The Halogens (OCR A Level Chemistry)

Exam Questions

3 hours40 questions
1a4 marks

Give the reagents required to test for and distinguish between the halide ions.

1b2 marks

Write an equation, including state symbols, to show the reaction between silver nitrate solution, AgNO3 (aq), and sodium iodide solution, NaI (aq).

1c1 mark

Table 1 shows the boiling points of some of the halogens.

Table 1

Halogen

Boiling Point (°C)

Fluorine

-188

Chlorine

-35

Bromine 

 

Iodine

183

Astatine

337

Using Table 1, suggest a value for the boiling point of bromine.

1d4 marks

Halogens can react with other halide salts.

i)
Write the equation, including state symbols, for the reaction between chlorine water, Cl2, and potassium iodide, KI.
[2 marks] 
ii)
State any observations that you may see in this reaction. 
[1 mark]
iii)
State the name of this type of reaction.

[1 mark]

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2a1 mark

State what is meant by the term disproportionation reaction.

2b3 marks

The reaction between chlorine and an alkali is an example of a disproportionation reaction.

Write an equation, including state symbols, for the reaction between cold dilute sodium hydroxide, NaOH (aq) and chlorine, Cl2 (g).

2c3 marks

Chlorine is added to drinking water and used in swimming pools to kill any potentially harmful bacteria. 

The equation below shows the reaction between chlorine and water. 

Cl2 (aq) + H2O (l) HCl (aq) + HOCl (aq)

Use the oxidation numbers to explain why this is an example of a disproportionation reaction. 

2d2 marks

Chlorine, Cl2, is a toxic substance. Suggest why adding chlorine to the drinking water supply is safe to do so.

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3a1 mark

Bromine, Br2, can displace iodine, I2, from a solution of sodium iodide, NaI.

Give the simplest ionic equation for this reaction.

3b1 mark

In the reaction outlined in part a), bromine, Br2, is acting as an oxidising agent. 

Order the halogens below from the weakest to strongest oxidising agent.

Bromine, Br2

Chlorine, Cl2

Iodine, I2

3c1 mark

The reducing power of the halide ions increases as you go down the group. 

State the meaning of the term reducing agent.

3d3 marks

Explain why an iodide ion, I-, is a better reducing agent than a bromide ion, Br -.

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4a1 mark

The presence of halide ions in solution can be detected by adding silver nitrate solution and dilute nitric acid.

State the purpose of the nitric acid in this test.

4b2 marks

Explain how the addition of an ammonia solution can be used to confirm that a precipitate is silver chloride.

4c2 marks

A student suspects a sample is a sulfate salt.

Describe the chemical test the student should perform to confirm the presence of sulfate ions.

4d2 marks

The student also suspects the cation present in the salt is ammonium.

Describe the chemical test the student should perform to confirm the presence of the ammonium ion.

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5a2 marks

Explain why fluorine has a lower boiling point than chlorine.

5b1 mark

Chlorine can be reacted with sodium hydroxide to form bleach.

Write an ionic equation for the reaction between chlorine and cold dilute sodium hydroxide solution.

5c1 mark

Give the oxidation states of chlorine in each of the chlorine-containing ions formed.

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1a4 marks

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.

1b2 marks

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.

1c1 mark

Chlorine will also react with cold, dilute sodium hydroxide. 

Provide the equation for this reaction.

1d6 marks

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. 

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2a6 marks

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.

2b3 marks

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.

2c2 marks

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?
2d3 marks

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.

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3a6 marks

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. 

3b3 marks

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

3c5 marks

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

  Boiling point/°C
ammonia, NH3 – 33
fluorine, F2 – 188
bromine, Br2 59

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.

3d3 marks

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.

Experiment number

Experiment details

Colour seen within the organic solvent
1 addition of Cl2 (aq) to I(aq) ions  
2 addition of Cl2 (aq) to Br(aq) ions orange
3 addition of Br2 (aq) to Cl(aq)ions  

i)
Complete the table to show the expected colours.

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

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4a5 marks

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.
4b3 marks

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.

4c5 marks

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.

4d2 marks

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

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5a6 marks

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

Unknown

Test

Result

X

Added acidified silver nitrate solution 


Then added concentrated ammonia

Yellow precipitate formed 


Precipitate remained

Y

Added barium chloride solution 

White precipitate formed

Z

Added acidified silver nitrate solution 


Then added concentrated ammonia

Cream precipitate formed 


Precipitate dissolved 

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. 

5b3 marks

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.

5c3 marks

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

4KClO3 (s) rightwards arrow 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.

5d3 marks

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

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

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1a6 marks

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.

1b3 marks

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)
1c2 marks

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.

1d4 marks

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. 

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2a4 marks

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.

2b2 marks

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

2c1 mark

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.

2d2 marks

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.

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3a3 marks

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.

3b2 marks

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.

3c2 marks

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-.

3d2 marks

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

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4a5 marks

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. 
4b5 marks

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).
4c3 marks

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.

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5a1 mark

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.

5b1 mark

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)
5c
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3 marks

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 IF7.

5d4 marks

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

  IF IF3 IF5 IF7
ΔHf / kJ mol−1  −95.4 −486 −843 −962.5

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.

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