Chemistry · Paper 2C · 2021

3 questions 🇹🇿 NECTA ✓ Official NECTA answers

CSEE 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2C · 2021

3 questions

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Questions (3)

Section A

1. 14 marks

The rate of reaction can be affected by concentration of reactants. The following experiment was conducted to investigate the effect of concentration on the rate of reaction between sodium thiosulphate solution and nitric acid.

Experimental Data
Experiment	Volume of Distilled water (cm3)	Volume of W (cm3)	Volume of V (cm3)
(i)	2	5	3
(ii)	3	5	2
(iii)	4	5	1
(iv)	5	5	0

(a) What was the volume of pipette used? 1 mk

(a) Complete the table by filling the empty column. 3 mk

(b) Write a balanced chemical equation for the reaction between sodium thiosulphate and nitric acid. 3 mk

(b) What was the colour change at the end point of titration? 2 mk

(d) Plot a graph of the volume of $Na_{2}S_{2}O_{3}$ (cm$^{3}$), y-axis against time (s), x-axis. 4 mk

(d) Showing your procedure clearly, determine the value of X (the number of molecules of water of crystallization) in the formula ($Na_2CO_3 \cdot XH_2O$). 19 mk

(e) What conclusion can you draw from the graph regarding the disappearance of the letter X? 2 mk

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Section B

2. 25 marks

Determine the effect of concentration on the rate of chemical reaction. The chemicals provided are labelled as W containing 79.0 g of sodium thiosulphate in 0.5 litre of solution and V containing 6.3 g/dm3 nitric acid solution. You are also provided with a stopwatch and a piece of white paper marked X on which a 50 cm3 beaker containing the reaction mixture. Proceed as follows: (i) Place a 50 cm3 beaker on top of letter X in such a way that the letter X is visible when viewed from above. (ii) Using measuring cylinder, measure 2 cm3 of solution W and pour it into a 50 cm3 beaker. Add 3 cm3 of distilled water. (iii) Measure 5 cm3 of solution V and put it into a 50 cm3 beaker containing solution W and distilled water and immediately start a stopwatch. (iv) Swirl the contents in the 50 cm3 beaker and stop the stopwatch when

(a) Complete the table by filling the empty column. 0 mk

(i) Place a 50 cm³ beaker on top of letter X in such a way that the letter X is visible when viewed from above. 0 mk

(ii) Using measuring cylinder, measure 2 cm³ of solution W and pour it into a 50 cm³ beaker. Add 3 cm³ of distilled water. 0 mk

(b) Write a balanced chemical equation for the reaction between sodium thiosulphate and nitric acid. 0 mk

(iii) Measure 5 cm³ of solution V and put it into a 50 cm³ beaker containing solution W and distilled water and immediately start a stopwatch. 0 mk

(iv) Swirl the contents in the 50 cm³ beaker and stop the stopwatch when 25 mk

(d) Plot a graph of the volume of Na2S2O3 (cm3), y-axis against time (s), x-axis. 0 mk

(e) What conclusion can you draw from the graph regarding the disappearance of the letter X? 0 mk

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