Group 17 (CIE AS Chemistry)

A group of students were completing test tube reactions to identify three samples, X, Y and Z, since their labels had fallen off the bottles. All of the samples were solutions of sodium salts. 

The positive results of the tests are shown in Table 1.1 below. 

Table 1.1

Using the results from Table 1.1 identify X, Y and Z and write ionic equations, including state symbols, to demonstrate the key reactions taking place in each of the test tube reactions. 

• X

[2] 

• Y
  

[2] 

• Z

[2] 

Two students are debating the oxidising ability of Group 17. 

Student A states that the oxidising ability of the halides increases down the group, but Student B states that it is the oxidising ability of the halogen molecules that increases down the group. 

Is student A, student B or neither correct? Justify your answer.

On gently heating, the compound KClO₃ reacts as shown in the equation.

4KClO₃ (s) KCl (s) + 3KClO₄ (s)

This reaction is an example of disproportionation.

State what is meant by disproportionation and use oxidation numbers to show that disproportionation has taken place.

Bromine is used to extract iodine from a solution containing iodide ions.

Explain why iodine is a weaker oxidising agent than than bromine.

The elements of Group 17 of the Periodic Table show trends in their properties.

Table 2.1

State the full electronic configuration for bromine.

Explain why bromine is a liquid at room temperature, but fluorine and chlorine are gases.

The bond enthalpy of the halogen molecules generally decreases down Group 17. Explain why the bond strength of fluorine is lower than that of chlorine. 

Chlorine will react with cold, dilute sodium hydroxide.

Chlorine is added to drinking water to ensure that it is safe to drink.

Use an appropriate chemical equation to explain why one of the products formed in the reaction in part (c) is classed as a weak acid. 

When solid NaCl reacts with concentrated sulfuric acid the only gas produced is HCl (g) and when solid NaBr reacts with concentrated sulfuric acid the gases produced are brown fumes of Br₂ (g) as well as HBr (g). 

Explain the difference in the reactions of concentrated sulfuric acid with NaBr and with NaCl. Your answer should refer to the role of the sulfuric acid in each reaction.

Solid sodium iodide, NaI (s) reacts with concentrated sulfuric acid to form purple fumes of I₂ (g), a yellow solid and hydrogen sulfide gas. H₂S (g).

Sodium chlorate(V) is a powerful oxidising agent formed by the reaction of chlorine and hot sodium hydroxide is used in the manufacture of matches and as a garden weedkiller.

Chlorine is added to drinking water to ensure that it is safe to drink. 

State a concern with adding chlorine to drinking water.

A group of students are asked to determine the order of reactivity of the halogens and are provided with solutions of potassium chloride, potassium bromide and potassium iodide, as well as aqueous samples of each of the halogens. 

Suggest one extra reagent the students could use to make their observations more clear.

Halogens react with hydrogen gas to form hydrogen halides. Chlorine reacts with hydrogen when exposed to ultraviolet light to form hydrogen chloride.

[3]

Solid NaI reacts with concentrated sulfuric acid to form purple fumes of I₂ (g) and hydrogen sulfide gas, H₂S (g). However, when solid NaCl reacts with concentrated sulfuric acid the only gas produced is HCl (g).

Explain the difference in the reactions of concentrated sulfuric acid with NaI and with NaCl. Your answer should refer to the role of the sulfuric acid in each reaction.

Chlorine is commonly used in water purification. When chlorine is added to water it reacts to produce a mixture of acids, one of which is chloric(I) acid, HClO, a powerful oxidising agent.

An acid base reaction occurs for the reaction between solid potassium bromide and concentrated sulfuric acid. The equation for this is:

2H₂SO₄ (aq) + 2KBr (s) → K₂SO₄ (aq) + SO₂ (g) + Br₂ (g) + 2H₂O (l)

In this reaction redox products are also formed. State the two redox products and possible observation for each one.

Explain why the reaction between solid potassium iodide and concentrated sulfuric acid produces hydrogen sulfide whereas the reaction between solid potassium bromide, and concentrated sulfuric acid does not. 

Hydrogen halides, H-X, are formed from the reaction of hydrogen and a halogen, X₂. Explain why in solution hydrogen fluoride is classed as a weak acid, whereas hydrogen chloride is classed as a strong acid.

The halogens can react with iron wool to produce various iron compounds including iron(III) chloride, iron(III) bromide and iron(II) iodide.

The Fe²⁺ ion can also react with chlorine, Cl₂ and bromine, Br₂. 

[1]

Bromate ions are toxic and fish eggs exposed to bromate can develop disorders in the brain and spine. This can also poison humans, where symptoms include acute gastrointestinal irritation and depression of the central nervous system. 

Another reaction in which bromate ions are formed is by the reaction with disinfection contaminants, such as hypochlorite, HClO, in sea water shown in the equation.

Br^– + 3HClO + 3OH^– → BrO₃^– + 3Cl^– + 3H₂O

State the oxidation number for Cl in HClO and Cl^–.

Identify the oxidising agent in the reaction given in part (c). Explain your answer.

Table 3.1 shows the boiling points of some halogenoalkanes.

Table 3.1 

Explain the trend in boiling points of the halogenoalkanes in Table 3.1.