1. 1
   A water treatment plant adds chlorine gas to drinking water to kill harmful microbes. The engineers running the plant explain to trainees that chlorine belongs to Group 7 of the periodic table, along with bromine and iodine.

   (a)

   What do chlorine, bromine and iodine have in common that places them all in Group 7?

   [1 mark]

   • AThey each have 1 electron in their outer shell
   • BThey each have 7 electrons in their outer shell
   • CThey are all unreactive gases
   • DThey each form positive ions

   (b)

   The engineers note that chlorine is a gas at room temperature, bromine is a liquid, and iodine is a solid. Which statement correctly describes this trend down Group 7?

   [1 mark]

   • AMelting and boiling points decrease going down the group
   • BOnly iodine has a measurable melting point
   • CMelting and boiling points stay the same throughout the group
   • DMelting and boiling points increase going down the group

   (c)

   Explain, in terms of relative molecular mass, why the melting and boiling points of the halogens increase going down Group 7 from chlorine to bromine to iodine.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A food scientist studying salt purification methods adds chlorine water to a colourless solution of potassium bromide in a test tube. The solution turns orange, showing that a chemical reaction has taken place.

   (a)

   A food scientist studying salt production adds chlorine water to a colourless solution of potassium bromide. The solution turns orange, showing that a reaction has taken place.
   
   What type of reaction has occurred between chlorine and potassium bromide?

   [1 mark]

   • AA displacement reaction, in which chlorine displaces bromine from potassium bromide
   • BA neutralisation reaction between an acid and an alkali
   • CAn electrolysis reaction
   • DA precipitation reaction between two solids

   (b)

   Why is chlorine able to displace bromine from potassium bromide solution, but bromine is not able to displace chlorine from potassium chloride solution?

   [1 mark]

   • ABromine is a more reactive halogen than chlorine
   • BChlorine has more protons than bromine
   • CChlorine is a more reactive halogen than bromine, as it is higher up Group 7
   • DDisplacement in Group 7 depends only on colour, not reactivity

   (c)

   Write a word equation for the reaction between chlorine and potassium bromide solution, and explain how this reaction demonstrates the trend in reactivity down Group 7.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A swimming pool maintenance company uses chlorine gas to sterilise pool water and explains to new staff how chlorine reacts with different substances, including metals and non-metals, to form compounds.

   (a)

   Describe what would be observed and produced if chlorine gas were passed over hot iron wool, and explain why this is described as chlorine's 'combining' reaction with a metal rather than a displacement reaction.

   [3 marks]

   (b)

   The company also wants new staff to understand what happens if chlorine gas comes into contact with hydrogen gas, forming hydrogen chloride, and how this compound behaves when dissolved in water. Describe the type of bonding in hydrogen chloride, and explain what happens to its properties when it dissolves in water.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical supply company is deciding which halogen, chlorine or iodine, to recommend to a client for use as a general-purpose disinfectant, given that more reactive halogens tend to be more effective at killing microbes.

   (a)

   Explain the general trends in physical and chemical properties across Group 7, and use electronic structure to explain why reactivity decreases going down the group from chlorine to iodine.

   [6 marks]

   (b)

   The client raises a concern: since iodine is less reactive than chlorine, would iodine ever be preferred as a disinfectant instead of chlorine? Evaluate the factors, beyond simple reactivity, that might make iodine a more suitable choice than chlorine in certain situations, such as for treating a wound versus sterilising a large public water supply.

   [6 marks]

   Total for question 4: 12 marks

End of questions