A LevelChemistryCIETopic Questions11. Group 1711.3 Some Reactions of the Halide Ions

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First teaching 2023

First exams 2025

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Some Reactions of the Halide Ions (CIE A Level Chemistry)

Topic Questions

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1a2 marks

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

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1b5 marks

Sodium iodide reacts with concentrated sulfuric acid via the following equation.

H2SO4 (l) + NaI (s) → HI (g) + NaHSO4 (s)

The hydrogen iodide gas, HI (g), formed can react in a number of ways. 

 
i)
The hydrogen iodide formed reacts with concentrated H2SO4 to form a yellow solid, a smell of bad eggs and a purple vapour. Name the three products responsible for these observations.
yellow solid ........................................

smell of bad eggs ........................................

purple vapour ........................................
 
[3]
iii)
Write the equation which forms the yellow solid and purple vapour from HI and H2SO4. Include state symbols in your answer.
 
[2]

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1c1 mark

When sodium bromide reacts with concentrated sulfuric acid, bromine and sulfur dioxide are formed. 

Write the half equation for the formation of bromine from bromide ions.

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1a4 marks
Silver nitrate solution, AgNO3 (aq), is added to separate solutions of NaI and NaCl and precipitates form. An excess of aqueous ammonia is then added to both precipitates.

i)
Complete Table 1.1 to give the colour and name of the precipitate formed in each reaction and the effect of the addition of an excess of aqueous ammonia to each of the precipitates formed.

Table 1.1

  NaI (aq) + AgNO3 (aq) NaCl (aq) + AgNO3 (aq)
Colour of precipitate    
Name of precipitate    
Effect of addition of an excess of aqueous ammonia to the precipitate    

[3]

ii)
Write an ionic equation, including state symbols, to show the reaction occurring when AgNO3 (aq) is added to NaI (aq).
[1]

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1b3 marks

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.

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1c4 marks

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.

i)
Explain the meaning of the term oxidising agent, in terms of electron transfer.
[1]
ii)
Suggest an equation for this reaction of chlorine with water.
[1]
iii)
Write an equation for the reaction of chlorine with hot aqueous sodium hydroxide.

Use oxidation numbers to explain why this is a redox reaction.

[2]

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2a2 marks

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.


Redox product 1 .....................................................

Observation ............................................................

Redox product 2 .....................................................

Observation .............................................................

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2b4 marks

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. 

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2c2 marks

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.

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1a2 marks

This question is about the redox reactions of halide ions.

Chlorine can displace iodine from potassium iodide.

i)
Write the balanced equation for this reaction.

[1]

ii)
Write the ionic equation for this reaction.

[1]

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1b2 marks

Using oxidation numbers, explain how iodide ions act as a reducing agent in this reaction.

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1c6 marks
i)
Chlorine also displaces bromine from potassium bromide but not as readily as it can displace iodine from potassium iodide.

Explain how you can use these results to determine which is the stronger reducing agent, bromide ions or iodide ions.
[2]
ii)
Explain why the halide ions show this trend in reducing power.

[4]

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1d4 marks

Iodide ions can also act as a reducing agent to reduce Fe3+ ions to Fe2+.

i)
Write the ionic equation for this reaction.

[2]

ii)
Explain, in terms of electrons, how Fe3+ ions act as an oxidising agent.

[2]

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2a6 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 1.1 below. 

Table 1.1

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 1.1 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] 

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2b3 marks

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.

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2c3 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.

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2d3 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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3a3 marks

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.

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3b3 marks

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).

i)
Write the half equation for the formation of the yellow solid.
[1]
ii)
Write the half equation for the formation of iodine gas.
[1]
iii)
Hence write the full equation from your answer to parts (b)(i) and (b)(ii).
[1]

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3c3 marks

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.

i)
Write the equation for this reaction. State symbols are not required. 
[1]
ii)
Use oxidation numbers to explain why this is a redox reaction. 
[1]
iii)
In terms of electrons state the meaning of the term oxidising agent.
[1]

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3d1 mark

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

State a concern with adding chlorine to drinking water.

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