2.3 Group 7 (17), the Halogens

Chlorine is added to swimming pools in controlled amounts to kill any bacteria that may cause illness to those using the pool.

If chlorine reacts with water in the absence of sunlight an equilibrium is established.

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

The uric acid in urine can react with one of the products in the reaction in part (a) producing a toxic compound known as cyanogen chloride. Explain why this should not be a concern for the swimming pool users.

When aqueous chlorine is added to cold, aqueous sodium hydroxide to produce a key ingredient to produce bleach.

Sodium bromate, NaBrO₃, is a compound that is mainly used in batch and continuous dyeing processes.

The hypobromite ion, BrO^-, and oxygen can be formed directly in drinking water via the ozonation of bromide ions which are already present. 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 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 this reaction for the reaction given in part (c). Justify your answer.

The structure for a tertiary halogenoalkane is shown in Figure 1. X can represent either a fluorine, chlorine, bromine or iodine atom.

Figure 1

With reference to bond enthalpy explain why the reaction with aqueous potassium hydroxide, KOH (aq), to produce a tertiary alcohol when X = F is the slowest.

Suggest a method to show how the rate of reaction is different for each molecule drawn in Figure 2 when they are reacted independently with aqueous potassium hydroxide.

Figure 2

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

Suggest a method different to your answer in part (b) that would enable a student to identify a hydrogen halide, H-X, when X could be Cl, Br or I. Give one observation and one equation, including state symbols, for the reaction that will occur with one of the hydrogen halide compounds. 

Lead dioxide reacts with hydrogen chloride to give lead chloride, chlorine and water.

Table 1 shows the boiling points of some halogenoalkanes.

Table 1

A student states that the boiling point increases due to the strength of the C-X bond in the molecule increasing down group 7.

State whether the student is correct and justify your answer.

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.

Chlorine gas can be reacted with solid iodine to produce iodine monochloride, ICl, which is a brown liquid at room temperature.

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.

Halide ions are formed when the halogens gain electrons. When the halogens react with sodium, they produce sodium halides. 

The sodium halides will react with concentrated sulfuric acid, to produce a variety of products. 

State the role of the sulfuric acid when it is reacted with solid sodium chloride.

State an observation that would be made when concentrated sulfuric acid is reacted with solid sodium chloride. 

State one additional observation which would be made from a further reaction between concentrated sulfuric acid and solid sodium iodide, and name the product responsible for this observation.

This question is about the reactions and properties of the elements in Group 7 of the Periodic Table. 

Define the term electronegativity and explain the trend in electronegativity seen down Group 7.

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. 

Chlorine is used to treat water to make sure that it is safe to use. When water is exposed to direct sunlight, the chlorine present in the water can react and be lost from the water. 

Write an equation for the reaction that occurs between chlorine and water in direct sunlight. 

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).

Sodium iodide is an example of a sodium halide, which will react with concentrated sulfuric acid in a redox reaction.

Name a solid sodium halide which will not undergo a redox reaction with concentrated sulfuric acid.

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.

C₃H₇Cl, C₃H₇Br and C₃H₇I will all react in the same way when water, ethanol and aqueous sodium hydroxide are added and the reaction mixture is gently warmed. During these hydrolysis reactions, acidified silver nitrate solution can also be added, which  results in a precipitate being formed.

State and explain which of the three halogenoalkanes will react the most rapidly, producing the precipitate the fastest.

A solid sample of compound Z from part (a), will react with concentrated sulfuric acid.

Write equations, including state symbols, for all reactions which occur.

State one observation that could be made. 

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.