Secondary 4 Pure Chemistry Practice Paper 5

Answer Key - Pure Chemistry Secondary 4 Practice Paper (Version 5)

Section A: Structured Questions

Question 1 (a) Add a dilute acid (e.g., $\text{HCl}$). Observation: Effervescence/bubbles of a colorless, odorless gas are produced. [2] (b) Observation: Effervescence / Bubbles of gas evolved. [1] Equation: $\text{Na}_2\text{CO}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$ [2]

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

Question 3 (a) Any two: Sulfate ($\text{SO}_4^{2-}$), Carbonate ($\text{CO}_3^{2-}$), Phosphate ($\text{PO}_4^{3-}$). [2] (b) For Sulfate: Add barium nitrate solution and dilute nitric acid. [1] Observation: A white precipitate of barium sulfate forms. [1] The nitric acid prevents other anions (like carbonates) from precipitating. [1]

Question 4 (a) $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$ [2] (b) Catalyst: Iron [1]. Temperature: $450^\circ\text{C}$ [1]. Pressure: $200\text{ atm}$ [1]. (c) Low temperature favors the exothermic forward reaction, increasing yield. [1] However, at very low temperatures, the rate of reaction is too slow to be commercially viable. [1] A compromise temperature ensures a reasonable yield at a reasonable rate. [1]

Question 5 (a) Lead(II) carbonate is insoluble. Reacting it with sulfuric acid would form lead(II) sulfate, which is also insoluble and would coat the carbonate particles, preventing further reaction. [2] (b) Method: Precipitation. [1] Salts: Lead(II) nitrate and sodium sulfate (or any soluble lead salt and soluble sulfate salt). [2]

Question 6 (a) A white precipitate is formed, which dissolves in excess sodium hydroxide to form a colorless solution. [2] (b) $\text{Al}^{3+}\text{(aq)} + 3\text{OH}^-\text{(aq)} \rightarrow \text{Al(OH)}_3\text{(s)}$ [2]

Question 7 (a) Magnesium (or Sodium/Potassium, though Mg is more common in this context). [1] (b) Hydrogen ($\text{H}_2$). [1] Test: Place a lighted splint at the mouth of the test tube; a "pop" sound is heard. [1]

Question 8 (a) Add alkaline substances to the soil to neutralize the excess acidity. [2] (b) Slaked lime / Calcium hydroxide / Calcium carbonate. [1] It reacts with the $\text{H}^+$ ions in the soil (neutralization), increasing the pH to a level suitable for plant growth. [2]

Question 9 (a) To identify the end-point of the titration (when neutralization is complete) via a color change. [1] (b) Moles of $\text{H}_2\text{SO}_4 = 0.100 \times (20/1000) = 0.002\text{ mol}$. [1] Moles of $\text{H}^+ = 0.002 \times 2 = 0.004\text{ mol}$. [1] Moles of $\text{NaOH} = 0.004\text{ mol}$. [1] Concentration of $\text{NaOH} = 0.004 / (25/1000) = 0.16\text{ mol/dm}^3$. [1]

Question 10 (a) Silver nitrate: Soluble [1]. Barium sulfate: Insoluble [1]. Potassium chloride: Soluble [1]. (b) Mix the two soluble salts to form a precipitate. [1] Filter the mixture to collect the insoluble salt. [1] Wash the residue with distilled water and dry it (e.g., in an oven). [1]

Section B: Free-Response Questions

Question 11 (a) Ammonia is alkaline. [1] It turns moist red litmus paper blue. [1] It reacts with acids (e.g., $\text{HCl}$) to form a salt (e.g., $\text{NH}_4\text{Cl}$). [2] (b) Nitrogen is a limiting nutrient for plant growth. [1] Fertilizers provide essential nitrogen for protein synthesis and chlorophyll production. [1] This increases crop yields, ensuring a stable food supply for a growing population. [1]

Question 12 (a) Add aqueous $\text{NaOH}$ to the mixture. [1] A blue precipitate forms, indicating $\text{Cu}^{2+}$. [1] A white precipitate also forms, indicating $\text{Zn}^{2+}$. [1] Add excess $\text{NaOH}$. [1] The blue precipitate remains insoluble, while the white precipitate dissolves to form a colorless solution. [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) Both reactions produce a colorless gas and are exothermic. [1] Magnesium reacts with $\text{HCl}$ to produce hydrogen gas ($\text{H}_2$). [2] Calcium carbonate reacts with $\text{HCl}$ to produce carbon dioxide gas ($\text{CO}_2$). [2] (b) The reaction is easily controlled (not as violent as alkali metals). [1] The gas produced ($\text{CO}_2$) is easily collected. [1] The reactants are cheap and readily available. [1]

Question 14 (a) For the same concentration, a strong acid has a lower pH than a weak acid. [1] This is because strong acids ionize completely, resulting in a higher concentration of $\text{H}^+$ ions. [2] Since $\text{pH} = -\log[\text{H}^+]$, a higher $\text{H}^+$ concentration leads to a lower pH value. [1] (b) Example: Aluminum chloride ($\text{AlCl}_3$) or Ammonium nitrate ($\text{NH}_4\text{NO}_3$). [1] These salts are formed from a strong acid and a weak base. [1] In water, the cations undergo hydrolysis, releasing $\text{H}^+$ ions into the solution. [1]