Chemistry · Paper 2A · 2023

3 questions 🇹🇿 NECTA ✓ Official NECTA answers

CSEE 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2A · 2023

3 questions

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Questions (3)

Section A

1. 0 marks

Table: Experimental Data Experiment 4 (f)(i) Plot a graph of volume of RR against time.

Experimental Data Experiment 4
Volume of RR (cm')	Volume of Distilled Water (cm)	Volume of SS (cm')
10	4	10
8	6	10
6		10
4		10

(a) What is the aim of this experiment? 0 mk

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

(b) Complete filling the table. 0 mk

(b) Calculate the average volume of the acid used. 3 mk

(d) Indicating the state symbols of the reactants and products, write a balanced chemical equation for the reaction between RR and SS. (ii) Write the ionic equation for the reaction. 0 mk

(d) Indicating all the state symbols, write a balanced chemical equation for the neutralization reaction between V1 and V2. 3 mk

(e) How does the concentration of RR affect the time for the mark M to disappear? 0 mk

(e) Write an ionic equation for the reaction. 2 mk

(f) Showing your procedures clearly, calculate the concentration in g/dm^3 of the claimed component (sodium hydroxide). 6 mk

(f) What can you conclude from the graph? 0 mk

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2. 13 marks

You are provided with the following: RR: a solution containing 0.2 M sodium thiosulphate (Na₂S₂O₃); SS: a solution containing 1.0 M hydrochloric acid (HCl); Distilled water; Plain paper marked M; Stopwatch. Procedure (i) Mark and measure 4.0 cm³ of solution RR and add to it 50 cm³ of distilled water. (ii) Place the beaker containing RR and distilled water on the paper marked M. (iii) Swirl the contents and add a drop of solution SS to the beaker. The contents of the beaker is then placed on the mark M on the plain paper. (iv) Switch off the stop watch when the mark M disappears. (v) Record the time taken for the letter M to disappear. (vi) Repeat the experiment using different data as shown in the following table.

(a) What is the purpose of the stopwatch in this experiment? 1 mk

(b) Write down the balanced chemical equation for the reaction occurring in the beaker. 3 mk

((a)(i) Divide the solution obtained in (ii) into three portions. To the first portion, add a few drops of aqueous sodium hydroxide. To the second portion, add a few drops of aqueous ammonia. To the third portion, add a few drops of barium chloride solution followed by dilute hydrochloric acid. 15 mk

(d) Explain why the mark M disappears. 2 mk

(e) If the concentration of HCl was increased, how would it affect the time taken for the mark M to disappear? Explain your answer. 3 mk

(f) Calculate the initial rate of reaction in the first experiment (using the data from the table) in mol dm^-3 s^-1. 5 mk

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