      (i)          Fe2O3 + 3CO ® 2Fe + 3CO2
          (ii)          CaCO3    ®     CaO   + CO2
                        CaO + SiO2 ® CaSiO3
          (iii)         C + O2      ® CO2
                        CO2 + C ® 2CO

(f)      (i)           Coke/Coal tar/Coal gas/Ammonical liquor

          (ii)          Coke        -       solid fuels/production of water gas, extraction of metals.
Coal tar   -       synthesis of chemicals - perfumes, dyes, paints, drugs, plastic and explosives etc.
Coal gas -       as fuels in homes and industries.
                        Ammonical liquor - production of (NH4)2 SO4 for fertilizer.