Secondary 3 Chemistry Semestral Assessment 2 (End of Year) Paper 4

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TuitionGoWhere Exam Practice (AI)

Subject: Chemistry Level: Secondary 3 Paper: SA2 (Version 4 of 5) Duration: 1 hour 45 minutes Total Marks: 80 Name: ____________________ Class: __________ Date: __________

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Instructions to Candidates

1. Answer all questions in the spaces provided.
2. Write in dark blue or black pen.
3. Where necessary, use a calculator.
4. Show all working for calculations.
5. For questions requiring a "dot-and-cross" diagram, use clear dots (•) and crosses (×).

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Section A: Structured Questions (50 Marks)

Question 1 A student is investigating the properties of an unknown oxide, Compound X. (a) Compound X reacts with dilute hydrochloric acid to form a salt and water. [1] (b) Compound X also reacts with sodium hydroxide solution to form a soluble salt and water. [1] (c) Based on the observations in (a) and (b), state the term used to describe Compound X. [1] (d) Suggest a possible identity for Compound X. [1] (e) Write a balanced chemical equation for the reaction of Compound X with hydrochloric acid. [2]

Question 2 The table below shows the melting points of three substances.

Substance	Melting Point (°C)
Magnesium oxide	2,852
Carbon tetrachloride	-23.1
Sodium chloride	801

(a) Explain, in terms of structure and bonding, why carbon tetrachloride has a significantly lower melting point than sodium chloride. [3] (b) Compare the melting point of magnesium oxide with that of sodium chloride. Explain the difference by referring to the charges of the ions present. [3]

Question 3 A solution of an acid, R, is titrated against a 0.100 mol/dm³ solution of sodium hydroxide. (a) The student records the following titration volumes of R:

• Rough: 22.50 cm³
• Trial 1: 21.10 cm³
• Trial 2: 21.20 cm³
• Trial 3: 21.10 cm³ Calculate the average volume of R required for complete neutralization. [1] (b) Calculate the number of moles of sodium hydroxide in 25.0 cm³ of the alkali solution. [1] (c) If the average volume of R used was 21.15 cm³, and the reaction is 1:1, calculate the concentration of acid R in mol/dm³. [2]

Question 4 (a) Which solid compound is commonly added to acidic soil to increase the pH? [1] (b) Explain how this compound increases the pH of the soil. [2] (c) State two other common salts that are soluble in water. [2]

Question 5 (a) Draw the "dot and cross" diagram to show the bonding in phosphorus trichloride ($\text{PCl}_3$). Show only the valence electrons. [2] (b) State whether phosphorus trichloride is a giant covalent structure or a simple molecular structure. [1] (c) Explain why phosphorus trichloride does not conduct electricity in any state. [2]

Question 6 (a) Name two compounds that can be reacted together to form an ammonium salt. [1] (b) Write the balanced chemical equation for the reaction between ammonia and sulfuric acid. [2] (c) Describe a chemical test to identify the presence of ammonium ions ($\text{NH}_4^+$) in a salt. [2]

Question 7 (a) Define the term monatomic. [1] (b) Give an example of a monatomic element. [1] (c) Draw the electronic structure of Neon. [1]

Question 8 (a) State the atom in chlorofluorocarbons (CFCs) responsible for the depletion of the ozone layer. [1] (b) Describe how this atom is released from the CFC molecule. [2]

Question 9 A salt is prepared by reacting an excess of zinc carbonate with dilute nitric acid. (a) Name the salt produced. [1] (b) Explain why an excess of zinc carbonate is used. [2] (c) Describe how the pure salt is obtained from the resulting mixture. [3]

Question 10 (a) Distinguish between a strong acid and a concentrated acid. [2] (b) Give an example of a weak acid. [1] (c) Explain why a weak acid has a higher pH than a strong acid of the same concentration. [2]

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Section B: Free-Response Questions (30 Marks)

Question 11 A student is tasked with preparing a pure sample of barium sulfate. (a) State the method of salt preparation that should be used. [1] (b) Name the two starting reagents required. [2] (c) Describe the experimental procedure to obtain the pure dry salt, including the steps for purification. [5] (d) Write the balanced ionic equation for this reaction. [2]

Question 12 (a) Compare the properties of diamond and graphite. Explain their differences in terms of their structure and bonding. [6] (b) Why is graphite used as a lubricant while diamond is used in cutting tools? [4]

Question 13 A sample of an organic acid has a relative molecular mass of 60. (a) Identify the functional group present in a carboxylic acid. [1] (b) Suggest the structural formula of the organic acid. [2] (c) Describe the reaction between this organic acid and ethanol. Name the product formed. [4]

Question 14 (a) Explain the process of the Haber Process for the manufacture of ammonia, including the necessary conditions (temperature, pressure, and catalyst). [6] (b) Explain why a compromise temperature is used in the Haber Process instead of a very low temperature. [4]

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Answer Key - Chemistry Secondary 3 SA2 (Version 4)

Section A: Structured Questions

Question 1 (a) It reacts with acid $\rightarrow$ Basic/Amphoteric property. [1] (b) It reacts with alkali $\rightarrow$ Acidic/Amphoteric property. [1] (c) Amphoteric oxide. [1] (d) Aluminum oxide ($\text{Al}_2\text{O}_3$) or Zinc oxide ($\text{ZnO}$). [1] (e) $\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$ (or $\text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}$). [2]

Question 2 (a) $\text{CCl}_4$ is a simple molecular structure with weak van der Waals forces between molecules. [1] $\text{NaCl}$ is a giant ionic lattice with strong electrostatic forces of attraction between $\text{Na}^+$ and $\text{Cl}^-$ ions. [1] Much more energy is required to break the ionic bonds than the intermolecular forces. [1] (b) $\text{MgO}$ has a higher melting point. [1] $\text{Mg}^{2+}$ and $\text{O}^{2-}$ have higher charges than $\text{Na}^+$ and $\text{Cl}^-$. [1] Stronger electrostatic attraction between ions of higher charge requires more energy to overcome. [1]

Question 3 (a) $(21.10 + 21.20 + 21.10) / 3 = 21.13\text{ cm}^3$ (Rough run excluded). [1] (b) $n = c \times V = 0.100 \times (25.0 / 1000) = 0.0025\text{ mol}$. [1] (c) $\text{Moles of acid} = 0.0025\text{ mol}$. [1] $\text{Conc} = 0.0025 / (21.15 / 1000) = 0.118\text{ mol/dm}^3$. [1]

Question 4 (a) Calcium oxide ($\text{CaO}$) / Calcium hydroxide ($\text{Ca(OH)}_2$) / Calcium carbonate ($\text{CaCO}_3$). [1] (b) These are basic compounds. [1] They neutralize the acid in the soil, thereby increasing the pH. [1] (c) Sodium chloride, Potassium nitrate (any two soluble salts). [2]

Question 5 (a) Diagram showing P in center with 3 shared pairs of electrons with 3 Cl atoms. P has 1 lone pair. [2] (b) Simple molecular structure. [1] (c) No free ions or electrons. [1] Covalent bonds hold electrons tightly between atoms. [1]

Question 6 (a) Ammonia (or ammonium hydroxide) and an acid (e.g., $\text{HCl}$). [1] (b) $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4$. [2] (c) Heat salt with aqueous $\text{NaOH}$. [1] Ammonia gas is evolved, which turns moist red litmus paper blue. [1]

Question 7 (a) Consisting of a single atom per particle/molecule. [1] (b) Helium / Neon / Argon / Krypton / Xenon. [1] (c) Nucleus with 10 electrons (2 in 1st shell, 8 in 2nd shell). [1]

Question 8 (a) Chlorine (Cl). [1] (b) UV radiation breaks the $\text{C-Cl}$ bond in the CFC molecule. [1] This releases a highly reactive chlorine free radical. [1]

Question 9 (a) Zinc nitrate. [1] (b) To ensure all the nitric acid is completely reacted/neutralized. [1] (c) Filtration to remove excess $\text{ZnCO}_3$. [1] Evaporation of filtrate to saturation point. [1] Crystallization and drying of crystals. [1]

Question 10 (a) Strong acid: Completely ionizes in aqueous solution. [1] Concentrated acid: High amount of solute (acid) per unit volume of solvent. [1] (b) Ethanoic acid / Citric acid. [1] (c) Weak acids only partially ionize in water. [1] This results in a lower concentration of $\text{H}^+$ ions, leading to a higher pH. [1]

Section B: Free-Response Questions

Question 11 (a) Precipitation. [1] (b) Barium chloride ($\text{BaCl}_2$) and Sodium sulfate ($\text{Na}_2\text{SO}_4$). [2] (c) Mix the two aqueous solutions to form a precipitate. [1] Filter the mixture to collect the precipitate. [1] Wash the precipitate with distilled water to remove impurities. [1] Filter again. [1] Dry the salt in an oven or between filter papers. [1] (d) $\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$. [2]

Question 12 (a) Diamond: Each C bonded to 4 others in a 3D tetrahedral lattice. [2] Very strong covalent bonds throughout. [2] Graphite: Each C bonded to 3 others in hexagonal layers. [2] Weak forces between layers. [2] (Max 6) (b) Graphite: Layers can slide over each other due to weak intermolecular forces. [2] Diamond: Extremely hard due to the rigid 3D network of strong covalent bonds. [2]

Question 13 (a) $-\text{COOH}$ (Carboxyl group). [1] (b) $\text{CH}_3\text{CH}_2\text{COOH}$ (Ethanoic acid). [2] (c) Reaction is esterification. [1] Heat with concentrated sulfuric acid as catalyst. [1] Product is ethyl ethanoate. [1] Sweet smelling liquid. [1]

Question 14 (a) $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$. [1] Temp: $450^\circ\text{C}$. [1] Pressure: $200\text{ atm}$. [1] Catalyst: Iron. [1] Nitrogen and hydrogen are reacted. [1] Ammonia is liquefied and removed. [1] (b) Low temperature favors the forward reaction (exothermic). [2] However, low temperature makes the rate of reaction too slow to be economically viable. [2]