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Secondary 4 Pure Chemistry Practice Paper 1

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TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4

TuitionGoWhere Practice Paper (AI)

Subject: Pure Chemistry (6092)
Level: Secondary 4
Topic: Acids, Bases and Salts
Paper: Practice Paper (Version 1 of 5)
Duration: 1 Hour
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.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a calculator.
A copy of the Periodic Table is provided (not shown here, assume standard access).

Section A: Structured Questions (30 Marks)

Answer all questions in this section.

1. Hydrochloric acid is a strong acid, while ethanoic acid is a weak acid. Both acids have a concentration of 0.1 mol/dm³.

(a) Define the term strong acid.

_________________________________________________________________________ [1]

(b) Explain, in terms of ionisation, why the pH of 0.1 mol/dm³ hydrochloric acid is lower than the pH of 0.1 mol/dm³ ethanoic acid.

_________________________________________________________________________ [2]

(c) Describe a simple chemical test, other than using a pH meter or indicator, to distinguish between these two acids of the same concentration. State the expected observation for each.
Test: __________________________________________________________________
Observation with HCl: __________________________________________________
Observation with CH₃COOH: ____________________________________________ [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 wants to prepare pure, dry crystals of magnesium sulfate ( $MgSO_4$ ) from dilute sulfuric acid and an insoluble base.

(a) Name a suitable insoluble base for this preparation.
_________________________________________________________________________ [1]

(b) Describe the method used to prepare the crystals. Your answer should include:

How the student knows the acid has been fully neutralised.
The steps to obtain pure, dry crystals.

_________________________________________________________________________ [4]

_________________________________________________________________________ [1]

4. The table below shows the results of adding aqueous sodium hydroxide and aqueous ammonia to three different metal salt solutions, P, Q, and R.

Solution	Add NaOH(aq)	Add NH₃(aq)
P	White ppt, soluble in excess	White ppt, insoluble in excess
Q	Green ppt, insoluble in excess	Green ppt, insoluble in excess
R	Blue ppt, insoluble in excess	Blue ppt, soluble in excess to form deep blue solution

(a) Identify the cation present in each solution.
P: __________________________
Q: __________________________
R: __________________________ [3]

(b) Write the ionic equation for the formation of the precipitate in solution Q.
_________________________________________________________________________ [1]

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) Although the forward reaction is exothermic, a temperature of 450°C is used instead of a lower temperature. Explain why.

_________________________________________________________________________ [2]

6. Copper(II) carbonate reacts with dilute nitric acid.

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

(b) Describe two observations made during this reaction.

______________________________________________________________________ [2]

(c) If 12.4 g of copper(II) carbonate ( $M_r = 124$ ) reacts with excess dilute nitric acid, calculate the volume of gas produced at room temperature and pressure (r.t.p.).
[Molar volume of gas at r.t.p. = 24 dm³/mol]

[3]

Section B: Free-Response Questions (20 Marks)

Answer all questions in this section.

7. Soil acidity is a major problem for farmers. Calcium hydroxide (slaked lime) is often added to soil to control pH.

(a) Explain why it is important to control soil pH for crop growth.

_________________________________________________________________________ [1]

(b) Write the ionic equation for the neutralisation reaction between calcium hydroxide and hydrogen ions in the soil.
_________________________________________________________________________ [1]

(c) A farmer adds calcium hydroxide to a patch of soil. Later, he adds ammonium sulfate fertilizer to the same patch.
(i) Write the formula of the gas produced when these two compounds react.
_________________________________________________________________________ [1]

(ii) Explain why this reaction is undesirable for the farmer.

_________________________________________________________________________ [2]

(d) Instead of calcium hydroxide, the farmer considers using calcium carbonate.
(i) Write the equation for the reaction between calcium carbonate and dilute hydrochloric acid (representing soil acid).
_________________________________________________________________________ [2]

(ii) Suggest one advantage of using calcium carbonate over calcium hydroxide.

_________________________________________________________________________ [1]

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

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

The graph below shows the results.

(Imagine a graph: Y-axis = Volume of $CO_2$ ( $cm^3$ ), X-axis = Time (s). The curve starts at 0, rises steeply, and levels off at 60 $cm^3$ after 120 seconds.)

(a) Explain, in terms of particles, why the rate of reaction is highest at the start of the experiment.

_________________________________________________________________________ [2]

(b) Calculate the average rate of reaction during the first 60 seconds if 45 $cm^3$ of gas was collected.
 [2]

(c) The experiment is repeated using the same mass of calcium carbonate powder instead of chips.
(i) Sketch the new curve on the grid provided in your mind (or describe it).

_________________________________________________________________________ [2]

(ii) Explain the difference in the rate using collision theory.

_________________________________________________________________________ [2]

(d) The experiment is repeated again using half the concentration of hydrochloric acid but the same volume.
(i) What is the final volume of gas collected?
_________________________________________________________________________ [1]

(ii) Explain your answer to (d)(i).

_________________________________________________________________________ [2]

Answers

TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4

Answer Key & Marking Scheme

Subject: Pure Chemistry (6092)
Topic: Acids, Bases and Salts
Version: 1

Section A: Structured Questions

1.
(a) An acid that is completely ionised / dissociated in water. [1]
(b) Hydrochloric acid is a strong acid and ionises completely to produce a high concentration of $H^+$ ions. Ethanoic acid is a weak acid and ionises partially, producing a lower concentration of $H^+$ ions. Since pH is a measure of $H^+$ concentration, the higher $[H^+]$ in HCl results in a lower pH. [2]
(1 mark for complete vs partial ionisation, 1 mark for link to $H^+$ concentration/pH)
(c) Test: Add magnesium ribbon / zinc granules / named reactive metal / named carbonate. [1]
Observation with HCl: Vigorous effervescence / faster bubbling. [0.5]
Observation with CH₃COOH: Slow effervescence / slower bubbling. [0.5]
(Alternatively: Measure electrical conductivity. HCl conducts better.) [2]

2.
(a) An oxide that reacts with both acids and bases to form salt and water. [1]
(b) (i) $ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l)$ [2]
(1 mark for correct formulae, 1 mark for balancing/states)
(ii) $ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$
(Accept $Na_2[Zn(OH)_4]$ ) [2]
(1 mark for correct formulae, 1 mark for balancing/states)

3.
(a) Magnesium oxide / Magnesium carbonate / Magnesium hydroxide. [1]
(Must be insoluble Mg compound)
(b) 1. Add excess base to the acid (warm the acid to speed up reaction). [1]
2. Filter the mixture to remove the unreacted excess base. [1]
3. Heat the filtrate to evaporate some water / until saturated. [1]
4. Allow to cool for crystallisation, then filter, wash with cold water, and dry. [1]
(c) Magnesium hydroxide is insoluble in water, so it cannot be placed in the burette for titration. Titration requires both reactants to be in solution (aqueous). [1]

4.
(a) P: Aluminium ion ( $Al^{3+}$ ) [1]
Q: Iron(II) ion ( $Fe^{2+}$ ) [1]
R: Copper(II) ion ( $Cu^{2+}$ ) [1]
(b) $Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)$ [1]
(State symbols required)

5.
(a) Iron / Fe. [1]
(b) High pressure favours the side with fewer moles of gas. The forward reaction produces 2 moles of gas from 4 moles of gas. Therefore, high pressure increases the yield of ammonia. [2]
(c) A lower temperature would increase the yield (as the reaction is exothermic) but would decrease the rate of reaction significantly. 450°C is a compromise temperature to ensure a reasonable rate of reaction while maintaining an acceptable yield. [2]
(1 mark for rate argument, 1 mark for compromise/yield argument)

6.
(a) $CuCO_3(s) + 2HNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + H_2O(l) + CO_2(g)$ [2]
(b) 1. Effervescence / bubbles of gas produced. [1]
2. The solid (green) disappears / dissolves to form a blue solution. [1]
(c) Moles of $CuCO_3 = 12.4 / 124 = 0.1$ mol. [1]
From equation, 1 mol $CuCO_3$ produces 1 mol $CO_2$ .
Moles of $CO_2 = 0.1$ mol. [1]
Volume = $0.1 \times 24 = 2.4$ dm³. [1]

Section B: Free-Response Questions

7.
(a) Enzymes in plants work best at specific pH levels. If soil is too acidic or alkaline, nutrient uptake is affected / enzymes denature, reducing crop yield. [1]
(b) $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$
(Or $2H^+ + Ca(OH)_2 \rightarrow Ca^{2+} + 2H_2O$ ) [1]
(c) (i) $NH_3$ (Ammonia). [1]
(ii) Ammonia is a gas and will escape into the air, leading to a loss of nitrogen fertilizer / reducing the effectiveness of the fertilizer. [2]
(d) (i) $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$ [2]
(ii) Calcium carbonate is less corrosive / safer to handle / reacts more slowly (less violent) than calcium hydroxide. [1]

8.
(a) At the start, the concentration of reactants (HCl) is highest. This means there are more reactant particles per unit volume, leading to a higher frequency of effective collisions. [2]
(1 mark for highest concentration, 1 mark for frequency of collisions)
(b) Rate = Change in volume / Change in time
Rate = $45 cm^3 / 60 s = 0.75 cm^3/s$ . [2]
(1 mark for substitution, 1 mark for answer with units)
(c) (i) The curve will be steeper at the beginning (higher initial rate) but will level off at the same final volume (60 $cm^3$ ). [2]
(1 mark for steeper initial gradient, 1 mark for same final height)
(ii) Powder has a larger total surface area than chips. This exposes more particles to collisions, increasing the frequency of effective collisions, thus increasing the rate. [2]
(1 mark for surface area, 1 mark for collision frequency)
(d) (i) 30 $cm^3$ . [1]
(ii) Halving the concentration halves the number of moles of HCl available. Since CaCO₃ is in excess, HCl is the limiting reagent. Therefore, half the amount of product ( $CO_2$ ) is formed. [2]
(1 mark for limiting reagent concept, 1 mark for proportional yield)