Secondary 4 Pure Chemistry Preliminary Examination Paper 4

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Secondary 4 Pure Chemistry Quiz - Acids Bases Salts

Name: ____________________
Class: ____________________
Date: ____________________
Score: ________ / 45

Duration: 60 Minutes
Total Marks: 45 Marks

Instructions:

• Answer all questions in the spaces provided.
• For chemical equations, ensure all formulae are balanced and state symbols are included where requested.
• Use a calculator where necessary.

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Section A: Short Answer and Conceptual Questions (1-10)

1. State the definition of a strong acid in terms of its ionisation in aqueous solution. [1]

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2. A gas is produced when a metal carbonate reacts with dilute hydrochloric acid. Identify the gas and state the observation when this gas is passed through limewater. [2] Gas: ____________________
   Observation: ________________________________________________________________

3. Write a balanced chemical equation, including state symbols, for the reaction between magnesium ribbon and dilute sulfuric acid. [2]

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4. Explain why a solution of ethanoic acid has a higher pH than a solution of hydrochloric acid, even though both have the same concentration. [2]

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5. A student reacts sulfur with oxygen to form a gas that contributes to acid rain. Name this gas and write the equation for its formation. [2] Gas: ____________________
   Equation: __________________________________________________________________

6. Define the term 'neutralisation'. [1]

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7. State the colour change observed when phenolphthalein indicator is added to a solution of sodium hydroxide. [1]

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8. Which of the following is a base but not an alkali? (A) Sodium hydroxide (B) Potassium hydroxide (C) Copper(II) oxide (D) Ammonia [1] Answer: ________

9. Write the balanced chemical equation for the reaction between carbon dioxide gas and aqueous sodium hydroxide. [2]

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10. Explain why an excess of acid is added when reacting a metal carbonate with acid to prepare a salt. [2]

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Section B: Structured Response and Application (11-15)

11. A student is tasked with preparing a pure sample of zinc sulfate. (a) Suggest a suitable starting material (metal or base) to react with dilute sulfuric acid. [1] ________________________________________________________________________ (b) Discuss the suitability of using zinc powder versus zinc granules in this reaction. [2] ________________________________________________________________________ ________________________________________________________________________ (c) Describe the process used to remove any unreacted zinc from the solution. [1] ________________________________________________________________________

12. Two colorless solutions, X and Y, contain either $\text{Al}^{3+}$ or $\text{Zn}^{2+}$ ions. (a) Describe a simple test using aqueous sodium hydroxide to differentiate between solution X and solution Y. [2] ________________________________________________________________________ ________________________________________________________________________ (b) If solution X contains $\text{Al}^{3+}$, describe the observation when excess sodium hydroxide is added. [1] ________________________________________________________________________

13. A salt is prepared by the precipitation method. (a) State the solubility rule that governs the formation of a precipitate. [1] ________________________________________________________________________ (b) Name the two types of salts that must be soluble for this method to be feasible. [2] ________________________________________________________________________ ________________________________________________________________________

14. The Haber Process is used to manufacture ammonia. (a) State the chemical equation for the production of ammonia. [2] ________________________________________________________________________ (b) Explain why a compromise temperature is used rather than a very low temperature, despite the reaction being exothermic. [2] ________________________________________________________________________ ________________________________________________________________________

15. A student tests the pH of three different solutions: A (pH 2), B (pH 7), and C (pH 13). (a) Identify which solution is a strong alkali. [1] ________________________________________________________________________ (b) If solution A is diluted with distilled water, explain what happens to its pH. [2] ________________________________________________________________________ ________________________________________________________________________

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Section C: Data Interpretation and Synthesis (16-20)

16. A titration is performed between $25.0\text{ cm}^3$ of $0.10\text{ mol/dm}^3$ $\text{NaOH}$ and $\text{HCl}$. (a) If $20.0\text{ cm}^3$ of $\text{HCl}$ is required to reach the endpoint, calculate the concentration of the $\text{HCl}$. [3] ________________________________________________________________________ ________________________________________________________________________ (b) State the purpose of the indicator in this experiment. [1] ________________________________________________________________________

17. Compare the properties of a salt of a strong acid and a strong base (e.g., $\text{NaCl}$) with a salt of a weak acid and a strong base (e.g., $\text{CH}_3\text{COONa}$). Which one is alkaline in aqueous solution? Explain why. [3]

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18. A sample of an unknown salt is heated in a test tube. A gas is evolved that turns damp red litmus paper blue. (a) Identify the gas evolved. [1] ________________________________________________________________________ (b) Suggest the identity of the salt. [1] ________________________________________________________________________

19. Describe the method of preparing a soluble salt using the reaction between an acid and an insoluble base (e.g., $\text{CuO}$ and $\text{H}_2\text{SO}_4$). [4]

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

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Secondary 4 Pure Chemistry Quiz - Acids Bases Salts (Answer Key)

1. A strong acid is one that completely ionises/dissociates in aqueous solution to produce $\text{H}^+$ ions. (1)

2. Gas: Carbon dioxide ($\text{CO}_2$). (1) Observation: Limewater turns milky/cloudy. (1)

3. $\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)}$ (2)

4. Ethanoic acid is a weak acid, meaning it only partially ionises in water. (1) Therefore, it has a lower concentration of $\text{H}^+$ ions compared to $\text{HCl}$ (a strong acid) of the same concentration, resulting in a higher pH. (1)

5. Gas: Sulfur dioxide ($\text{SO}_2$). (1) Equation: $\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}$ (1)

6. A reaction between an acid and a base to produce a salt and water. (1)

7. Colorless to pink/magenta. (1)

8. (C) Copper(II) oxide (It is a base but insoluble in water, thus not an alkali). (1)

9. $\text{CO}_2\text{(g)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{CO}_3\text{(aq)} + \text{H}_2\text{O(l)}$ (2)

10. To ensure that all the carbonate has reacted completely. (1) This ensures the resulting salt is not contaminated with unreacted carbonate. (1)

11. (a) Zinc oxide / Zinc carbonate. (1) (b) Zinc powder has a larger surface area than granules. (1) This increases the rate of reaction, making the preparation faster. (1) (c) Filtration. (1)

12. (a) Add aqueous $\text{NaOH}$ to both. Both will form white precipitates. (1) Add excess $\text{NaOH}$; the precipitate of $\text{Zn}^{2+}$ will dissolve to form a colorless solution, while $\text{Al}^{3+}$ will also dissolve (Wait: both dissolve in excess NaOH). Correction based on syllabus: Both $\text{Al}^{3+}$ and $\text{Zn}^{2+}$ form white precipitates that dissolve in excess $\text{NaOH}$. To differentiate, use aqueous ammonia ($\text{NH}_3$). $\text{Zn}^{2+}$ dissolves in excess $\text{NH}_3$, $\text{Al}^{3+}$ does not. (2) (b) The white precipitate dissolves to form a colorless solution. (1)

13. (a) The salt formed must be insoluble in water. (1) (b) The starting salts (reactants) must both be soluble in water. (2)

14. (a) $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$ (2) (b) A very low temperature would shift the equilibrium to the right (increasing yield) but would make the rate of reaction too slow to be economically viable. (2)

15. (a) Solution C. (1) (b) The pH will increase. (1) Dilution decreases the concentration of $\text{H}^+$ ions per unit volume. (1)

16. (a) $\text{Moles of NaOH} = 0.10 \times (25/1000) = 0.0025\text{ mol}$. (1) $\text{Moles of HCl} = \text{Moles of NaOH} = 0.0025\text{ mol}$ (1:1 ratio). (1) $\text{Concentration of HCl} = 0.0025 / (20/1000) = 0.125\text{ mol/dm}^3$. (1) (b) To indicate the endpoint/neutralisation point by changing color. (1)

17. Salt of weak acid and strong base (e.g., $\text{CH}_3\text{COONa}$) is alkaline. (1) The conjugate base of the weak acid (ethanoate ion) undergoes hydrolysis in water, producing $\text{OH}^-$ ions. (2)

18. (a) Ammonia ($\text{NH}_3$). (1) (b) Ammonium salt (e.g., Ammonium chloride or Ammonium sulfate). (1)

19. 1. Add excess $\text{CuO}$ to warm $\text{H}_2\text{SO}_4$ and stir. (1)
    2. Filter the mixture to remove unreacted $\text{CuO}$. (1)
    3. Evaporate the filtrate to the point of crystallization. (1)
    4. Allow the salt to crystallize, filter, wash with distilled water, and dry. (1)

20. Lime ($\text{CaO}$) is a basic oxide. (1) It reacts with the $\text{H}^+$ ions in the acidic soil (neutralisation). (1) This raises the pH of the soil to a level suitable for the specific crop's growth. (1)