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

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TuitionGoWhere Practice Paper - Chemistry Secondary 3

TuitionGoWhere Secondary School (AI)
Subject: Chemistry
Level: Secondary 3 (Express)
Paper: SA2 Practice Paper (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.
You may use a calculator.

Section A: Structured Questions [40 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
P	1
Q	7
R	13
S	5

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

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

(c) Solution R is added to Solution S. State what happens to the pH of Solution S.
........................................................................................................................................ [1]

(d) Suggest a possible identity for Solution P.
........................................................................................................................................ [1]

2. Calcium oxide is used in agriculture to treat acidic soil.

(a) Write the chemical formula for calcium oxide.
........................................................................................................................................ [1]

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

(c) Farmers often prefer using calcium carbonate (limestone) instead of calcium oxide. Suggest one reason for this preference.
........................................................................................................................................
........................................................................................................................................ [1]

3. A student investigates the reaction between dilute sulfuric acid and excess zinc granules. The volume of hydrogen gas produced is measured every 30 seconds.

(a) Write a balanced chemical equation for this reaction, including state symbols.
........................................................................................................................................ [2]

(b) Sketch a graph of volume of hydrogen gas (y-axis) against time (x-axis) for this reaction. Label the curve A.
 [2]

(c) The experiment is repeated using the same mass of zinc powder instead of granules. On the same grid, sketch the curve for this second experiment and label it B.
[1]

(d) Explain, in terms of particles, why the initial rate of reaction in experiment B is faster than in experiment A.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

4. Barium sulfate is an insoluble salt used in medical X-rays. It can be prepared by mixing aqueous barium chloride and aqueous sodium sulfate.

(a) Name the type of reaction used to prepare barium sulfate.
........................................................................................................................................ [1]

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

(c) Describe how you would obtain a pure, dry sample of barium sulfate from the reaction mixture.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [3]

5. Ammonia is manufactured industrially by the Haber Process.

(a) Write the balanced chemical equation for the Haber Process.
........................................................................................................................................ [2]

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

(c) The reaction is reversible. Explain what is meant by a reversible reaction.
........................................................................................................................................
........................................................................................................................................ [1]

(d) Ammonia reacts with sulfuric acid to form a salt. Name this salt.
........................................................................................................................................ [1]

6. Solution X is known to be either dilute hydrochloric acid or dilute ethanoic acid. Both solutions have the same concentration of $0.1 \text{ mol/dm}^3$ .

(a) Describe a simple chemical test, using a named reagent, to distinguish between the two acids. State the expected observation for each.
Reagent: ....................................................................................................................
Observation with Hydrochloric Acid: ........................................................................
Observation with Ethanoic Acid: ............................................................................... [3]

(b) Explain, in terms of ionisation, why hydrochloric acid is classified as a strong acid while ethanoic acid is a weak acid.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

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

(a) Why is excess copper(II) carbonate used?
........................................................................................................................................ [1]

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

(c) After the reaction is complete, the mixture is filtered. What is removed by filtration?
........................................................................................................................................ [1]

(d) Describe the steps required to obtain dry crystals of copper(II) sulfate from the filtrate.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

8. The diagram below shows the electronic structure of a water molecule ( $H_2O$ ).

(Note: In a real exam, a dot-and-cross diagram would be shown here. Assume O is central with 2 lone pairs and 2 bonding pairs shared with H.)

(a) What type of bonding is present in a water molecule?
........................................................................................................................................ [1]

(b) Water can act as a base in certain reactions. Define a base in terms of proton transfer.
........................................................................................................................................ [1]

(c) When water reacts with ammonia, ammonium ions and hydroxide ions are formed. Write the equation for this reaction.
........................................................................................................................................ [1]

9. A student titrates $25.0 \text{ cm}^3$ of $0.10 \text{ mol/dm}^3$ sodium hydroxide solution against dilute hydrochloric acid.

(a) Name the apparatus used to measure exactly $25.0 \text{ cm}^3$ of the sodium hydroxide solution.
........................................................................................................................................ [1]

(b) Name a suitable indicator for this titration and state the colour change at the end-point.
Indicator: ...................................................................................................................
Colour Change: ......................................................................................................... [2]

(c) The student finds that $20.0 \text{ cm}^3$ of hydrochloric acid is required for neutralisation. Calculate the concentration of the hydrochloric acid in $\text{mol/dm}^3$ .
 [3]

10. Zinc oxide is an amphoteric oxide.

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

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

(c) Write the balanced chemical equation for the reaction between zinc oxide and aqueous sodium hydroxide.
........................................................................................................................................ [2]

Section B: Free-Response Questions [10 marks]

Answer all questions in this section.

11. Magnesium reacts with two different acids, A and B, of equal concentration.

Acid A is nitric acid ( $HNO_3$ ).
Acid B is carbonic acid ( $H_2CO_3$ ).

(a) Predict which acid will react faster with magnesium. Explain your answer in terms of pH and ion concentration.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [3]

(b) Describe a test to confirm that the gas produced in these reactions is hydrogen.
........................................................................................................................................
........................................................................................................................................ [2]

(c) Nitric acid is often not used to prepare hydrogen gas in the laboratory. Suggest a reason why.
........................................................................................................................................
........................................................................................................................................ [1]

(d) Carbonic acid is unstable and decomposes easily. Write the equation for the decomposition of carbonic acid.
........................................................................................................................................ [1]

(e) If $0.1 \text{ mol}$ of magnesium reacts completely with excess nitric acid, calculate the maximum volume of hydrogen gas produced at room temperature and pressure (r.t.p.).
[Molar volume of gas at r.t.p. = $24 \text{ dm}^3$ ]
 [3]

[End of Paper]

Answers

TuitionGoWhere Practice Paper - Chemistry Secondary 3 (SA2 Version 2)

Marking Scheme & Answer Key

Section A: Structured Questions

1. (a) Q [1] (b) P [1] (c) The pH increases (becomes less acidic / moves towards 7 / becomes alkaline). [1] (d) Hydrochloric acid / Sulfuric acid / Nitric acid (Any strong mineral acid). [1]

2. (a) CaO [1] (b) Calcium oxide reacts with water in the soil to form calcium hydroxide (a base), which neutralises the acid.
Equation: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$
OR
$CaO(s) + 2H^+(aq) \rightarrow Ca^{2+}(aq) + H_2O(l)$
[1 for explanation, 1 for correct equation] (c) Calcium carbonate reacts more slowly/gently, preventing a sudden rise in pH which might harm plants. OR Calcium carbonate is cheaper/more readily available. [1]

3. (a) $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$
[1 for correct formulae, 1 for balancing and state symbols] (b) Graph: Curve starts at origin, gradient decreases, becomes horizontal (plateau). [1 for shape, 1 for labels/axis] (c) Curve B starts with a steeper gradient than A but reaches the same final volume (same plateau height). [1] (d) Zinc powder has a larger total surface area than granules. This leads to more frequent collisions between zinc and hydrogen ions per unit time, increasing the rate of reaction. [1 for surface area, 1 for collision frequency]

4. (a) Precipitation (or Double Decomposition). [1] (b) $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$
[1 for correct ions, 1 for state symbols and balancing] (c)

Filter the mixture to collect the residue (barium sulfate).
Wash the residue with distilled water to remove soluble impurities (sodium chloride).
Dry the residue between filter papers or in an oven.
[1 for each correct step, max 3]

5. (a) $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$
[1 for correct formulae, 1 for balancing and reversible sign] (b) Iron (Fe). [1] (c) A reaction where the products can react to reform the original reactants. [1] (d) Ammonium sulfate. [1]

6. (a) Reagent: Magnesium ribbon / Zinc granules / Calcium Carbonate.
Obs (HCl): Vigorous effervescence / fast bubbling.
Obs (Ethanoic): Slow effervescence / slow bubbling.
OR
Reagent: pH meter / Universal Indicator.
Obs (HCl): pH 1 / Red colour.
Obs (Ethanoic): pH 3-4 / Orange-Yellow colour.
[1 for reagent, 1 for each correct observation] (b) Hydrochloric acid ionises completely in water to produce a high concentration of $H^+$ ions. Ethanoic acid ionises only partially in water, producing a low concentration of $H^+$ ions. [1 for complete ionisation, 1 for partial ionisation]

7. (a) To ensure all the sulfuric acid reacts. [1] (b) $CuCO_3(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l) + CO_2(g)$
[1 for correct formulae, 1 for balancing] (c) Unreacted/excess copper(II) carbonate. [1] (d)

Heat the filtrate to evaporate some water (until saturated/crystallisation point).
Allow the solution to cool slowly to form crystals.
Filter and dry the crystals.
[1 for evaporation/concentration, 1 for cooling/crystallisation]

8. (a) Covalent bonding. [1] (b) A base is a proton ( $H^+$ ) acceptor. [1] (c) $NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$ [1]

9. (a) Pipette. [1] (b) Indicator: Methyl Orange OR Phenolphthalein.
Colour Change:
If Methyl Orange: Yellow to Orange/Red.
If Phenolphthalein: Pink to Colourless.
[1 for indicator, 1 for correct colour change] (c) Moles of NaOH = $0.10 \times \frac{25.0}{1000} = 0.0025 \text{ mol}$
Ratio NaOH : HCl is 1 : 1.
Moles of HCl = $0.0025 \text{ mol}$
Concentration of HCl = $\frac{0.0025}{\frac{20.0}{1000}} = \frac{0.0025}{0.020} = 0.125 \text{ mol/dm}^3$
[1 for moles of NaOH, 1 for mole ratio/moles HCl, 1 for final concentration]

10. (a) An amphoteric substance reacts with both acids and bases. [1] (b) $ZnO(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l)$ [2] (c) $ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$
(Accept $Na_2[Zn(OH)_4]$ ) [2]

Section B: Free-Response Questions

11. (a) Nitric acid (Acid A) will react faster.
Nitric acid is a strong acid and ionises completely, producing a higher concentration of hydrogen ions ( $H^+$ ) compared to carbonic acid, which is a weak acid and ionises partially.
Higher $[H^+]$ leads to more frequent effective collisions with magnesium.
[1 for identifying Nitric Acid, 1 for linking strong/weak to $[H^+]$ , 1 for collision theory link]

(b) Place a lighted splint at the mouth of the test tube.
The gas burns with a 'pop' sound.
[1 for method, 1 for observation]

(c) Nitric acid is an oxidising agent. It may oxidise the hydrogen produced to water, or produce nitrogen oxides instead of hydrogen gas. [1]

(d) $H_2CO_3(aq) \rightarrow H_2O(l) + CO_2(g)$ [1]

(e) Equation: $Mg(s) + 2HNO_3(aq) \rightarrow Mg(NO_3)_2(aq) + H_2(g)$
Ratio Mg : $H_2$ is 1 : 1.
Moles of $H_2$ produced = $0.1 \text{ mol}$ .
Volume = $0.1 \times 24 = 2.4 \text{ dm}^3$ .
[1 for mole ratio, 1 for calculation, 1 for unit and value]

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