The structures of six compounds are shown below.
Answer the following questions about these substances.
Each compound may be used once, more than once or not at all.
(a) Which substance, A, B, C, D, E or F,
(i) gives a white precipitate on addition of an aqueous solution of sodium sulfate,
(ii) is a component of many fertilisers,
(iii) contains a Group III element,
(iv) is an acidic gas at room temperature,
(v) turns anhydrous cobalt chloride pink,
(vi) is the main component of natural gas?
(b) Compound A can be made by direct combination of chlorine and aluminium using the apparatus shown below.
(i) On the diagram above, draw an arrow to show where heat is applied.
(ii) Suggest one safety precaution that should be taken when carrying out this experiment.
(iii) Complete the symbol equation for this reaction.
\(2Al + ……..Cl_2 \rightarrow Al_2Cl_6\)
▶️ Answer/Explanation
(a) Identification of substances:
(i) F (BaCl2) – Barium chloride reacts with sodium sulfate to form a white precipitate of barium sulfate.
(ii) D ((NH4)2SO4) – Ammonium sulfate is a common fertiliser due to its nitrogen and sulfur content.
(iii) A (Al2Cl6) – Aluminium (Al) is a Group III element.
(iv) E (HCl) – Hydrogen chloride is an acidic gas at room temperature.
(v) B (H2O) – Water turns anhydrous cobalt chloride from blue to pink.
(vi) C (CH4) – Methane is the main component of natural gas.
(b) Synthesis of aluminium chloride:
(i) Heat is applied under the aluminium foil to initiate the reaction with chlorine gas.
(ii) Use a fume cupboard – Chlorine gas is toxic and must be handled in a well-ventilated area.
(iii) The balanced equation is:
\[ 2Al + 3Cl_2 \rightarrow Al_2Cl_6 \] (3 moles of Cl2 are required to react with 2 moles of Al.)
A student used chromatography to separate the dyes in the blue ink from a ball-point pen. She used the equipment shown in the diagrams below.
(a) Complete the diagram below to show how she set up the apparatus.
(b) Describe how chromatography could be used by the student to separate the dyes.
(c) The student used water as a solvent. Suggest a different solvent that she could use.
(d) The diagram below shows the results of the chromatography using the blue ink, B, and several pure dyes, W, X, Y and Z.
(i) Which of the dyes, W, X, Y and Z, were in the blue ink?
(ii) How many dyes in the ink had been separated by this chromatography?
(e) The diagram shows the ball-point pen used in the experiment.
(i) The cap of the pen is made of poly(ethene).
Describe the formation of poly(ethene) from ethene. In your answer, include the words:
- monomer,
- polymer.
(ii) The tip of the pen is made from an alloy.
What is meant by the term alloy ?
(f) The table shows some properties of four alloys.
(i) How does the strength of the steel and aluminum alloys vary with their thermal conductivity?
(ii) Which one of these alloys is the best one to use to make the body of an aircraft? Give two reasons for your answer.
▶️ Answer/Explanation
(a) The setup includes:
– A chromatography paper not touching the sides of the beaker.
– The solvent (e.g., water) at the bottom, with the paper dipping into it.
– A watch glass covering the beaker to prevent solvent evaporation.
(b) Steps to separate dyes:
1. Place a small spot of ink above the solvent level on the paper.
2. Allow the solvent to rise up the paper, carrying the dyes at different rates.
3. Dyes separate based on their solubility and affinity for the paper.
(c) Alternative solvents: ethanol, propanone, or ethyl acetate (any organic solvent).
(d)(i) The blue ink contains dyes W, X, and Y (their spots align with B’s separated components).
(d)(ii) The ink contains 4 dyes (W, X, Y, and an unidentified component).
(e)(i) Poly(ethene) forms when ethene monomers (C₂H₄) undergo addition polymerization, linking into long polymer chains.
(e)(ii) An alloy is a mixture of metals (or a metal with a non-metal) to enhance properties like strength.
(f)(i) Higher strength alloys have lower thermal conductivity (e.g., steel is stronger but less conductive than aluminum).
(f)(ii) Best aircraft alloy: High-strength aluminum.
Reasons:
1. High strength-to-weight ratio (lightweight but strong).
2. Low density (reduces overall aircraft weight).