Secondary 4 Combined Science Chemistry Preliminary Examination Paper 3

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TuitionGoWhere Exam Practice (AI)

Subject: Combined Science Chemistry
Level: Secondary 4
Paper: Preliminary Examination (Version 3 of 5)
Duration: 1 hour 15 minutes
Total Marks: 65

Name: __________________________ Class: __________ Date: __________

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Instructions to Candidates:

1. Answer all questions in the spaces provided.
2. Write your answers clearly and use a blue or black pen.
3. For calculation questions, show all working and provide answers to 3 significant figures.
4. State symbols must be included in chemical equations where requested.

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Section A: Multiple Choice Questions (10 Marks)

Answer all questions. Each question carries 1 mark.

1. Which of the following is a mixture? A. Pure water B. Limestone C. Vinegar D. Glucose

2. Which apparatus is most suitable for delivering exactly 25.0 cm³ of sodium hydroxide solution into a conical flask? A. Beaker B. Burette C. Pipette D. Measuring cylinder

3. Which of the following describes a property of an ionic compound? A. Low melting point B. Conducts electricity in the solid state C. Soluble in organic solvents D. Conducts electricity when molten

4. An element X has the electronic structure 2, 8, 1. To which group and period does it belong? A. Group 1, Period 2 B. Group 1, Period 3 C. Group 3, Period 1 D. Group 2, Period 3

5. What is the relative molecular mass of urea, $\text{CO}(\text{NH}_2)_2$? [$\text{C}=12, \text{O}=16, \text{N}=14, \text{H}=1$] A. 44 B. 60 C. 72 D. 86

6. Which of the following reactions is a neutralisation reaction? A. $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$ B. $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ C. $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ D. $\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$

7. Which of the following is the correct observation when bromine water is added to an unsaturated organic acid? A. No observable change B. Orange-brown colour turns colourless C. Colourless solution turns blue D. Effervescence is observed

8. Which statement about the atomic radius of elements in Period 2 is correct? A. It increases from left to right. B. It remains constant across the period. C. It decreases from left to right. D. It fluctuates randomly.

9. Which of the following is a property of an alkali? A. Turns blue litmus paper red B. pH value less than 7 C. Reacts with metals to produce hydrogen gas D. Tastes sour

10. Which of the following is the correct formula for the salt formed from the reaction between magnesium and nitric acid? A. $\text{MgNO}_3$ B. $\text{Mg}(\text{NO}_3)_2$ C. $\text{Mg}_2\text{NO}_3$ D. $\text{MgNO}_2$

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Section B: Structured Questions (55 Marks)

Question 11 A student is studying the properties of a substance, Compound Z. Compound Z has a high melting point, is soluble in water, and conducts electricity only when in aqueous or molten state. (a) State the bonding type of Compound Z. [1]

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(b) Explain why Compound Z does not conduct electricity in the solid state. [2]

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(c) Draw a dot-and-cross diagram to show the bonding in $\text{MgCl}_2$, a substance with similar properties to Z. Show outer electrons only. [2]

Question 12 The reaction between dilute hydrochloric acid and sodium carbonate is represented by the following equation: $\text{Na}_2\text{CO}_3(s) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$ (a) Classify this reaction using the term "neutralisation". Is this a correct classification? Explain your answer. [2]

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(b) Calculate the mass of $\text{Na}_2\text{CO}_3$ required to react completely with $100\text{ cm}^3$ of $0.5\text{ mol/dm}^3$ $\text{HCl}$. [3] [$\text{Na}=23, \text{C}=12, \text{O}=16$]

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Question 13 (a) Describe a chemical test to determine if a sample of an organic acid is unsaturated. State the reagent and the observation for a positive result. [2]

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(b) Carbon monoxide ($\text{CO}$) is a pollutant often produced by incomplete combustion. Describe the effects of $\text{CO}$ on human beings. [2]

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Question 14 (a) Complete the table below for the following particles. [4]

Particle	Protons	Electrons	Neutrons
$\text{Na}$ (Atomic No: 11, Mass No: 23)
$\text{Na}^+$
$\text{Cl}^-$ (Atomic No: 17, Mass No: 35)
$\text{Ca}^{2+}$ (Atomic No: 20, Mass No: 40)

(b) Explain the trend in atomic radius as you move down Group 1 of the Periodic Table. [2]

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Question 15 A reaction is carried out by adding zinc granules to dilute sulfuric acid. (a) Write the balanced chemical equation for this reaction, including state symbols. [2]

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(b) The student observes that the reaction rate increases when the temperature is raised. Using collision theory, explain why this happens. [3]

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Question 16 (a) Define the term "strong acid". [2]

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(b) Compare the pH of a $0.1\text{ mol/dm}^3$ solution of $\text{HCl}$ and a $0.1\text{ mol/dm}^3$ solution of $\text{CH}_3\text{COOH}$. Which one has a lower pH? Explain your answer. [3]

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Question 17 (a) Suggest why aluminium is widely recycled rather than being extracted from its ore, bauxite. [1]

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(b) Describe the method used to prepare a pure, dry sample of copper(II) sulfate crystals from copper(II) oxide and sulfuric acid. [5]

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Question 18 (a) An element Y is in Period 3 and Group 2. (i) State the electronic structure of element Y. [1]

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(ii) Name element Y. [1]

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(b) Explain how the electronic structure of element Y determines its group. [2]

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Question 19 The following terms are used to describe chemical reactions: {addition, neutralisation, redox, decomposition, substitution}. Classify the following reactions: [5] (i) $\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$: ______________________ (ii) $\text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g)$: ______________________ (iii) $\text{KOH}(aq) + \text{HNO}_3(aq) \rightarrow \text{KNO}_3(aq) + \text{H}_2\text{O}(l)$: ______________________ (iv) $\text{CH}_4 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{HCl}$: ______________________ (v) $\text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g)$: ______________________

Question 20 (a) Calculate the concentration in $\text{mol/dm}^3$ of a solution containing $4.0\text{ g}$ of $\text{NaOH}$ dissolved in $250\text{ cm}^3$ of water. [3] [$\text{Na}=23, \text{O}=16, \text{H}=1$]

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(b) If $20\text{ cm}^3$ of this $\text{NaOH}$ solution is neutralized by $15\text{ cm}^3$ of $\text{HCl}$, calculate the concentration of the $\text{HCl}$ solution. [4]

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Answer Key - Combined Science Chemistry Prelim (Version 3)

Section A: Multiple Choice

1. C (Vinegar is a solution of acetic acid in water)
2. C (Pipette is used for precise fixed volumes)
3. D (Ionic compounds conduct when ions are free to move)
4. B (3 shells = Period 3; 1 valence electron = Group 1)
5. B ($12 + 16 + 2(14 + 2) = 60$)
6. C (Acid + Alkali $\rightarrow$ Salt + Water)
7. B (Bromine water decolorizes in the presence of C=C)
8. C (Increased nuclear charge pulls electrons closer)
9. C (Alkalis react with amphoteric/active metals to release $\text{H}_2$)
10. B ($\text{Mg}^{2+}$ and $\text{NO}_3^-$ ions)

Section B: Structured Questions

Question 11 (a) Ionic bonding [1] (b) In solid state, ions are held in a rigid giant lattice by strong electrostatic forces [1]. Ions are not free to move to carry charge [1]. (c) Diagram showing $\text{Mg}$ losing 2 electrons to two $\text{Cl}$ atoms. $\text{Mg}$ ion with $[\text{Ne}]$ core and $2+$ charge; $\text{Cl}$ ions with $[\text{Ar}]$ core and $1-$ charge. [2]

Question 12 (a) Yes [1]. It involves a reaction between an acid ($\text{HCl}$) and a carbonate/base ($\text{Na}_2\text{CO}_3$) to produce a salt and water [1]. (b) Moles of $\text{HCl} = 0.5 \times (100/1000) = 0.05\text{ mol}$ [1] Mole ratio $\text{Na}_2\text{CO}_3 : \text{HCl} = 1 : 2$. Moles of $\text{Na}_2\text{CO}_3 = 0.05 / 2 = 0.025\text{ mol}$ [1] Molar mass $\text{Na}_2\text{CO}_3 = (23\times2) + 12 + (16\times3) = 106\text{ g/mol}$ [1] Mass $= 0.025 \times 106 = 2.65\text{ g}$ [1]

Question 13 (a) Reagent: Bromine water [1]. Observation: Orange-brown colour turns colourless [1]. (b) $\text{CO}$ binds irreversibly to haemoglobin in blood [1]. This reduces the capacity of blood to transport oxygen to tissues, leading to fatigue or death [1].

Question 14 (a)

• $\text{Na}$: 11, 11, 12 [1]
• $\text{Na}^+$: 11, 10, 12 [1]
• $\text{Cl}^-$: 17, 18, 18 [1]
• $\text{Ca}^{2+}$: 20, 18, 20 [1] (b) Atomic radius increases [1] because the number of occupied electron shells increases, increasing the distance between the nucleus and valence electrons [1].

Question 15 (a) $\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$ [2] (b) Higher temperature increases the average kinetic energy of particles [1]. Particles collide more frequently [1] and with energy greater than or equal to the activation energy, leading to more effective collisions [1].

Question 16 (a) An acid that completely dissociates/ionizes into ions in aqueous solution [2]. (b) $\text{HCl}$ has a lower pH [1]. $\text{HCl}$ is a strong acid and ionizes completely, producing a higher concentration of $\text{H}^+$ ions [1] compared to $\text{CH}_3\text{COOH}$ which is a weak acid and only partially ionizes [1].

Question 17 (a) Recycling requires significantly less energy than extracting aluminium from bauxite [1]. (b)

1. Add excess copper(II) oxide to warm sulfuric acid [1].
2. Filter the mixture to remove unreacted copper(II) oxide [1].
3. Heat the filtrate to evaporate excess water (crystallization) [1].
4. Allow the solution to cool and crystals to form [1].
5. Filter and dry the crystals between filter papers [1].

Question 18 (a) (i) 2, 8, 2 [1] (ii) Magnesium [1] (b) The number of valence electrons (2) corresponds to the Group number (Group 2) [2].

Question 19 (i) Decomposition [1] (ii) Redox [1] (iii) Neutralisation [1] (iv) Substitution [1] (v) Addition [1]

Question 20 (a) Molar mass $\text{NaOH} = 40\text{ g/mol}$ [1] Moles $= 4.0 / 40 = 0.1\text{ mol}$ [1] Conc $= 0.1 / (250/1000) = 0.4\text{ mol/dm}^3$ [1] (b) Moles of $\text{NaOH} = 0.4 \times (20/1000) = 0.008\text{ mol}$ [1] Mole ratio $\text{NaOH} : \text{HCl} = 1 : 1 \rightarrow$ Moles of $\text{HCl} = 0.008\text{ mol}$ [1] Conc $\text{HCl} = 0.008 / (15/1000) = 0.533\text{ mol/dm}^3$ [2]