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FTNA 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2 · 2020

27 questions

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Questions (27)

Section A

1. 1 mark

Which decision should be made if the results of an experiment do not support the stated hypothesis? A To use the results as an idea for further testing. B To ignore the results and set a new experiment. C To repeat the experiment in the same way. D To identify a new problem.

A. To use the results as an idea for further testing.
B. To ignore the results and set a new experiment.
C. To repeat the experiment in the same way.
D. To identify a new problem.
2. 5 marks

(b) Answer the given questions by writing the correct answer in the blank spaces provided. (i) In which stage is the hypothesis tested during scientific investigation? ……………………. (ii) On which factor does the physical state of a molecule depend? ……………………. (iii) Which properties depend on the proportions of mixing substances in a mixture? ……………………. (iv) What are the building blocks of matter? ……………………. (v) Which particles are transferred during chemical reactions? …………………….

((a)(i) Match Vertical columns of the Periodic Table with the correct response in List B. 1 mk
((b)(i) What are the building blocks of matter? 1 mk
((a)(v) Match Elements which react vigorously with water with the correct response in List B. 1 mk
((b)(v) Which particles are transferred during chemical reactions? 1 mk

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Section B

3. 5 marks

(b) Differentiate luminous from non-luminous flame by giving five points. S/N Luminous flame Non luminous flame (i) (ii) (iii) (iv) (v)

((a)(i) Give a second reason why candles are not suitable for heating in the laboratory. 1 mk
((b)(i) Point 4 differentiating luminous and non-luminous flame. 1 mk
((b)(v) Point 5 differentiating luminous and non-luminous flame. 1 mk

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4. 5 marks

(b) Classify each of the following elements into their respective groups and periods. S/N Element Group Period (i) Calcium (ii) Hydrogen (iii) Chlorine (iv) Boron (v) Aluminium

((a)(i) State the fourth main idea of Dalton’s atomic theory of matter. 1 mk
((b)(i) Classify Boron into its respective group and period. 1 mk
((a)(v) State the fifth main idea of Dalton’s atomic theory of matter. 1 mk
((b)(v) Classify Aluminium into its respective group and period. 1 mk

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5. 6 marks

Categorize the three factors (variables) which affect the problem being investigated during scientific investigation.

(b(i)) Identify and explain the first factor (variable). 2 mk
(a(i)) Give one field in which the scientific procedure is applied with an example. 2 mk
(a(ii)) Give another field in which the scientific procedure is applied with an example. 2 mk
(b(ii)) Identify and explain the second factor (variable). 2 mk
(b(iii) Identify and explain the third factor (variable). 2 mk

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6. 6 marks

A mass of 20.0 g of petrol was burnt in air. The heat produced was used to heat 2.5 litres of water. Given that the heat value of petrol is 43,640 kJ/kg, by how much the temperature of water could have changed? (The specific heat capacity of water = 4.18 kJ kg-1 K-1, Density of water = 1000 kg/m3).

(a) Atom 3 mk
(b) Calculate the change in temperature of the water. 6 mk
(a(i)) Explain why the use of charcoal is harmful to the environment. 2 mk
(b) Molecule 3 mk
(a(ii)) Explain why charcoal is still being used by the majority of Tanzanians for domestic purposes. 2 mk
(c) Ion 4 mk

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7. 5 marks

Consider the reaction: 2H₂ + O₂ → 2H₂O. Calculate the number of moles of water produced when 4 moles of hydrogen react completely with oxygen.

(none) 5 mk

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8. 10 marks

(a) Giving a reason, state whether rust will form or not in each of the situations (i) - (vi). (i) Iron bar is dipped into boiling water. (ii) Painted iron is dipped into un-boiled water. (iii) Iron bar is dipped in un-boiled water. (iv) Oiled bar is left outside the room over nights. (v) Aluminium wire is dipped in un-boiled water. (vi) A dry iron bar is wrapped with cotton wool. (b) Briefly explain any four methods of preventing rusting. (i) (ii) (iii) (iv)

(none) 10 mk
((a)(i) Giving a reason, state whether rust will form or not in the situation: Oiled bar is left outside the room over nights. 1 mk
((a)(v) Giving a reason, state whether rust will form or not in the situation: A dry iron bar is wrapped with cotton wool. 1 mk
((b)(i) Briefly explain one method of preventing rusting. 2 mk

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