Trends in properties of the halogensAQA A-Level Chemistry: Subtopic test

1. 1
   A student is comparing the halogens using data from a textbook. The Pauling electronegativity values given are fluorine 4.0, chlorine 3.0, bromine 2.8 and iodine 2.5.

   (a)

   Which statement correctly defines electronegativity?

   [1 mark]

   • AThe energy required to remove one electron from each atom in a mole of gaseous atoms
   • BThe energy released when one mole of gaseous atoms each gain one electron
   • CThe ability of an atom to attract the bonding pair of electrons in a covalent bond
   • DThe tendency of an atom to lose electrons and form a positive ion

   (b)

   Which statement best explains why electronegativity decreases from chlorine to iodine?

   [1 mark]

   • AThe nuclear charge decreases down the group
   • BThe atomic radius and shielding increase, so the nucleus attracts the bonding pair less strongly
   • CThe number of outer-shell electrons decreases down the group
   • DThe halogen molecules become more polar down the group

   (c)

   Hydrogen has a Pauling electronegativity of 2.2. State and explain which is the more polar bond, H–Cl or H–I.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chlorine, bromine and iodine are a gas, a liquid and a solid respectively at room temperature. Their boiling points are approximately 239 K, 332 K and 457 K.

   (a)

   Which description of solid iodine is correct?

   [1 mark]

   • AA simple molecular lattice of I₂ molecules with London forces between the molecules
   • BA giant covalent lattice of iodine atoms joined by covalent bonds
   • CAn ionic lattice of I⁺ and I⁻ ions
   • DA metallic lattice of I⁺ ions in a sea of delocalised electrons

   (b)

   Which equation represents the change that occurs when liquid bromine boils?

   [1 mark]

   • ABr₂(l) → 2Br(g)
   • BBr₂(l) → Br⁺(g) + Br⁻(g)
   • C2Br(l) → Br₂(g)
   • DBr₂(l) → Br₂(g)

   (c)

   Explain why the boiling point of iodine is higher than that of chlorine.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates displacement reactions of the halogens in aqueous solution. In each test she adds a few drops of a halogen water to 1 cm³ of an aqueous potassium halide, adds 1 cm³ of cyclohexane, shakes the tube and allows the layers to separate.

   (a)

   Chlorine water is added to aqueous potassium bromide. State the colour of the cyclohexane layer, write an ionic equation for the reaction and identify the species that is oxidised.

   [3 marks]

   (b)

   Bromine water is added to aqueous potassium iodide and, in a separate test, to aqueous potassium chloride. State the colour of the cyclohexane layer with potassium iodide and write an ionic equation for the reaction. Explain, in terms of atomic structure, why bromine does not react with potassium chloride.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher has three unlabelled aqueous solutions, known to be potassium chloride, potassium bromide and potassium iodide. She has chlorine water, bromine water and cyclohexane available and wants her class to identify each solution and then to discuss the trends in the properties of the halogens.

   (a)

   Describe how chlorine water and cyclohexane can be used to identify which solution is which. Include the observations expected, ionic equations for any reactions and an explanation of why chlorine reacts with only two of the three solutions.

   [6 marks]

   (b)

   A student makes three claims. Claim 1: iodine displaces bromide ions from aqueous potassium bromide because iodine atoms are larger. Claim 2: bromine displaces iodide ions from aqueous potassium iodide. Claim 3: the boiling points of the halogens increase down the group because the covalent bonds get stronger. Evaluate each claim, using ideas about oxidising ability, structure and bonding.

   [6 marks]

   Total for question 4: 12 marks

End of questions