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Chemistry Paper 3,Nov/Dec 2009  
Questions:         1 2 3       Main
General Comments
Weakness/Remedies
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Question 3

  1. A compound G on heating gives a crackling sound, reddish brown fumes H, a gas J which relights a glowing spling and a residue K.

The residue K is reddish brown when hot and yellow when cold.
            (i)        Identify G, H, J and K.
            (ii) What name is given to the crackling sound? [5 marks]

  1. The following diagram represents a set-up used to determine the heat of neutralization:    

(i)  Identify the apparatus labelled X, Y and Z.
(ii)  State the function of X, Y and Z in the experiment.  [6 marks]

(c)  In the experiment in 3(b), 100 cm# of 0.50 mol dm-3 HCl was reacted with
100 cm3 of 0.50 mol dm-3  NaOH solution.  The following results were recorded
           
Initial temperature of HCl(aq)     (T1)   =  25.0oC 

Initial temperature of NaOH(aq)  (T2)   =  25.2oC

Final temperature of mixture  (T3) 28.5oC;

From the data determine the:
(i) change in temperature;
(ii) total heat evolved;
(iii) heat of neutralization.

Equation for the reaction is

NaOH(aq) + HCl(aq)NaCl(aq)+  H2O(1)

[specific heat capacity of water = 4.2 Jg-1 K-1 ]
[11 marks]

(d) State what would be observed for each of the following reactions:
(i) Ethanol is warmed with acidified K2Cr2O7:
(ii) Propane is bubbled through bromine water;
(iii) Freshly precipitated Fe(OH)2 is left to stand in air for some time.

_____________________________________________________________________________________________________
OBSERVATION

The question was attempted by most candidates and the performance was poor.

In part (a)(i) and (ii), only few candidates correctly identified G , H, J and K and gave the name of the crackling sound because  good number of them lacked laboratory exposure. 
The expected responses from the candidates were as written thus:

(i) G Lead (II) trioxonitrate (V)
HNitrogen (IV) oxide
JOxygen
K Lead (II) oxide
IUPAC name or correct formula was also accepted in place of name

(ii) Decrepitation

In part (b)(i) and (ii), candidates correctly identified X, Y and Z as thermometer, stiner/stirring rod and pads of cotton cool/insulator/lagging material respectively and stated their respective functions in the set-up thus:

Xto take or measure the temperature of the system.
Y – to mix the solutions
Z – to reduce heat loss from the system.

However, some of them lost marks because they could neither correctly identify Z as insulator but wrote “ice block” nor spell rightly each of the apparatus.

In part (c), only very few candidates could determine change in temperature, total heat evolved and heat of neutralization because many of them did not understand the demand of the question.
The expected answers from candidates were as follows

(i)   Average temperature = 25.0 + 25.2    =  25.1o C
2

Change in temperature = (301.5 298.1) K / 28.5o C
=  3.4K/ 3.4o C /3.4

(ii) Total heat evolved = mass x specific heat capacity x change in
temperature
200 g x 4.2 Jg-1K-1 3.4K
2856J

(iii) From the equation:
HCl(aq)+ NaOH(aq) →  NaCl(aq) + H2O(L)

1 mole of HCl reacts with 1 mole of NaOH to form 1 mole of water

100 cm3 of 0.5 mol dm-3 contain 100 x 0.5  = 0.05 mol of HCl
1000
there4; 0.05 mol of acid react with 0.05 mol of alkali to produce 0.05 mol water.
i.e. 0.05 mole of water evolved 2856J of heat

∴ 1 mole of water will evolve 2856
0.05
= 57120J / 57.1kJ mol-1.
Heat of neutralization = 57.1kj mol-1   [check sign to score].

In sub-question (d), most candidates could not correctly state what was observed in (i) – (iii) because they lacked practical exposure hence gave theoretical answers.

The expected answers from candidates were:

(i)Orange K2Cr2O7(aq) turns green

(ii)Bromine water is decolourized/turns colourless

(iii) Turns brown/reddish brown.

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