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Secondary 3 Chemistry Practice Paper 2

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TuitionGoWhere Practice Paper - Chemistry Secondary 3

TuitionGoWhere Practice Paper (AI)

Subject: Chemistry
Level: Secondary 3 (Express)
Paper: Practice Paper 2 (Version 2 of 5)
Duration: 1 hour 15 minutes
Total Marks: 50

Name: __________________________
Class: __________________________
Date: __________________________

Instructions to Candidates

Write your name, class, and date in the spaces provided.
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 view, assume standard access).
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 different aqueous solutions, P, Q, R, and S.

Solution	pH Value
P	1.0
Q	5.5
R	7.0
S	13.0

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

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

(c) Solution S is added to Solution P until the pH of the mixture becomes 7.0.
(i) Name the type of reaction that occurs.
........................................................................................................................... [1]
(ii) Write a balanced chemical equation for the reaction between hydrochloric acid and sodium hydroxide, including state symbols.
........................................................................................................................... [2]

2. A student investigates the reaction between excess calcium carbonate and dilute hydrochloric acid. The equation for the reaction is:

$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$

(a) Describe the observation when calcium carbonate is added to the acid.
........................................................................................................................... [1]

(b) The student repeats the experiment using the same mass of calcium carbonate but with a higher concentration of hydrochloric acid.
Explain, in terms of particles, why the rate of reaction increases.
...........................................................................................................................
...........................................................................................................................
........................................................................................................................... [2]

(c) The student wants to prepare a pure, dry sample of calcium chloride crystals.
(i) Why is titration not a suitable method for preparing this salt?
........................................................................................................................... [1]
(ii) Describe the steps required to obtain pure, dry calcium chloride crystals from the reaction mixture in (a).
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
........................................................................................................................... [3]

3. Zinc oxide is an amphoteric oxide.

(a) Define the term amphoteric.
........................................................................................................................... [1]

(b) Write balanced chemical equations for the reactions of zinc oxide with:
(i) Dilute sulfuric acid.
........................................................................................................................... [2]
(ii) Aqueous sodium hydroxide. (Note: The product is sodium zincate, $Na_2ZnO_2$ , and water).
........................................................................................................................... [2]

4. Ammonium sulfate, $(NH_4)_2SO_4$ , is a common fertiliser.

(a) Calculate the relative molecular mass ( $M_r$ ) of ammonium sulfate.
(Relative atomic masses: $H = 1, N = 14, O = 16, S = 32$ )

Answer space

$M_r =$ ................................................... [2]

(b) Ammonium salts decompose when heated with alkalis.
Describe the test for ammonia gas, including the observation.
...........................................................................................................................
........................................................................................................................... [2]

(c) Explain why farmers should not add calcium hydroxide (lime) to soil at the same time as applying ammonium sulfate fertiliser.
...........................................................................................................................
...........................................................................................................................
........................................................................................................................... [2]

5. Barium sulfate is an insoluble salt.

(a) Name two aqueous solutions that can be mixed to prepare a precipitate of barium sulfate.

...................................................
................................................... [2]

(b) Write the ionic equation for this precipitation reaction, including state symbols.
........................................................................................................................... [2]

(c) After the precipitate forms, it must be purified.
Explain why the precipitate is washed with distilled water.
........................................................................................................................... [1]

Section B: Free-Response Questions (20 Marks)

Answer all questions in this section.

6. A student is given three unlabelled bottles containing white solids. The solids are known to be:

Sodium chloride
Zinc carbonate
Copper(II) sulfate (anhydrous)

(a) Describe how the student can identify each solid using only distilled water. Include observations for each.

Answer space

...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
........................................................................................................................... [4]

(b) The student takes the solution of sodium chloride identified in (a) and performs electrolysis using inert electrodes.
(i) Name the product formed at the anode.
........................................................................................................................... [1]
(ii) Name the product formed at the cathode.
........................................................................................................................... [1]
(iii) Write the half-equation for the reaction at the anode.
........................................................................................................................... [1]

7. Sulfuric acid is a strong diprotic acid. Ethanoic acid is a weak monoprotic acid.

(a) Explain the difference between a strong acid and a weak acid in terms of ionisation.
...........................................................................................................................
...........................................................................................................................
........................................................................................................................... [2]

(b) Both acids have a concentration of $1.0 \text{ mol/dm}^3$ .
(i) Which acid has a lower pH? Explain your answer.
...........................................................................................................................
........................................................................................................................... [2]
(ii) Describe a simple chemical test (other than using a pH meter or indicator) to distinguish between the two acids based on their rate of reaction. Include the reagent used and the expected observations.

Answer space

(c) Sulfuric acid is used in the manufacture of fertilisers. One step involves the reaction of ammonia with sulfuric acid.
(i) Write the balanced chemical equation for this reaction.
........................................................................................................................... [2]
(ii) Why is ammonia gas not directly added to soil as a fertiliser?
........................................................................................................................... [1]

8. Magnesium reacts with hydrochloric acid to produce hydrogen gas.

$Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$

In an experiment, $0.12 \text{ g}$ of magnesium ribbon was added to $50 \text{ cm}^3$ of $1.0 \text{ mol/dm}^3$ hydrochloric acid. The volume of hydrogen gas produced was measured every 10 seconds.

(a) Calculate the number of moles of magnesium used.
( $A_r: Mg = 24$ )

Answer space

Moles of Mg = ................................................... [2]

(b) Determine which reactant is in excess. Show your working.
(Molar volume of gas at r.t.p. = $24 \text{ dm}^3/\text{mol}$ )

Answer space

Volume = ................................................... $\text{cm}^3$ [2]

End of Paper

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Answers

TuitionGoWhere Practice Paper - Chemistry Secondary 3

Answer Key & Marking Scheme (Version 2)

Section A: Structured Questions

1.
(a) R [1]
(b) P [1]
(c)
(i) Neutralisation [1]
(ii) $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$ [2]
(1 mark for correct formulae, 1 mark for balancing and state symbols)

2.
(a) Effervescence / Bubbles of gas produced; Solid dissolves (eventually). [1]
(Accept "fizzing")
(b) Higher concentration means more acid particles per unit volume [1]; leading to more frequent collisions between acid particles and calcium carbonate particles [1]. [2]
(c)
(i) Calcium carbonate is insoluble in water / is a solid base (not an alkali) [1]. Titration is used for soluble bases (alkalis).
(ii) Filter the mixture to remove excess calcium carbonate [1]; Heat the filtrate to the point of crystallisation (saturation) [1]; Allow to cool to form crystals [1]; Dry crystals between filter papers / in a warm oven [1].
(Max 3 marks. Must mention filtration of excess solid first.)

3.
(a) An oxide (or hydroxide) that reacts with both acids and bases (alkalis) to form salt and water. [1]
(b)
(i) $ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l)$ [2]
(1 mark for formulae, 1 mark for balancing)
(ii) $ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$ [2]
(1 mark for formulae, 1 mark for balancing)

4.
(a) $N: 14 \times 2 = 28$ ; $H: 1 \times 8 = 8$ ; $S: 32 \times 1 = 32$ ; $O: 16 \times 4 = 64$ .
Total = $28 + 8 + 32 + 64 = 132$ [2]
(1 mark for correct working/summing parts, 1 mark for final answer)
(b) Warm the salt with aqueous sodium hydroxide (or alkali) [1]; Ammonia gas is produced which turns damp red litmus paper blue [1]. [2]
(c) Calcium hydroxide is a base/alkali [1]; It will react with ammonium ions to release ammonia gas [1]; This causes loss of nitrogen from the fertiliser (reducing effectiveness) [1].
(Max 2 marks. Must link base + ammonium $\rightarrow$ ammonia gas loss.)

5.
(a) Any soluble barium salt (e.g., Barium chloride, Barium nitrate) [1] AND Any soluble sulfate (e.g., Sodium sulfate, Potassium sulfate, Dilute sulfuric acid) [1]. [2]
(b) $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$ [2]
(1 mark for correct ions, 1 mark for state symbols and balancing)
(c) To remove soluble impurities / residual ions from the filtrate (e.g., sodium ions, chloride ions) [1].

Section B: Free-Response Questions

6.
(a) Add distilled water to a sample of each solid.

Copper(II) sulfate: Dissolves to form a blue solution. [1]
Zinc carbonate: Does not dissolve (remains as a white solid/suspension). [1]
Sodium chloride: Dissolves to form a colourless solution. [1]
(1 mark for correct observation for each. Must distinguish between blue solution, colourless solution, and insoluble.)
(b)
(i) Chlorine [1]
(ii) Hydrogen [1]
(iii) $2Cl^- \rightarrow Cl_2 + 2e^-$ [1]

7.
(a) A strong acid is fully ionised/dissociated in water [1]; A weak acid is partially ionised/dissociated in water [1]. [2]
(b)
(i) Sulfuric acid [1]; Because it produces a higher concentration of hydrogen ions ( $H^+$ ) due to full ionisation (and is diprotic) [1]. [2]
(ii) Add a reactive metal (e.g., Magnesium ribbon) or a carbonate (e.g., Sodium carbonate) to equal volumes/concentrations of each acid [1]; The strong acid (sulfuric) will react more vigorously / produce bubbles faster [1]; The weak acid (ethanoic) will react more slowly / produce bubbles slower [1]. [3]
(Alternative: Use pH paper. Strong acid pH ~0-1, Weak acid pH ~3-4. But question asks for rate-based distinction implicitly by mentioning "rate of reaction" in prompt context, though "simple chemical test" allows pH. However, "rate of reaction" distinction is better tested with Mg/Carbonate. If student uses pH paper, award marks if they correctly identify pH difference.)
(Correction: The question asks to distinguish based on rate. So Mg/Carbonate is the expected answer.)
(c)
(i) $2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4$ [2]
(1 mark for formulae, 1 mark for balancing)
(ii) Ammonia is a gas / volatile / easily lost to the atmosphere [1]; Or it is too alkaline/caustic for direct soil application in high concentrations. [1]

8.
(a) Moles = $\frac{\text{Mass}}{M_r} = \frac{0.12}{24} = 0.005 \text{ mol}$ [2]
(1 mark for substitution, 1 mark for answer)
(b)
Moles of HCl = $\text{Conc} \times \text{Vol} = 1.0 \times \frac{50}{1000} = 0.05 \text{ mol}$ [1]
From equation: 1 mol Mg reacts with 2 mol HCl.
0.005 mol Mg requires $0.005 \times 2 = 0.01 \text{ mol}$ HCl [1].
Since 0.05 mol HCl is available (which is > 0.01 mol), HCl is in excess [1].
(Therefore Mg is the limiting reactant.) [3]
(c)
From equation: 1 mol Mg produces 1 mol $H_2$ .
0.005 mol Mg produces 0.005 mol $H_2$ [1].
Volume = $\text{Moles} \times 24 \text{ dm}^3 = 0.005 \times 24 = 0.12 \text{ dm}^3$ [1].
Convert to $\text{cm}^3$ : $0.12 \times 1000 = 120 \text{ cm}^3$ [1].
(Wait, marks allocation: 2 marks total. 1 mark for moles of gas, 1 mark for volume conversion/calc.)
Volume = 120 $\text{cm}^3$ [2]