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
K Lead (II) oxide
IUPAC name or correct formula was also accepted in place of name
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
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
200 g x 4.2 Jg-1K-1 3.4K
(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
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
= 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.