The Halogens (OCR A Level Chemistry A): Exam Questions

A group of students is discussing the properties of Group 7 elements and their compounds. 

Student A states that astatine would have a higher boiling point than iodine, but student B disagrees. Student B states that fluorine would have the highest boiling point of the elements in Group 7. 

State whether Student A, Student B or neither of the students is correct. Explain your answer.

The students also discussed the reactions and uses of elements of Group 7. Chlorine is added to drinking water to ensure that it is safe to drink.  

State a concern with adding chlorine to drinking water and provide an equation for the reaction of chlorine with cold water.

Chlorine will also react with cold, dilute sodium hydroxide. 

Provide the equation for this reaction.

The students had to plan a method that they could use, to determine the trend in reactivity down Group 7 using displacement reactions.
In their plan, they had to use potassium chloride solution, potassium bromide solution and potassium iodide solution, as well as aqueous samples of each of the halogens.  

Describe how the students would use the reagents named above to determine the trend in reactivity down Group 7 and suggest the trend that the students would find from their results.

Explain your answer, using relevant ionic equations. 

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.

Step 1

• She bubbles some chlorine gas through a sample of the solution.

• The mixture changes colour.

Step 2

• The student then adds an organic solvent, cyclohexane, to the mixture.

• She shakes the contents and allows the layers to separate.

i) Write the ionic equation for the reaction that takes place in step 1

ii) What colour does the cyclohexane layer turn in step 2?

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.

i) Complete the table to show the expected colours.

ii) Write the ionic equation for the reaction taking place in experiment 2.

Chlorine will react with cold, dilute sodium hydroxide. 

i) Write the equation for this reaction and give an important use of the solution which is formed.

ii) State the IUPAC name of the product formed which contains both chlorine and oxygen, and explain why this reaction is an example of a disproportionation reaction.

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).

i) State why it is necessary to use nitric acid in this test.

ii) Explain how ammonia can be used as a further test to distinguish between the presence of different halide ions and state why this test might be necessary.

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.

This question is about testing for anions.

Aqueous samples of potassium chloride, potassium bromide, and potassium sulfate were accidentally mixed up in a laboratory. Explain what chemical tests a student could perform to successfully identify each compound in the test tubes. Include ionic equations, with state symbols, for any reactions that may take place.

A student reacted solid sodium bromide with concentrated sulfuric acid. 

i) Give the half equations and the full equation for the conversion of H₂SO₄ to S and Br^- to Br₂. 

ii) Give the full equation from the half equations outlined in part (b) (i)

The reaction between solid sodium bromide and concentrated sulfuric acid is an example of an acid-base reaction. The equation for this is:

2H₂SO₄ (aq) + 2NaBr (s) → Na₂SO₄ (aq) + SO₂ (g) + Br₂ (g)
+ 2H₂O (l)

In this reaction, redox products are also formed. List all of the redox products produced from the reaction between solid sodium bromide and concentrated sulfuric acid and give observations for any products.

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

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.

i) Write an equation for the reaction that occurs.

ii) State the name of the type of reaction that is taking place and explain your answer using oxidation numbers.

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. 

i) Give the ionic equation for this reaction.

ii) Explain why it is important not to add any household cleaning agents that contain hydrochloric acid to a solution of 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.

i) Write the half equation for the production of BrO^- from the BrO₃^- ion. 

ii) Explain whether bromine has been oxidised or reduced in this reaction.

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.

i) Write the overall equation for this reaction. 

ii) By using oxidation numbers explain which compound is the oxidising agent and which compound is the reducing agent. 

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. 

i) State the half equation for the conversion of Br₂ to Br^-

ii) State the half equation for the conversion of Fe²⁺ to Fe³⁺

iii) Hence, give the full equation for the reaction of Br₂ and Fe²⁺

iv) Explain why I₂ cannot convert Fe²⁺ to Fe³⁺

v) Write an alternative equation involving Fe²⁺ and Fe³⁺ ions that justifies your answer to part (iv).

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. 

i) Give the equation for this reaction.

ii) A further equilibrium can be established by reacting ICl with chlorine producing a yellow solid, ICl₃, as the only product. ICl is placed in a U tube and chlorine gas is passed over it. Suggest one observation if the chlorine supply was increased and justify your answer.

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.

2AgCl(s) + 2ClF₃(l) → 2AgF₂(s) + Cl₂(g) + 2ClF(g)

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.

i) Use the data to calculate the standard enthalpy change for each of the reactions in part c).

ii) Only one of IF, IF₃ and IF₅, does not disproportionate – suggest which one.