Some elements are shown in the order they appear in the reactivity series. The most reactive element is at the top.
sodium
calcium
magnesium
aluminium
zinc
iron
hydrogen
copper
(a) Answer the questions using the list of elements. Each element may be used once, more than once or not at all.
Identify:
(i) a non-metal
(ii) a metal which is stored under oil
(iii) the main component of steel
(iv) a metal with three electrons in the outer shell of its atoms
(v) a metal found in brass
(vi) a metal that forms chlorides of the type \(XCl_2\) and \(XCl_3\).
(b) Name the main ores of:
(i) zinc.
(ii) aluminium.
(c) In an experiment, a sample of aluminium appeared less reactive than expected.
Explain why.
(d) Name two metals from the list which are extracted by reduction of their ores using carbon.
(e) When zinc granules are added to aqueous copper(II) sulfate, a reaction occurs. During the reaction, a red-pink solid is formed and the solution becomes colourless.
(i) Name the red-pink solid.
(ii) Name the colourless solution.
(iii) Explain, in terms of particles, why the rate of this reaction increases when the temperature is increased.
(iv) Suggest two other ways of increasing the rate of this reaction.
▶️ Answer/Explanation
(a)
(i) Hydrogen (only non-metal in the list).
(ii) Sodium (highly reactive, stored under oil to prevent reactions).
(iii) Iron (main component of steel).
(iv) Aluminium (has 3 valence electrons).
(v) Zinc or copper (both are used in brass).
(vi) Iron (forms both \(FeCl_2\) and \(FeCl_3\)).
(b)
(i) Zinc blende (main ore of zinc).
(ii) Bauxite (main ore of aluminium).
(c) Aluminium forms a protective oxide layer that prevents further reaction, making it appear less reactive.
(d) Two metals extracted using carbon:
1. Zinc
2. Iron (or copper).
(e)
(i) The red-pink solid is copper (displaced by zinc).
(ii) The colourless solution is zinc sulfate.
(iii) At higher temperatures, particles move faster, increasing collision frequency and energy, leading to more successful reactions.
(iv) Two ways to increase reaction rate:
1. Increase concentration of copper(II) sulfate.
2. Use powdered zinc (greater surface area).
(a) Aqueous ammonium sulfate, \(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4\), is warmed with aqueous sodium hydroxide. The pungent-smelling gas ammonia, \(\mathrm{NH}_3\), is produced. Balance the equation for this reaction.
\[ \left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 + \ldots \mathrm{NaOH} \rightarrow \ldots \mathrm{NH}_3 + \ldots \mathrm{H}_2 \mathrm{O} + \mathrm{Na}_2 \mathrm{SO}_4 \]
(b) A \(2.8 \mathrm{~g}\) sample of impure ammonium sulfate is found to contain \(0.7 \mathrm{~g}\) of impurities.
Calculate the percentage of ammonium sulfate in this sample.
(c) Describe a test for ammonia gas.
(d) Ammonia gas is prepared at the front of a laboratory.
The pungent smell of ammonia spreads throughout the laboratory slowly.
(i) Name the process that occurs when ammonia gas spreads throughout the laboratory.
(ii) Explain, using ideas about particles, why ammonia gas spreads throughout the laboratory.
(iii) Explain why carbon dioxide gas, \(\mathrm{CO}_2\), will spread throughout the laboratory at a slower rate than ammonia gas, \(\mathrm{NH}_3\).
(e) Ammonia is produced in the Haber process.
The equation for the reaction is shown.
\[ \mathrm{N}_2(\mathrm{~g}) + 3 \mathrm{H}_2(\mathrm{~g}) \rightarrow 2 \mathrm{NH}_3(\mathrm{~g}) \]
(i) In the Haber process, a temperature of \(450^{\circ} \mathrm{C}\) and a pressure of 200 atmospheres are used in the presence of finely-divided iron.
A larger equilibrium yield of ammonia would be produced if a lower temperature and a higher pressure are used.
Explain why a lower temperature and a higher pressure are not used.
(ii) State the role of iron in the Haber process.
(f) Ammonia is a weak base.
(i) Explain the meaning of the term base.
(ii) Suggest the pH of aqueous ammonia.
▶️ Answer/Explanation
(a) The balanced equation is:
\[ \left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 + 2 \mathrm{NaOH} \rightarrow 2 \mathrm{NH}_3 + 2 \mathrm{H}_2 \mathrm{O} + \mathrm{Na}_2 \mathrm{SO}_4 \]
Explanation: Two \(\mathrm{NH}_3\) molecules and two \(\mathrm{H}_2\mathrm{O}\) molecules balance the ammonium and hydroxide ions.
(b) Percentage of ammonium sulfate:
\[ \text{Percentage} = \left(\frac{2.8 – 0.7}{2.8}\right) \times 100 = 75\% \]
Explanation: Subtract impurities from the total mass and calculate the percentage purity.
(c) Test for ammonia:
Test: Use damp red litmus paper.
Result: Turns blue (ammonia is alkaline).
(d) (i) The process is diffusion.
(ii) Ammonia particles move randomly from high to low concentration until evenly distributed.
(iii) \(\mathrm{CO}_2\) diffuses slower because it has a higher molecular mass (\(44 \mathrm{~g/mol}\)) than \(\mathrm{NH}_3\) (\(17 \mathrm{~g/mol}\)).
(e) (i) Lower temperature slows the reaction rate, reducing efficiency. Higher pressure increases costs due to specialized equipment.
(ii) Iron acts as a catalyst to speed up the reaction.
(f) (i) A base is a proton acceptor (according to Brønsted-Lowry theory).
(ii) The pH of aqueous ammonia is between 10 and 12 (weakly alkaline).