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Secondary 4 Pure Chemistry Preliminary Examination Paper 5

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Secondary 4 Pure Chemistry - Preliminary Examination

Subject: Pure Chemistry
Level: Secondary 4
Paper: Preliminary Paper 2 (Version 5 of 5)
Duration: 1 hour 45 minutes
Total Marks: 80

Name: ___________________________ Class: ___________ Date: ___________

Instructions to Candidates:

Answer all questions in the spaces provided.
Write your answers clearly and concisely.
For calculations, show all working. Use 2 decimal places unless specified otherwise.
State symbols must be included in chemical equations where required.

Section A: Structured Questions (50 Marks)

Question 1 A student is investigating the properties of an unknown gas, Gas X. When Gas X is bubbled through a solution of sodium hydroxide, a salt and water are formed. (a) If Gas X is sulfur dioxide, write the balanced chemical equation for its reaction with sodium hydroxide. [2]

(b) Explain why sulfur dioxide is described as a gas that contributes to acid rain. [2]

Question 2 A chemist needs to prepare a pure sample of magnesium sulfate. (a) Discuss the suitability of reacting magnesium ribbon with dilute hydrochloric acid to obtain this salt. [2]

(b) State one observation that would be seen during the reaction in part (a). [1]

Question 3 Two colorless solutions, Solution A containing $\text{Al}^{3+}$ ions and Solution B containing $\text{Pb}^{2+}$ ions, are provided. (a) Describe a simple chemical test using aqueous sodium hydroxide to differentiate between Solution A and Solution B. [3]

(b) Write the ionic equation for the formation of the precipitate in Solution A. [2]

Question 4 The Haber Process is used for the industrial manufacture of ammonia. (a) State the conditions (temperature, pressure, and catalyst) used in the Haber Process. [3]

(b) Explain why a compromise temperature is used rather than a very low temperature, despite the reaction being exothermic. [2]

Question 5 A sample of an insoluble salt is reacted with an acid to produce a soluble salt. (a) If the insoluble salt is calcium carbonate and the acid is dilute nitric acid, state the formula of the salt produced. [1]

(b) Describe the method to obtain a pure, dry sample of the salt from the resulting solution. [3]

Question 6 (a) Define a "strong acid" in terms of its ionization in aqueous solution. [2]

(b) Compare the pH of a $0.1 \text{ mol/dm}^3$ solution of hydrochloric acid with a $0.1 \text{ mol/dm}^3$ solution of ethanoic acid. Explain your answer. [3]

Question 7 A student performs a titration to determine the concentration of a sodium hydroxide solution using $0.10 \text{ mol/dm}^3$ sulfuric acid. (a) Name a suitable indicator for this titration and state the color change at the end-point. [2]

(b) If $25.0 \text{ cm}^3$ of $\text{NaOH}$ required $20.0 \text{ cm}^3$ of $\text{H}_2\text{SO}_4$ , calculate the concentration of the $\text{NaOH}$ solution in $\text{mol/dm}^3$ . [4]

Question 8 (a) State the solubility of the following salts: [2] i. Silver chloride: ____________________ ii. Potassium nitrate: ____________________ (b) Describe how to prepare a pure sample of lead(II) iodide. [3]

Question 9 A gas is evolved when a metal carbonate reacts with an acid. (a) Identify the gas and describe the test to confirm its identity. [2]

(b) Write the general word equation for the reaction between a metal carbonate and an acid. [2]

Question 10 Explain how the addition of lime ( $\text{CaO}$ ) to acidic soil helps in improving crop yield. [3]

Section B: Free-Response Questions (30 Marks)

Question 11 (a) Describe the process of preparing a soluble salt using the titration method. Use the preparation of sodium chloride from sodium hydroxide and hydrochloric acid as an example. [6]

(b) Why is the titration method preferred over the direct reaction of a metal with an acid when the starting material is a soluble base? [2]

Question 12 A student is given three unknown salts: $\text{ZnSO}_4$ , $\text{CuSO}_4$ , and $\text{FeSO}_4$ . (a) Describe how the student can identify each salt using only aqueous sodium hydroxide. [6]

(b) Write the balanced chemical equation for the reaction of $\text{CuSO}_4$ with $\text{NaOH}$ . [2]

Question 13 (a) Explain the difference between a salt and an acid. [3]

(b) A salt is formed by the reaction of a strong acid and a weak base. Predict whether the resulting aqueous solution of this salt will be acidic, alkaline, or neutral. Explain your reasoning. [4]

Question 14 (a) Describe the role of the catalyst in the Haber Process. [2]

(b) Discuss the environmental impact of the production of ammonia, specifically regarding the energy required for high pressure. [3]

Answers

Answer Key - Pure Chemistry Preliminary Paper 2 (Version 5)

Section A: Structured Questions

Question 1 (a) $\text{SO}_2\text{(g)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{SO}_3\text{(aq)} + \text{H}_2\text{O(l)}$ [2] (b) $\text{SO}_2$ is an acidic oxide. It dissolves in rainwater to form sulfurous acid ( $\text{H}_2\text{SO}_3$ ), which lowers the pH of rain, making it acidic. [2]

Question 2 (a) Suitable. Magnesium is reactive enough to react steadily with dilute $\text{HCl}$ to produce $\text{MgSO}_4$ and $\text{H}_2$ gas, but not so reactive (like $\text{Na}$ or $\text{K}$ ) that the reaction is explosive/uncontrollable. [2] (b) Effervescence / bubbles of gas produced / magnesium ribbon disappears. [1]

Question 3 (a) Add aqueous $\text{NaOH}$ to both solutions. Both form white precipitates. [1] Add excess $\text{NaOH}$ . The precipitate in Solution A ( $\text{Al}^{3+}$ ) dissolves to form a colorless solution, while the precipitate in Solution B ( $\text{Pb}^{2+}$ ) remains insoluble/does not dissolve. [2] (b) $\text{Al}^{3+}\text{(aq)} + 3\text{OH}^-\text{(aq)} \rightarrow \text{Al(OH)}_3\text{(s)}$ [2]

Question 4 (a) Temperature: $450^\circ\text{C}$ , Pressure: $200 \text{ atm}$ , Catalyst: Iron [3] (b) Low temperature favors the exothermic forward reaction (higher yield), but the rate of reaction would be too slow to be economically viable. A compromise temperature ensures a reasonable rate and yield. [2]

Question 5 (a) $\text{Ca(NO}_3)_2$ [1] (b) Filter the mixture to remove unreacted calcium carbonate. [1] Heat the filtrate to evaporate water until the point of crystallization. [1] Allow to cool and crystallize, then filter and dry the crystals. [1]

Question 6 (a) A strong acid is one that completely ionizes/dissociates in aqueous solution to produce $\text{H}^+$ ions. [2] (b) $\text{HCl}$ has a lower pH than ethanoic acid. [1] $\text{HCl}$ is a strong acid and fully ionizes, producing a higher concentration of $\text{H}^+$ ions. [1] Ethanoic acid is a weak acid and only partially ionizes. [1]

Question 7 (a) Methyl orange (Red to Yellow) OR Phenolphthalein (Pink to Colorless). [2] (b) $\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$ $\text{moles of } \text{H}_2\text{SO}_4 = 0.10 \times (20/1000) = 0.002 \text{ mol}$ [1] $\text{moles of } \text{NaOH} = 0.002 \times 2 = 0.004 \text{ mol}$ [1] $\text{Concentration of } \text{NaOH} = 0.004 / (25/1000) = 0.16 \text{ mol/dm}^3$ [2]

Question 8 (a) i. Insoluble; ii. Soluble [2] (b) Mix aqueous lead(II) nitrate and potassium iodide. [1] A yellow precipitate of lead(II) iodide forms. [1] Filter the precipitate, wash with distilled water, and dry. [1]

Question 9 (a) Carbon dioxide. [1] Bubble the gas through limewater; it turns cloudy/milky. [1] (b) Metal carbonate + Acid $\rightarrow$ Salt + Water + Carbon dioxide [2]

Question 10 $\text{CaO}$ is a basic oxide. [1] It reacts with the $\text{H}^+$ ions in acidic soil to neutralize them. [1] This raises the soil pH to a level suitable for plant growth, improving nutrient availability. [1]

Section B: Free-Response Questions

Question 11 (a) Use a pipette to transfer a fixed volume of $\text{NaOH}$ into a conical flask. [1] Add a few drops of indicator. [1] Titrate with $\text{HCl}$ from a burette until the indicator changes color (end-point). [1] Repeat to find a concordant average volume. [1] Prepare a second mixture using the average volume of $\text{HCl}$ and $\text{NaOH}$ to ensure exact neutralization. [1] Evaporate the water and crystallize the $\text{NaCl}$ . [1] (b) Because the base is already soluble, the reaction is instantaneous and the titration allows for the exact stoichiometric amount of acid to be added, preventing the final salt solution from being too acidic. [2]

Question 12 (a) Add $\text{NaOH}$ to each. $\text{CuSO}_4$ forms a blue precipitate. [2] $\text{ZnSO}_4$ and $\text{FeSO}_4$ both form precipitates ( $\text{Zn}$ white, $\text{Fe}$ green). [2] Add excess $\text{NaOH}$ : the white precipitate of $\text{Zn(OH)}_2$ dissolves, whereas the green precipitate of $\text{Fe(OH)}_2$ does not. [2] (b) $\text{CuSO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Cu(OH)}_2\text{(s)} + \text{Na}_2\text{SO}_4\text{(aq)}$ [2]

Question 13 (a) An acid is a substance that produces $\text{H}^+$ ions in aqueous solution. [1] A salt is an ionic compound formed when the hydrogen ions of an acid are partially or totally replaced by a metal ion or ammonium ion. [2] (b) Acidic. [1] A strong acid fully dissociates, while a weak base only partially dissociates. [1] The resulting salt contains the conjugate base of the strong acid and the conjugate acid of the weak base. [1] The hydrolysis of the salt ions results in a net increase of $\text{H}^+$ ions in solution. [1]

Question 14 (a) The iron catalyst provides an alternative reaction pathway with a lower activation energy. [2] (b) High pressure requires massive energy for compressors. [1] This energy is often derived from burning fossil fuels. [1] This leads to the emission of $\text{CO}_2$ , contributing to the greenhouse effect and global warming. [1]