F321: Atoms, Bonds and Groups Group 7

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Bathsheba French
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1 F321: Atoms, Bonds and Groups Group 7 93 Marks 1. Chlorine and bromine are elements in Group 7 of the Periodic Table. Chlorine is used in water treatment. State one advantage and one disadvantage of using chlorine in water treatment. advantage: disadvantage: [Total 2 marks] 2. The electron configuration of bromine contains outermost electrons in the 4th shell. Using your knowledge of Group 7 elements, complete the electron configuration of bromine. 1s 2 2s 2 2p 6 3s 2 3p Displacement reactions can be used to detect bromide ions in solution. A student has a solution that contains bromide ions. The student carries out the following experiment. Step 1 Step 2 She bubbles some chlorine gas through a sample of the solution. The mixture changes colour. The student then adds an organic solvent, cyclohexane, to the mixture. She shakes the contents and allows the layers to separate. [Total 1 mark] Write the ionic equation for the reaction that takes place in step 1. Plymstock School 1

2 (ii) What colour does the cyclohexane layer turn in step 2? [Total 2 mark] 4. Chlorine reacts differently with dilute and concentrated aqueous solutions of sodium hydroxide. When chlorine reacts with dilute sodium hydroxide, one of the products is sodium chlorate(i). This is the reaction that is used to manufacture bleach. When chlorine is reacted with hot concentrated sodium hydroxide, a different reaction takes place. One of the products is NaClO 3, used as a weedkiller. In each reaction, chlorine has been both oxidised and reduced. What term is used to describe a redox reaction in which an element is both oxidised and reduced? (ii) Write equations for these two reactions of chlorine with sodium hydroxide: equation for reaction with dilute sodium hydroxide, equation for reaction with hot concentrated sodium hydroxide. (iii) Chlorine forms another chlorate called sodium chlorate(vii), used in the manufacture of matches. Suggest the formula of sodium chlorate(vii). [Total 5 mark] Plymstock School 2

3 5. The boiling points of some Group 7 elements are shown below. Group 7 element boiling point / C chlorine 35 bromine 59 iodine 184 Explain why the halogens show this trend in boiling points. In your answer, you should use appropriate technical terms, spelt correctly. [Total 3 marks] 6. During the extraction of bromine industrially, chlorine is bubbled through a solution of bromide ions. A student thought this principle would also work for extracting iodine and carried out the experiment below. Stage 1 The student bubbled some chlorine through an aqueous solution of potassium iodide. Stage 2 The student added an organic solvent and shook the mixture. What would the student see at stage 1? (ii) Name the products and write an ionic equation for the reaction in stage 1. names of products:... ionic equation:... Plymstock School 3

4 (iii) Why does the reaction in stage 1 occur? (iv) What would the student see at stage 2? 7. The Group 7 element bromine was discovered by Balard in Bromine gets its name from the Greek bromos meaning stench. Bromine consists of a mixture of two isotopes, 79 Br and 81 Br. A student added an aqueous solution of sodium iodide to a solution of bromine. The colour turned from orange to a deep brown. The student then added an aqueous solution of sodium chloride to a solution of bromine. The orange colour was unchanged. [Total 5 marks] Explain these observations. In your answer, you should use appropriate technical terms, spelled correctly. (ii) Write an ionic equation for the reaction that has taken place. [Total 4 marks] Plymstock School 4

5 8. Chlorine is used in the production of safe drinking water throughout the world. Drinking water is purified by addition of small amounts of chlorine. The reaction that takes place can be represented by the equation below. Cl 2 (g) + H 2 O(l) 2H + (aq) + ClO (aq) + Cl (aq) (a) Describe a simple test that would confirm the presence of chloride ions in drinking water. Include an equation in your answer. (b) Chlorine can also be converted into bleach. How is bleach made from chlorine? Include an equation in your answer. 9. At room temperature and pressure, r.t.p., chlorine is a gas, bromine is a liquid and iodine is a solid. Explain why these halogens have different physical states at r.t.p [Total 5 marks] [Total 3 marks] Plymstock School 5

6 10. A student carried out an experiment using chlorine gas, Cl 2 (g). The student bubbled chlorine through an aqueous solution of potassium bromide, KBr(aq). A reaction took place. What colour is the solution after the reaction has taken place? (ii) Write an equation for this reaction. (iii) This reaction takes place because chlorine has a stronger oxidising power than bromine. Explain why chlorine has a stronger oxidising power than bromine. 11. A student carried out two experiments using chlorine gas, Cl 2 (g). [Total 6 marks] (a) In the first experiment, the student bubbled chlorine through 120 cm 3 of an aqueous solution of mol dm 3 sodium hydroxide, NaOH(aq). The equation for this reaction is shown below. Cl 2 (g) + 2NaOH(aq) NaCl(aq) + NaClO(aq) + H 2 O(l) Under the reaction conditions, 1 mole of Cl 2 (g) occupies 24.0 dm 3. What is meant by the term the mole? Plymstock School 6

7 (ii) How many moles of NaOH were in the 120 cm 3 volume of NaOH(aq)? answer... mol (iii) Calculate the volume of Cl 2 (g) that was needed to react with the NaOH(aq) used. answer... (iv) What is a common use for the solution that the student prepared?... (b) In the second experiment, the student repeated the procedure in (a) but with hot concentrated sodium hydroxide. A different reaction took place in which sodium chlorate (V) was formed instead of NaClO. Suggest the formula of sodium chlorate (V). [Total 6 marks] 12. Aqueous silver nitrate can be used as a test for halide ions. A student decided to carry out this test on a solution of magnesium chloride. The bottle of magnesium chloride that the student used showed the formula MgCl 2.6H 2 O. The student dissolved a small amount of MgCl 2.6H 2 O in water and added aqueous silver nitrate to the aqueous solution. What is the molar mass of MgCl 2.6H 2 O? molar mass =... g mol 1 (ii) What would the student see after adding the aqueous silver nitrate, AgNO 3 (aq)? (iii) Write an ionic equation for this reaction. Include state symbols. Plymstock School 7

8 (iv) Using aqueous silver nitrate, it is sometimes difficult to distinguish between chloride, bromide and iodide ions. How can aqueous ammonia be used to distinguish between these three ions? 13. Domestic tap water has been chlorinated. Chlorine reacts with water as shown below. Cl 2 (g) + H 2 O(l) HOCl (aq) + HCl (aq) [Total 7 marks] State the oxidation number of chlorine in Cl 2... HOCl... HCl... (ii) When carrying out halide tests with aqueous silver nitrate, it is important that distilled or deionised water is used for all solutions, rather than tap water. Suggest why. [Total 4 marks] 14. Aqueous silver nitrate was added to the solution of CaCl 2. State what you would expect to see. Plymstock School 8

9 (ii) Write an ionic equation for this reaction. [Total 2 marks] 15. In this question, one mark is available for the quality of spelling, punctuation and grammar. The halogens chlorine, bromine and iodine each exist as diatomic molecules at room temperature and pressure. The halogens all have van der Waals forces. Explain how van der Waals forces are formed. Explain the trend in volatilities of the halogens chlorine, bromine and iodine. [Total 6 marks] Plymstock School 9

10 16. Describe and explain the relative reactivity of the halogens chlorine, bromine and iodine in displacement reactions involving halides, using reactions on a test tube scale. Include equations and observations in your answer. [5] Quality of Written Communication [Total 6 marks] 17. Chlorine is used in the preparation of many commercially important materials such as bleach and iodine. Bleach is a solution of sodium chlorate(l), NaOCl, made by dissolving chlorine in aqueous sodium hydroxide. Cl 2 (g) + 2NaOH(aq) NaOCl(aq) + NaCl(aq) + H 2 O(l) Determine the changes in oxidation number of chlorine during the preparation of bleach and comment on your results [Total 3 marks] 18. Iodine is extracted commercially from seawater with chlorine gas. Seawater contains very small quantities of dissolved iodide ions, which are oxidised to iodine by the chlorine gas. Write an ionic equation for the reaction that has taken place. (ii) Use your understanding of electronic structure to explain why chlorine is a stronger oxidising agent than iodine. [Total 4 marks] Plymstock School 10

11 19. In this question, one mark is available for the quality of written communication. This question is about the Group 7 elements chlorine, bromine and iodine. Explain why chlorine, bromine and iodine have different physical states at room temperature and pressure. [4] An aqueous solution of bromine was added to an aqueous solution of chloride ions and also to an aqueous solution of iodide ions. State what you would see in each case. Include an equation for any chemical reaction that takes place. Explain the trend shown by these observations. [4] Quality of Written Communication [Total 9 marks] 20. A household bleach contains sodium chlorate(i), NaClO, as its active ingredient. The concentration of NaClO in the bleach can be found by using its reaction with hydrogen peroxide, H 2 O 2. NaClO(aq) + H 2 O 2 (aq) O 2 (g) + NaCl (aq) + H 2 O(l) (a) Chlorine has been reduced in this reaction. Use oxidation numbers to prove this. (b) A student added an excess of aqueous hydrogen peroxide to 5.0 cm 3 of the bleach. 84 cm 3 of oxygen gas were released. How many moles of O 2 were released? Assume that, under the laboratory conditions, 1.00 mol of gas molecules occupies 24 dm 3. answer... mol Plymstock School 11

12 (ii) How many moles of NaClO were in 5.0 cm 3 of the bleach? answer... mol (iii) What was the concentration, in mol dm 3, of NaClO in the bleach? answer... mol dm 3 (c) The label on the bottle of household bleach states that the bleach contains a minimum of 4.5 g per 100 cm 3 of NaClO. Use your answer to (b)(iii) to decide whether or not the information on the label is correct. (d) It is extremely important that household bleach is not used with acids. This is because a reaction takes place that releases toxic chlorine gas. Suggest an equation for the reaction of an excess of hydrochloric acid with household bleach. [Total 10 marks] Plymstock School 12

State one advantage and one disadvantage of using chlorine in water treatment. advantage: disadvantage: [2]

1 Chlorine and bromine are elements in Group 7 of the Periodic Table. (a) Chlorine is used in water treatment. State one advantage and one disadvantage of using chlorine in water treatment. advantage:...