3.3 The Halogens

This question is about the use of chlorine in swimming pools to kill bacteria.

An equilibrium is established when chlorine reacts with water in the absence of sunlight.

A student states that in the reaction in part (a) chlorine oxidises the water. Is the student correct? Justify your answer.

Urine contains uric acid that reacts with one of the products in the reaction in part (a) to produce a highly toxic compound called cyanogen chloride.

Explain why this should not be a concern for swimming pool users.

The addition of aqueous chlorine to cold, aqueous sodium hydroxide produces a key ingredient to produce bleach. 

This question is about ions containing bromine.

Potassium bromate, KBrO₃, is a compound used in baking bread to help strengthen the dough and allow higher rising.

The hypobromite ion, BrO^-, is similar to the ClO^- ion found in bleaches. When water has been sterilised with ozone, both oxygen and BrO^- are formed via the reaction of any bromide ions in the water with residual ozone. The hypobromite formed can then react further with ozone, O₃, to produce the bromate ion, BrO₃^-

Write the equation for both of these processes, state symbols are not required.

Bromate ions are undesirable in drinking water due to their toxicity. Some people who have ingested large amounts of bromate show acute gastrointestinal irritation.

Naturally occurring bromine in seawater can react with disinfection contaminants, such as hypochlorite, HClO, forming bromate ions as shown in the equation.

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

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

Identify the reducing agent for the reaction given in part (c). Justify your answer.

This question is about various reactions involving halide ions.

Hydrogen chloride can react with lead(IV) oxide producing lead(II) chloride, chlorine and water.

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. 

An equilibrium is established when chlorine gas is reacted with solid iodine to produce iodine monochloride, ICl, which is a brown liquid at room temperature. 

This question is about chlorine trifluoride.

Chlorine trifluoride, ClF₃, is one of the most reactive substances known: it causes sand and asbestos to explode and it reacts with xenon. It has been investigated as rocket fuel.

ClF₃ is also used to turn uranium into uranium hexafluoride, UF₆, which is used to separate the isotopes of uranium. Chlorine monofluoride, ClF, is a side-product in this reaction.
Write a balanced equation for the reaction between uranium and chlorine trifluoride.

ClF₃ is a powerful oxidising agent. Chlorine trifluoride is able to react with silver chloride as shown in the reaction below.

Identify the species on the left-hand side of the equation that have been oxidised.

Iodine forms the fluorides IF, IF₃, IF₅ and IF₇.  It is theoretically possible that these molecules can disproportionate to the form a compound with iodine in its next highest oxidation number and elemental iodine. For example, IF₃ might disproportionate to give IF₅ and I₂. 

Give the equations for the theoretical disproportionation reactions of IF, IF₃, IF₅ and IF_7.

The standard enthalpy change of formation of the fluorine iodides are shown in the table below.

Chlorine, iodine monochloride and sodium chloride all contain chlorine atoms.  

Predict the order of the melting points for each of the compounds named above. Justify your predictions.

The halides act as reducing agents in certain reactions. 

When chlorine and bromide ions react together, the bromide ions act as reducing agents.

Deduce the half equation to demonstrate the bromide ions acting as reducing agents and explain why they are stronger reducing agents than chloride ions.

A student has a solution that contains bromide ions. The student carries out the following experiment.

Iodine can be used for the small-scale purification of drinking water. 

Iodine reacts with water as shown below:

I₂ (aq) + H₂O (l) → HI (aq) + HIO (aq)

Using oxidation numbers, explain why this reaction is an example of a disproportionation reaction.

State how an aqueous of solution of a halogen could be used to successfully distinguish between solutions containing potassium chloride, potassium bromide and potassium iodide.

Provide relevant equations to justify your answer. 

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

The table shows the boiling points of ammonia, fluorine and bromine.

Explain the different boiling points of NH₃, F₂ and Br₂.
Include the names of any relevant forces and particles.
In your answer you should use appropriate technical terms, spelled correctly.

Ions of Group 7 elements take part in displacement reactions. These reactions can be used to compare the reactivities of the elements within Group 7.

A student adds aqueous solutions of halogens to test-tubes containing solutions of halide ions. The resulting mixtures are then shaken with cyclohexane, an organic solvent.
One of the student’s results is shown in the table.

Chlorine will react with cold, dilute sodium hydroxide. 

There are many compounds which contain the chloride ion, Cl^-. 

State a simple test tube reaction you could do to determine whether chloride ions were present in an unknown solution.
Provide the simplest ionic equation, with state symbols, in your answer.

Nitric acid must be used before the key reagent is added in the test tube reaction in part (b).

Chlorine is used in water treatment.
State one advantage and one disadvantage of using chlorine in water treatment

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 below. 

Table 1

Using the results from Table 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. 

Two students are debating the oxidising ability of Group 7. 

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 student 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.