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Secondary 4 Pure Chemistry Preliminary Examination Paper 3

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TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4

TuitionGoWhere Secondary School (AI)
PRELIMINARY EXAMINATION 2024
Version 3 of 5

Subject: Pure Chemistry
Level: Secondary 4
Paper: 2 (Structured and Free-Response)
Duration: 1 hour 45 minutes
Total Marks: 80
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.
You may use a calculator.

Section A

Answer all questions in this section. [50 marks]

1 The diagram below shows the pH curve obtained when aqueous sodium hydroxide is added to 25.0 cm³ of dilute sulfuric acid.

(Imagine a graph: x-axis is Volume of NaOH added (cm³), y-axis is pH. The curve starts at pH 1, rises slowly, then shoots up vertically at 20.0 cm³ to pH 13, then levels off.)

(a) State the initial pH of the dilute sulfuric acid.
........................................................................................................................... [1]

(b) Determine the volume of aqueous sodium hydroxide required to completely neutralise the acid.
........................................................................................................................... [1]

(c) Write the balanced chemical equation for the reaction between sulfuric acid and sodium hydroxide. Include state symbols.
...........................................................................................................................
........................................................................................................................... [2]

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

2 Three white solids, A, B, and C, are known to be sodium chloride, calcium carbonate, and ammonium nitrate. The following tests were carried out.

Test	Observation for A	Observation for B	Observation for C
Add dilute nitric acid	No visible change	Effervescence; gas turns limewater milky	No visible change
Add aqueous sodium hydroxide and warm	No visible change	White precipitate formed	Pungent gas evolved; turns damp red litmus blue
Flame test	Golden-yellow flame	Brick-red flame	Golden-yellow flame

(a) Identify solids A, B, and C.
A: .........................................................................................................................
B: .........................................................................................................................
C: ......................................................................................................................... [3]

(b) Write the ionic equation for the reaction occurring in Test 1 for solid B.
........................................................................................................................... [1]

(c) Describe a confirmatory test for the cation present in solid A, using aqueous ammonia.
...........................................................................................................................
........................................................................................................................... [2]

3 Zinc oxide is an amphoteric oxide.

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

(b) Write balanced chemical equations for the reaction of zinc oxide with:
(i) Dilute hydrochloric acid
......................................................................................................................... [1]
(ii) Aqueous sodium hydroxide
......................................................................................................................... [1]

(c) A student attempts to prepare zinc sulfate crystals by reacting zinc oxide with dilute sulfuric acid.
(i) Why is excess zinc oxide added to the acid?
......................................................................................................................... [1]
(ii) Name the method used to remove the excess zinc oxide.
......................................................................................................................... [1]
(iii) Describe how pure, dry zinc sulfate crystals are obtained from the filtrate.
.........................................................................................................................
......................................................................................................................... [2]

4 Barium sulfate is an insoluble salt used in medical imaging.

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

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

(b) Write the ionic equation for the formation of barium sulfate.
........................................................................................................................... [1]

(c) Explain why barium sulfate is safe for patients to swallow, whereas other barium salts (like barium chloride) are toxic.
...........................................................................................................................
........................................................................................................................... [1]

5 The table below shows the properties of two acids, Acid X and Acid Y, at the same concentration (0.1 mol/dm³).

Property	Acid X	Acid Y
pH	1.0	2.9
Electrical Conductivity	High	Low
Reaction with Magnesium	Vigorous effervescence	Slow effervescence

(a) Identify which acid is the strong acid and which is the weak acid.
Strong acid: ..........................................................................................................
Weak acid: ............................................................................................................ [1]

(b) Explain, in terms of ionisation, why Acid Y has a higher pH than Acid X.
...........................................................................................................................
........................................................................................................................... [2]

(c) Despite having different pH values, 25.0 cm³ of Acid X and 25.0 cm³ of Acid Y require the same volume of 0.1 mol/dm³ sodium hydroxide for complete neutralisation. Explain why.
...........................................................................................................................
........................................................................................................................... [2]

6 Copper(II) sulfate solution is electrolysed using inert carbon electrodes.

(a) Write the half-equation for the reaction at the:
(i) Cathode
......................................................................................................................... [1]
(ii) Anode
......................................................................................................................... [1]

(b) State and explain the change in pH of the solution during electrolysis.
...........................................................................................................................
........................................................................................................................... [2]

(c) If the carbon anode is replaced with a copper anode, state what happens to the mass of the anode and the concentration of the copper(II) sulfate solution.
Mass of anode: ......................................................................................................
Concentration of solution: .................................................................................... [2]

7 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 conditions of temperature and pressure used in the Haber Process.
Temperature: .........................................................................................................
Pressure: ............................................................................................................... [2]

(b) Explain why a higher temperature is not used, even though it would increase the rate of reaction.
...........................................................................................................................
........................................................................................................................... [2]

(c) Ammonia is 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 chemical equation for this reaction.
......................................................................................................................... [1]

8 A student investigates the rate of reaction between calcium carbonate and dilute hydrochloric acid by measuring the loss in mass of the reaction flask over time.

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

(a) Explain why the mass of the flask decreases during the reaction.
........................................................................................................................... [1]

(b) Sketch a graph of loss in mass against time for the reaction. Label the curve A.
(Space for sketch)
 [2]

(c) On the same axes, sketch the curve expected if the experiment is repeated using the same mass of calcium carbonate but in the form of powder instead of lumps. Label this curve B. [1]

(d) Explain the difference in the initial rate of reaction between curve A and curve B in terms of collision theory.
...........................................................................................................................
........................................................................................................................... [2]

Section B

Answer all questions in this section. [30 marks]

9 Soil acidity affects crop growth. Farmers often add slaked lime (calcium hydroxide) to acidic soil.

(a) Write the chemical formula for slaked lime.
........................................................................................................................... [1]

(b) Explain why slaked lime is preferred over calcium oxide (quicklime) for treating soil, despite calcium oxide being cheaper.
...........................................................................................................................
........................................................................................................................... [2]

(c) A sample of soil is shaken with water and filtered. The filtrate has a pH of 4.5.
(i) Suggest the colour of universal indicator in this filtrate.
......................................................................................................................... [1]
(ii) Calculate the concentration of hydrogen ions, $[H^+]$ , in the filtrate.
......................................................................................................................... [1]

(d) Acid rain contributes to soil acidity.
(i) Name two gases that cause acid rain.

.....................................................................................................................
..................................................................................................................... [2]
(ii) Describe how one of these gases is formed in the atmosphere.
.........................................................................................................................
......................................................................................................................... [2]

10 Compound P is a salt that contains only iron, sulfur, and oxygen. When heated strongly, it decomposes to form iron(III) oxide, sulfur dioxide, and sulfur trioxide.

(a) Write the balanced chemical equation for the decomposition of Compound P.
...........................................................................................................................
........................................................................................................................... [2]

(b) The gases produced are passed through water.
(i) Name the acids formed when sulfur dioxide and sulfur trioxide dissolve in water.
Sulfur dioxide: ...................................................................................................
Sulfur trioxide: .................................................................................................. [2]
(ii) State the effect of these gases on damp blue litmus paper.
......................................................................................................................... [1]

(c) A different salt, Iron(II) sulfate, is dissolved in water. Aqueous sodium hydroxide is added dropwise until in excess.
(i) Describe the observations.
.........................................................................................................................
......................................................................................................................... [2]
(ii) Write the ionic equation for the formation of the precipitate.
......................................................................................................................... [1]

(d) Iron(II) sulfate solution is left standing in air for several days. It changes from pale green to yellow-brown.
(i) Name the type of reaction that has occurred.
......................................................................................................................... [1]
(ii) Identify the reagent in the air responsible for this change.
......................................................................................................................... [1]

11 Esters are organic compounds with pleasant smells, often used in food flavourings. They are prepared by reacting a carboxylic acid with an alcohol.

(a) Name the ester formed when ethanoic acid reacts with ethanol.
........................................................................................................................... [1]

(b) Draw the structural formula of the ester named in (a). Show all atoms and bonds.
 [2]

(c) The preparation of this ester requires a catalyst.
(i) Name the catalyst used.
......................................................................................................................... [1]
(ii) Why is the reaction mixture heated in a water bath rather than directly with a Bunsen burner?
......................................................................................................................... [1]

(d) Another ester, propyl ethanoate, is hydrolysed by boiling with aqueous sodium hydroxide.
(i) Name the two products formed.

.....................................................................................................................
..................................................................................................................... [2]
(ii) Why is this process called saponification when performed with fats/oils?
......................................................................................................................... [1]

12 The reactivity of metals can be determined by their reaction with water or steam.

(a) Arrange the following metals in order of decreasing reactivity: Copper, Magnesium, Sodium, Iron.
........................................................................................................................... [1]

(b) Describe the reaction of sodium with cold water. Include two observations.

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

(c) Write the balanced chemical equation for the reaction of magnesium with steam.
........................................................................................................................... [1]

(d) Aluminium appears to be unreactive with water, even though it is high in the reactivity series. Explain why.
...........................................................................................................................
........................................................................................................................... [2]

(e) Zinc is used to protect iron from rusting (galvanising).
(i) Name this method of protection.
......................................................................................................................... [1]
(ii) Explain how zinc protects iron even if the coating is scratched.
.........................................................................................................................
......................................................................................................................... [2]

End of Paper

Answers

TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4

Answer Key and Marking Scheme
Version 3 of 5

Section A

1
(a) 1 [1]
(b) 20.0 cm³ [1]
(c) $H_2SO_4(aq) + 2NaOH(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l)$ [2]
(1 mark for correct formulae, 1 mark for balancing and state symbols)
(d) Indicator: Phenolphthalein (or Methyl Orange) [1]
Colour change: Colourless to pink (for phenolphthalein) OR Red to yellow (for methyl orange) [1]
(Must match indicator chosen)

2
(a) A: Sodium chloride [1]
B: Calcium carbonate [1]
C: Ammonium nitrate [1]
(b) $CaCO_3(s) + 2H^+(aq) \rightarrow Ca^{2+}(aq) + H_2O(l) + CO_2(g)$ [1]
(Accept molecular equation if ionic not specified, but ionic is preferred for "ionic equation")
(c) Add aqueous ammonia to a solution of A. [1]
No precipitate is formed (or white precipitate forms and dissolves in excess is incorrect for Na+, correct is no ppt).
Correction: Sodium ions do not form a precipitate with aqueous ammonia. The flame test is the primary ID. However, if forced to use NaOH/NH3: "No visible change / No precipitate" distinguishes it from many others.
Better Answer for Cation Test (Na+): Flame test is already done. If using NaOH/NH3: "No precipitate forms with aqueous sodium hydroxide or aqueous ammonia." [1]
(Note: The question asks for a confirmatory test for the cation in A using aqueous ammonia. Since Na+ does not react, the observation "no precipitate" is the test result, but it's not unique. However, in the context of distinguishing from Al/Zn/Pb/Cu/Fe, "no precipitate" is the key observation.)
Alternative interpretation: If A was identified as something else, check ID. A is NaCl. Cation is Na+. Test with NH3: No ppt. [1] for observation.

3
(a) An amphoteric oxide reacts with both acids and bases to form salt and water. [1]
(b) (i) $ZnO(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l)$ [1]
(ii) $ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$ [1]
(Accept $Na_2[Zn(OH)_4]$ )
(c) (i) To ensure all the acid is reacted / neutralised. [1]
(ii) Filtration. [1]
(iii) Heat the filtrate to evaporate some water / until saturated. [1]
Allow to cool for crystals to form. Filter and wash with cold water. Dry between filter papers. [1]

4
(a) Barium chloride (or nitrate) and Sodium sulfate (or potassium sulfate/magnesium sulfate). [2]
(Must be soluble barium salt and soluble sulfate)
(b) $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$ [1]
(c) Barium sulfate is insoluble, so barium ions are not released into the body. [1]
(Toxic barium salts are soluble and release toxic $Ba^{2+}$ ions)

5
(a) Strong acid: X [0.5]
Weak acid: Y [0.5]
(b) Acid Y is partially ionised / dissociated in water. [1]
Therefore, the concentration of $H^+$ ions is lower than in Acid X (which is fully ionised). [1]
(c) Both acids have the same volume and concentration, so they contain the same number of moles of acid molecules. [1]
Neutralisation depends on the total amount of potential $H^+$ ions (stoichiometry), not just the free $H^+$ ions present initially. As $H^+$ is removed, the weak acid ionises further until all reacts. [1]

6
(a) (i) $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ [1]
(ii) $4OH^-(aq) \rightarrow O_2(g) + 2H_2O(l) + 4e^-$ [1]
(Or $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ )
(b) The pH decreases. [1]
$OH^-$ ions are discharged at the anode, leaving $H^+$ ions in solution (or $H^+$ produced at anode). The solution becomes more acidic. [1]
(c) Mass of anode: Decreases. [1]
Concentration of solution: Remains unchanged. [1]

7
(a) Temperature: 450°C [1]
Pressure: 200 atm [1]
(b) The forward reaction is exothermic. [1]
Higher temperature favours the endothermic (reverse) reaction, decreasing the yield of ammonia. [1]
(c) (i) Nitric acid [1]
(ii) $NH_3 + HNO_3 \rightarrow NH_4NO_3$ [1]

8
(a) Carbon dioxide gas is produced and escapes from the flask. [1]
(b) Graph: Curve starts steep and levels off horizontally. [2]
(1 mark for shape, 1 mark for plateau)
(c) Curve B starts steeper than A and levels off at the same final mass loss. [1]
(d) Powder has a larger surface area than lumps. [1]
This leads to a higher frequency of effective collisions between reactant particles. [1]

Section B

9
(a) $Ca(OH)_2$ [1]
(b) Calcium oxide reacts violently/exothermically with water/moisture in soil, which can damage plants/soil structure. Slaked lime is less reactive/safer to handle. [2]
(c) (i) Red / Orange / Pink (depending on exact shade, pH 4.5 is acidic) [1]
(Accept Red/Orange)
(ii) $[H^+] = 10^{-pH} = 10^{-4.5}$ mol/dm³ [1]
(Accept $3.16 \times 10^{-5}$ mol/dm³)
(d) (i) Sulfur dioxide ( $SO_2$ ) and Nitrogen oxides ( $NO_x$ ) [2]
(ii) $SO_2$ : Formed by burning fossil fuels containing sulfur impurities. [1]
$NO_x$ : Formed by reaction of nitrogen and oxygen in car engines/high temp combustion. [1]

10
(a) $2FeSO_4(s) \rightarrow Fe_2O_3(s) + SO_2(g) + SO_3(g)$ [2]
(1 mark for formulae, 1 mark for balancing)
(b) (i) Sulfur dioxide: Sulfurous acid ( $H_2SO_3$ ) [1]
Sulfur trioxide: Sulfuric acid ( $H_2SO_4$ ) [1]
(ii) Turns red. [1]
(c) (i) Green precipitate formed. [1]
Precipitate does not dissolve in excess. [1]
(Note: Fe(OH)2 oxidises to brown eventually, but initial ppt is green)
(ii) $Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)$ [1]
(d) (i) Oxidation [1]
(ii) Oxygen [1]

11
(a) Ethyl ethanoate [1]
(b)

      H   O
      |   ||
  H - C - C - O - C - H
      |       |   |
      H       H   H

[2]
(1 mark for ester linkage -COO-, 1 mark for correct alkyl groups)
(c) (i) Concentrated sulfuric acid [1]
(ii) Ethanol and ethanoic acid are flammable. [1]
(d) (i) Sodium ethanoate (or Ethanoic acid salt) [1]
Propanol [1]
(ii) Saponification is the alkaline hydrolysis of esters (fats/oils) to make soap. [1]

12
(a) Sodium > Magnesium > Iron > Copper [1]
(b) 1. Fizzes vigorously / moves rapidly on surface. [1]
2. Melts into a ball / disappears. [1]
(Accept: Lilac flame, universal indicator turns purple)
(c) $Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)$ [1]
(d) Aluminium has a protective layer of aluminium oxide on its surface. [1]
This layer is impermeable and prevents water from reacting with the metal underneath. [1]
(e) (i) Sacrificial protection [1]
(ii) Zinc is more reactive than iron. [1]
Zinc loses electrons/oxidises in preference to iron, protecting the iron from rusting. [1]