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Secondary 4 Pure Chemistry Practice Paper 3

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TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4

TuitionGoWhere Practice Paper (AI)
Version: 3 of 5
Subject: Pure Chemistry
Level: Secondary 4
Paper: Practice Paper (Topic: Acids, Bases and Salts)
Duration: 1 hour 15 minutes
Total Marks: 50

Name: _________________________
Class: _________________________
Date: _________________________

Instructions to Candidates

Write your Name, Class, and Date in the spaces above.
Answer all questions.
Write your answers in the spaces provided on the question paper.
The number of marks is given in brackets [ ] at the end of each question or part question.
A copy of the Periodic Table is printed on page 12 (not included in this extract, assume standard availability).
You may use a calculator.

Section A: Structured Questions (30 Marks)

Answer all questions in this section.

1. The table below shows the pH values of four aqueous solutions, A, B, C, and D.

Solution	pH Value
A	1.0
B	5.5
C	7.0
D	13.0

(a) Which solution is neutral?
_________________________________________________________________________ [1]

(b) Which solution contains the highest concentration of hydrogen ions, $H^+$ ?
_________________________________________________________________________ [1]

(c) Solution D is formed by dissolving sodium hydroxide in water. Write the ionic equation for the formation of the hydroxide ions in this solution.
_________________________________________________________________________ [1]

(d) Solution A is a strong acid, while Solution B is a weak acid. Explain the difference between a strong acid and a weak acid in terms of ionisation.

_________________________________________________________________________ [2]

2. Zinc oxide is an amphoteric oxide.

(a) Define the term amphoteric oxide.

_________________________________________________________________________ [1]

(b) Write balanced chemical equations, including state symbols, for the reaction of zinc oxide with:
(i) Dilute sulfuric acid.
_________________________________________________________________________ [2]

(ii) Aqueous sodium hydroxide.
_________________________________________________________________________ [2]

3. A student investigates the reaction between excess calcium carbonate and dilute hydrochloric acid. The volume of carbon dioxide gas collected is measured every 30 seconds.

(a) Write the balanced chemical equation for this reaction.
_________________________________________________________________________ [2]

(b) The student repeats the experiment using the same mass of calcium carbonate but with a higher concentration of hydrochloric acid.
(i) Sketch the expected graph of volume of $CO_2$ against time for the second experiment on the axes below. Label this line Y. The original experiment is labeled X.

(Imagine axes: Y-axis = Volume of $CO_2$ / $cm^3$ , X-axis = Time / s)
[Graph space: 12 lines] [2]

(ii) Explain, using collision theory, why the initial rate of reaction is higher in experiment Y.

_________________________________________________________________________ [2]

4. Barium sulfate is an insoluble salt.

(a) Name two aqueous solutions that can be mixed to prepare a pure, dry sample of barium sulfate by precipitation.

_________________________ [2]

(b) Describe the steps required to obtain a pure, dry sample of barium sulfate from the mixture in (a).

_________________________________________________________________________ [3]

5. Ammonia is manufactured industrially by the Haber Process.

$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \text{ kJ/mol}$

(a) State the catalyst used in the Haber Process.
_________________________________________________________________________ [1]

(b) Explain why a high pressure (200 atm) is used in this process.

_________________________________________________________________________ [2]

(c) The ammonia produced is often used to make fertilisers such as ammonium nitrate.
(i) Name the acid required to react with ammonia to produce ammonium nitrate.
_________________________________________________________________________ [1]

(ii) Write the balanced chemical equation for this neutralisation reaction.
_________________________________________________________________________ [2]

Section B: Free-Response Questions (20 Marks)

Answer all questions in this section.

6. A white solid, X, is known to be either sodium carbonate or sodium hydrogen carbonate. When heated, solid X decomposes to produce a colourless gas that turns limewater milky.

(a) Identify the gas produced.
_________________________________________________________________________ [1]

(b) Solid X is sodium hydrogen carbonate ( $NaHCO_3$ ). Write the balanced chemical equation for its thermal decomposition.
_________________________________________________________________________ [2]

(c) Describe a chemical test, other than heating, to distinguish between aqueous sodium carbonate and aqueous sodium chloride. Include the reagent used and the expected observations for both solutions.

_________________________________________________________________________ [3]

7. Sulfuric acid is a strong diprotic acid.

(a) Explain what is meant by the term diprotic acid.

_________________________________________________________________________ [1]

(b) 25.0 $cm^3$ of 0.10 mol/ $dm^3$ sulfuric acid is neutralised by aqueous sodium hydroxide.
(i) Calculate the number of moles of sulfuric acid used.
_________________________________________________________________________ [1]

(ii) Calculate the volume of 0.20 mol/ $dm^3$ sodium hydroxide required to completely neutralise the acid.
$H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$

_________________________________________________________________________ [3]

(c) In a separate experiment, ethanoic acid ( $CH_3COOH$ ) is used instead of sulfuric acid. Both acids have the same concentration (0.10 mol/ $dm^3$ ).
Explain why the initial pH of the ethanoic acid solution is higher than that of the sulfuric acid solution.

_________________________________________________________________________ [2]

8. Copper(II) sulfate crystals can be prepared by reacting excess copper(II) oxide with dilute sulfuric acid.

(a) Why is excess copper(II) oxide used?
_________________________________________________________________________ [1]

(b) After the reaction is complete, the mixture is filtered.
(i) What is retained on the filter paper?
_________________________________________________________________________ [1]

(ii) What is contained in the filtrate?
_________________________________________________________________________ [1]

(c) The filtrate is heated to evaporate some water and then left to cool. Crystals form.
(i) Why are the crystals not obtained by evaporating the solution to dryness?

_________________________________________________________________________ [2]

(ii) Suggest a method to dry the crystals after they have been filtered off.
_________________________________________________________________________ [1]

9. The table below shows the results of adding aqueous sodium hydroxide and aqueous ammonia to solutions containing different cations.

Cation	Add NaOH(aq)	Add NH $_3$ (aq)
$Cu^{2+}$	Light blue ppt, insoluble in excess	Light blue ppt, soluble in excess to give deep blue solution
$Fe^{2+}$	Green ppt, insoluble in excess	Green ppt, insoluble in excess
$Al^{3+}$	White ppt, soluble in excess	White ppt, insoluble in excess
$Zn^{2+}$	White ppt, soluble in excess	White ppt, soluble in excess

(a) A solution contains one of the cations listed above.

Adding NaOH(aq) produces a white precipitate.
Adding excess NaOH(aq) causes the precipitate to dissolve.
Adding NH $_3$ (aq) produces a white precipitate.
Adding excess NH $_3$ (aq) causes the precipitate to dissolve.

Identify the cation.
_________________________________________________________________________ [1]

(b) Another solution contains iron(III) ions, $Fe^{3+}$ .
Describe the observation when aqueous sodium hydroxide is added to this solution.
_________________________________________________________________________ [1]

(c) Write the ionic equation for the formation of the precipitate when aqueous sodium hydroxide is added to iron(III) ions.
_________________________________________________________________________ [2]

[End of Practice Paper]

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Answers

TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4 (Answer Key)

Version: 3 of 5
Subject: Pure Chemistry
Topic: Acids, Bases and Salts

Section A: Structured Questions

1.
(a) Solution C [1]
(b) Solution A [1]
(c) $NaOH(s) \rightarrow Na^+(aq) + OH^-(aq)$ OR $H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq)$ is incorrect context; accept dissociation of base:
$NaOH \rightarrow Na^+ + OH^-$ [1]
(Note: Question asks for formation of hydroxide ions in the solution from NaOH. Simple dissociation is expected.)
(d) A strong acid ionises/dissociates completely in water [1]. A weak acid ionises/dissociates only partially in water [1].

2.
(a) An oxide that reacts with both acids and bases to form salt and water [1].
(b)
(i) $ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l)$ [1 for formulae, 1 for balancing/states]
(ii) $ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$ [1 for formulae, 1 for balancing/states]
(Note: Sodium zincate is often written as $Na_2ZnO_2$ or $Na_2[Zn(OH)_4]$ . Both are acceptable at O-Level if balanced.)

3.
(a) $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$ [1 for formulae, 1 for balancing]
(b)
(i) Graph Y should have a steeper initial gradient than X [1] and reach the same final volume (horizontal plateau at same height) [1].
(ii) Higher concentration means more particles per unit volume [1]. This leads to a higher frequency of effective collisions [1].

4.
(a) Barium chloride (or barium nitrate) AND Sodium sulfate (or potassium sulfate/magnesium sulfate) [1 for each correct soluble salt pair containing $Ba^{2+}$ and $SO_4^{2-}$ ].
(b)

Filter the mixture to collect the precipitate [1].
Wash the residue with distilled water [1].
Dry the residue between filter papers or in an oven [1].

5.
(a) Iron [1]
(b) High pressure favours the forward reaction because there are fewer moles of gas on the right (2 moles) than on the left (4 moles) [1]. This increases the yield of ammonia [1].
(c)
(i) Nitric acid [1]
(ii) $NH_3(aq) + HNO_3(aq) \rightarrow NH_4NO_3(aq)$ [1 for formulae, 1 for balancing]

Section B: Free-Response Questions

6.
(a) Carbon dioxide ( $CO_2$ ) [1]
(b) $2NaHCO_3(s) \rightarrow Na_2CO_3(s) + H_2O(l) + CO_2(g)$ [1 for formulae, 1 for balancing]
(c)
Reagent: Dilute acid (e.g., HCl) [1].
Observation for Sodium Carbonate: Effervescence/bubbles of gas produced [1].
Observation for Sodium Chloride: No observable change / No effervescence [1].
(Alternative: Use Calcium Chloride. Carbonate gives white ppt, Chloride gives no ppt. Must specify reagent and both observations.)

7.
(a) A diprotic acid is an acid that can donate two protons ( $H^+$ ions) per molecule [1].
(b)
(i) Moles = $Concentration \times Volume(dm^3)$
$= 0.10 \times \frac{25.0}{1000} = 0.0025 \text{ mol}$ [1]
(ii) From equation, 1 mol $H_2SO_4$ reacts with 2 mol $NaOH$ .
Moles of $NaOH$ required $= 2 \times 0.0025 = 0.0050 \text{ mol}$ [1].
Volume of $NaOH = \frac{Moles}{Concentration} = \frac{0.0050}{0.20} = 0.025 \text{ dm}^3$ [1].
Volume $= 25.0 \text{ cm}^3$ [1 for correct unit/conversion].
(Total 3 marks: 1 for mole ratio, 1 for calc moles NaOH, 1 for final volume)
(c) Ethanoic acid is a weak acid and only partially ionises in water [1], resulting in a lower concentration of $H^+$ ions compared to sulfuric acid (which is strong and fully ionises) [1]. Lower $[H^+]$ means higher pH [1]. (Max 2 marks)

8.
(a) To ensure all the sulfuric acid is reacted / neutralised [1].
(b)
(i) Excess copper(II) oxide [1].
(ii) Copper(II) sulfate solution / Filtrate contains dissolved copper(II) sulfate [1].
(c)
(i) Copper(II) sulfate crystals are hydrated ( $CuSO_4 \cdot 5H_2O$ ). Evaporating to dryness would remove the water of crystallisation, leaving anhydrous white powder, or may cause decomposition [1]. Also, impurities would remain in the solid [1].
(ii) Press between filter papers [1] OR place in a warm oven/desiccator [1].

9.
(a) Zinc ion / $Zn^{2+}$ [1]
(b) Reddish-brown precipitate [1] (Note: Iron(III) hydroxide is reddish-brown. Iron(II) is green.)
(c) $Fe^{3+}(aq) + 3OH^-(aq) \rightarrow Fe(OH)_3(s)$ [1 for formulae/charges, 1 for balancing/state symbols]

[End of Answer Key]