Some Reactions of the Halide Ions (Cambridge (CIE) AS Chemistry)

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]

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

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]

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]

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

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] 

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.

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.

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.

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]

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]

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

State a concern with adding chlorine to drinking water.

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

[3]

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

[1]

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.

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]

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 .............................................................

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. 

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.