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Secondary 3 Chemistry Semestral Assessment 2 (End of Year) Paper 5

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TuitionGoWhere Practice Paper - Chemistry Secondary 3 (SA2)

TuitionGoWhere Exam Practice (AI)

Subject: Chemistry
Level: Secondary 3 (Express)
Paper: SA2 Practice Paper (Version 5 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 above.
Answer all questions.
Write your answers in the spaces provided in this booklet.
The number of marks is given in brackets [ ] at the end of each question or part question.
A copy of the Periodic Table is provided on page 12 (not included in this extract).
You may use a calculator.

Section A: Structured Questions (35 Marks)

Answer all questions in this section.

1. Calcium oxide is commonly used in agriculture to treat acidic soil. (a) State the chemical formula of calcium oxide. [1] ........................................................................................................................................

(b) Explain, with the aid of a chemical equation, how calcium oxide increases the pH of soil. [2] ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

2. A student investigates the reaction between dilute sulfuric acid and excess zinc powder. (a) Describe two observations the student would make during this reaction. [2]

.................................................................................................................................
.................................................................................................................................

(b) The student repeats the experiment using the same mass of zinc but with excess dilute hydrochloric acid of the same concentration. State and explain how the initial rate of reaction compares to the first experiment. [2] ........................................................................................................................................ ........................................................................................................................................

3. Barium sulfate is an insoluble salt used in medical imaging. It can be prepared by mixing aqueous barium chloride and aqueous sodium sulfate. (a) Write the ionic equation for this precipitation reaction, including state symbols. [2] ........................................................................................................................................

(b) Describe the steps required to obtain a pure, dry sample of barium sulfate from the reaction mixture. [3] ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

4. Ammonia is manufactured industrially via the Haber Process. $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \text{ kJ/mol}$ (a) Define the term dynamic equilibrium. [2] ........................................................................................................................................ ........................................................................................................................................

(b) Explain why a high pressure is used in the Haber Process, referring to both yield and rate. [2] ........................................................................................................................................ ........................................................................................................................................

5. Solution A is 0.1 mol/dm³ hydrochloric acid. Solution B is 0.1 mol/dm³ ethanoic acid. (a) Which solution has a lower pH? [1] ........................................................................................................................................

(b) Explain your answer to (a) in terms of ionisation. [2] ........................................................................................................................................ ........................................................................................................................................

6. Zinc oxide is an amphoteric oxide. (a) Define the term amphoteric. [1] ........................................................................................................................................

(b) Write a balanced chemical equation for the reaction between zinc oxide and dilute sulfuric acid. [2] ........................................................................................................................................

(c) Write a balanced chemical equation for the reaction between zinc oxide and aqueous sodium hydroxide. [2] (Note: The product is sodium zincate, $Na_2ZnO_2$ , and water) ........................................................................................................................................

7. A student performs a titration to determine the concentration of a potassium hydroxide solution. 25.0 cm³ of potassium hydroxide solution is pipetted into a conical flask. Phenolphthalein indicator is added. The solution is titrated against 0.050 mol/dm³ sulfuric acid. The burette readings are recorded below:

Titration	Rough	1	2	3
Final reading (cm³)	24.50	23.80	47.90	24.10
Initial reading (cm³)	0.00	0.00	23.80	0.00
Volume used (cm³)	24.50	23.80	24.10	24.10

(a) Identify the concordant results. [1] ........................................................................................................................................

(b) Calculate the average volume of sulfuric acid used. [1] ........................................................................................................................................

(c) Calculate the number of moles of sulfuric acid in this average volume. [1] ........................................................................................................................................

(d) The equation for the reaction is: $2KOH(aq) + H_2SO_4(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)$ Calculate the concentration of the potassium hydroxide solution in mol/dm³. [3] ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

8. Copper(II) carbonate reacts with dilute nitric acid to form copper(II) nitrate, water, and carbon dioxide. (a) Describe the test for carbon dioxide gas, including the expected observation. [2] ........................................................................................................................................ ........................................................................................................................................

(b) Calculate the maximum mass of copper(II) nitrate, $Cu(NO_3)_2$ , that can be formed when 5.0 g of copper(II) carbonate reacts with excess dilute nitric acid. [Relative atomic masses: Cu = 64, C = 12, O = 16, N = 14] [4] ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

9. The table below shows the pH values of four different solutions.

Solution	P	Q	R	S
pH	1	6	8	13

(a) Which solution is a strong alkali? [1] ........................................................................................................................................

(b) Which solution could be pure water? [1] ........................................................................................................................................

(c) Solution P is diluted with water. State what happens to its pH. [1] ........................................................................................................................................

10. Magnesium sulfate crystals, $MgSO_4 \cdot 7H_2O$ , can be prepared by reacting magnesium carbonate with dilute sulfuric acid. (a) Why is magnesium carbonate added in excess? [1] ........................................................................................................................................

(b) How is the excess magnesium carbonate removed? [1] ........................................................................................................................................

(c) Why is the solution not evaporated to dryness? [1] ........................................................................................................................................

Section B: Free-Response Questions (15 Marks)

Answer all questions in this section.

11. Iron(II) sulfate is a green solid that decomposes on strong heating to form iron(III) oxide, sulfur dioxide, and sulfur trioxide. $2FeSO_4(s) \rightarrow Fe_2O_3(s) + SO_2(g) + SO_3(g)$

(a) Describe the colour change observed in the solid during this reaction. [1] ........................................................................................................................................

(b) Sulfur dioxide is an acidic gas. (i) Describe the effect of sulfur dioxide on damp blue litmus paper. [1] ........................................................................................................................................

(ii) Sulfur dioxide contributes to acid rain. State one harmful effect of acid rain on buildings. [1] ........................................................................................................................................

(c) A student wants to prepare a pure sample of iron(II) sulfate crystals from iron filings and dilute sulfuric acid. (i) Write the chemical equation for this reaction. [2] ........................................................................................................................................

(ii) Explain why iron filings are preferred over iron powder for this preparation in a school laboratory, considering the rate of reaction. [2] ........................................................................................................................................ ........................................................................................................................................

(d) Another student attempts to prepare copper(II) sulfate crystals using copper metal and dilute sulfuric acid. No reaction occurs. Explain why no reaction occurs and suggest an alternative copper compound that would react with dilute sulfuric acid to form copper(II) sulfate. [3] ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

12. Sodium chloride and silicon(IV) oxide are both solids at room temperature. (a) Sodium chloride has a high melting point (801 °C), while silicon(IV) oxide also has a very high melting point (1710 °C). Explain, in terms of structure and bonding, why sodium chloride has a high melting point. [3] ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................

(b) Explain why sodium chloride conducts electricity when molten but not when solid. [2] ........................................................................................................................................ ........................................................................................................................................

(c) Silicon(IV) oxide does not conduct electricity in any state. Explain why. [2] ........................................................................................................................................ ........................................................................................................................................

(End of Paper)

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Answers

Marking Scheme - Chemistry Secondary 3 (SA2) Version 5

Total Marks: 50

Section A: Structured Questions

1. (a) CaO [1] (b) Calcium oxide is a basic oxide / reacts with acid in soil. [1] Equation: $CaO + 2H^+ \rightarrow Ca^{2+} + H_2O$ OR $CaO + H_2SO_4 \rightarrow CaSO_4 + H_2O$ (Accept any valid acid neutralization). [1]

2. (a) Effervescence / bubbles produced. [1] Zinc dissolves / solid disappears. [1] (Note: "Heat produced" is an observation of energy change, but physical observations are preferred. Accept "colourless solution formed" if zinc was the only solid). (b) Rate is faster. [1] Hydrochloric acid is a monoprotic acid while sulfuric is diprotic? No, concentration is same. Correction for standard Sec 3 logic: If concentrations are both 0.1 mol/dm³ of acid molecules, HCl provides 0.1 mol/dm³ $H^+$ , while $H_2SO_4$ provides 0.2 mol/dm³ $H^+$ . Therefore, the initial rate with sulfuric acid (Exp 1) would be faster due to higher $[H^+]$ . The question asks about Exp 2 (HCl) compared to Exp 1 ( $H_2SO_4$ ). So, the rate with HCl will be slower. [1] Because the concentration of hydrogen ions $[H^+]$ in 0.1 mol/dm³ HCl is lower than in 0.1 mol/dm³ $H_2SO_4$ (which ionises to give 2 $H^+$ per molecule). [1]

3. (a) $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$ [1 for formulae, 1 for state symbols] (b) 1. Filter the mixture to collect the residue. [1] 2. Wash the residue with distilled water to remove soluble impurities (NaCl). [1] 3. Dry the residue between filter papers or in an oven. [1]

4. (a) The rate of the forward reaction equals the rate of the backward reaction. [1] The concentrations of reactants and products remain constant. [1] (b) High pressure increases the yield because there are fewer moles of gas on the product side (2 moles vs 4 moles), so equilibrium shifts to the right. [1] High pressure increases the rate because particles are closer together, leading to more frequent collisions. [1]

5. (a) Solution A (Hydrochloric acid). [1] (b) Hydrochloric acid is a strong acid and ionises completely in water to produce a high concentration of $H^+$ ions. [1] Ethanoic acid is a weak acid and ionises partially, producing a lower concentration of $H^+$ ions. [1]

6. (a) An amphoteric substance reacts with both acids and bases. [1] (b) $ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O$ [1 for formulae, 1 for balancing] (c) $ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O$ [1 for formulae, 1 for balancing]

7. (a) Titres 2 and 3 (24.10 cm³ and 24.10 cm³). [1] (Note: Titre 1 is 23.80, which is >0.10 cm³ different from 2 and 3. Rough is ignored). (b) Average volume = $\frac{24.10 + 24.10}{2} = 24.10$ cm³. [1] (c) Moles of $H_2SO_4 = \frac{24.10}{1000} \times 0.050 = 0.001205$ mol. [1] (d) Mole ratio KOH : $H_2SO_4$ is 2 : 1. Moles of KOH = $2 \times 0.001205 = 0.00241$ mol. [1] Volume of KOH = 25.0 cm³ = 0.025 dm³. Concentration of KOH = $\frac{0.00241}{0.025} = 0.0964$ mol/dm³. [2 for calculation, 1 for unit/answer]

8. (a) Bubble gas through limewater. [1] Limewater turns milky / cloudy / white precipitate forms. [1] (b) $M_r$ of $CuCO_3 = 64 + 12 + (3 \times 16) = 124$ . [1] Moles of $CuCO_3 = \frac{5.0}{124} = 0.04032$ mol. Ratio $CuCO_3 : Cu(NO_3)_2$ is 1 : 1. Moles of $Cu(NO_3)_2 = 0.04032$ mol. $M_r$ of $Cu(NO_3)_2 = 64 + 2 \times [14 + (3 \times 16)] = 64 + 124 = 188$ . [1] Mass = $0.04032 \times 188 = 7.58$ g. [2 for correct answer, allow ecf]

9. (a) S [1] (b) Q [1] (Pure water is pH 7, Q is closest at 6, likely due to dissolved $CO_2$ , but in multiple choice context Q is the neutral-ish one. P is acid, R/S are alkali. If strict, none is exactly 7, but Q is the intended answer for "neutral/near neutral" vs strong acid/alkali). Correction: In many Sec 3 contexts, pH 6 is slightly acidic (rainwater), pH 7 is pure water. If forced to choose, Q is the only non-alkaline, non-strong-acid option. However, strictly, pure water is 7. Let's assume Q represents a very weak acid or near neutral. (c) pH increases (becomes less acidic / moves towards 7). [1]

10. (a) To ensure all the sulfuric acid reacts. [1] (b) Filtration. [1] (c) To prevent the loss of water of crystallisation / to obtain hydrated crystals. [1]

Section B: Free-Response Questions

11. (a) Green solid turns brown / reddish-brown. [1] (b)(i) Turns red. [1] (ii) Corrodes limestone / marble statues / buildings. [1] (c)(i) $Fe + H_2SO_4 \rightarrow FeSO_4 + H_2$ [1 for products, 1 for balancing/state symbols optional but good practice] (ii) Iron powder has a larger surface area, leading to a very fast/exothermic reaction which may be dangerous/hard to control. Iron filings react at a manageable rate. [2] (d) Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute acids. [1] Alternative: Copper(II) oxide / Copper(II) carbonate / Copper(II) hydroxide. [1] Equation (e.g., for oxide): $CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$ . [1]

12. (a) Sodium chloride has a giant ionic lattice structure. [1] There are strong electrostatic forces of attraction between the oppositely charged ions ( $Na^+$ and $Cl^-$ ). [1] A large amount of heat energy is required to overcome these strong forces. [1] (b) In the molten state, the ions are free to move and carry charge. [1] In the solid state, the ions are fixed in position and cannot move. [1] (c) Silicon(IV) oxide has a giant covalent structure. [1] All electrons are held in covalent bonds / there are no free electrons or mobile ions to conduct electricity. [1]