ASChemistryEdexcelExam Questions2. Inorganic Chemistry2.3 Group 7

Group 7 (Edexcel AS Chemistry)

Exam Questions

1 hour5 questions
Medium
Hard
1a5 marks

This question is about some redox reactions of chlorine, bromine and iodine.

An excess of aqueous potassium bromide was added to chlorine water and the solution turned orange.

i)
Write an equation for this reaction. State symbols are not required.

(1)

ii)
Silver nitrate solution was added to the mixture in (a) and excess dilute ammonia solution was then added to the precipitate formed.
Only some of the precipitate dissolved.

Deduce why only some of the precipitate dissolved.

(3)

iii)
Aqueous potassium bromide was added to aqueous iodine, instead of chlorine water. There was no reaction.

Give a reason why no reaction occurred.
(1)

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1b3 marks

Chlorine undergoes disproportionation when it reacts with hot aqueous sodium hydroxide solution.

i)
Complete the ionic equation for this reaction.
State symbols are not required.

(1)

...........Cl2 + ........... OH– → ........... Cl– + ...........ClO3– + ........... H2O

ii)
Explain, in terms of oxidation numbers, why this is a disproportionation reaction.

(2)

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2a
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The halogens are elements in Group 7 of the Periodic Table.

Chlorine compounds have many uses, including water treatment.

i)
Chlorine and phosphorus (P4) can react to form phosphorus(V) chloride.

The structure of a molecule of phosphorus is

q5a-9cho-al-1-oct-2021-edexcel-a-level-chem

Some mean bond enthalpy values are shown in the table.

Bond Mean bond enthalpy / kJ mol–1
P—P +198
Cl—Cl +243
P—Cl +326

Calculate the enthalpy change for the reaction between chlorine and phosphorus to form phosphorus(V) chloride.

10Cl2 + P4 → 4PCl5

(3)

ii)
Give a reason why bond enthalpy values are always positive.

(1)

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2b4 marks

Sodium chlorate(I) is a bleaching agent.

i)

Sodium chlorate(I) can be made by the reaction of chlorine with sodium hydroxide.

Show, by using oxidation numbers, that this reaction is disproportionation.

2NaOH + Cl2 → NaClO + NaCl + H2O

(2)

ii)

A different bleaching agent can be made by the reaction of chlorine with sodium hydroxide under different conditions.

Balance this equation.

............NaOH + ............Cl2 → NaClO3 + ............NaCl + ............H2O

(1)

iii)
What conditions are required for the reaction in (b)(ii)?
(1)
  ☐ A cold and dilute alkali
  ☐ B cold and concentrated alkali
  ☐ C hot alkali
  ☐ D excess chlorine

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2c1 mark

The halogens can be identified by their colour in an organic solvent such as hexane or cyclohexane.

Which sequence of colours is correct for chlorine, bromine and iodine dissolved in an organic solvent?

      Chlorine Bromine Iodine
  ☐ A orange red-brown black
  ☐ B pale green orange black
  ☐ C orange red-brown purple
  ☐ D pale green orange purple

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2d4 marks

Halide ions can be identified by their reaction with silver nitrate.

i)

Write the ionic equation for the reaction between aqueous solutions of sodium iodide and silver nitrate.

Include state symbols.

(2)

ii)

A solution containing 0.010 mol of a halide ion was reacted with excess silver nitrate and produced 1.88 g of precipitate.

Identify the halide ion.
Justify your answer.

(2)

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1a2 marks

This question is about the reactions of the halogens and halide ions.

i)
When chlorine gas is bubbled through an aqueous solution of potassium iodide, the reaction involves
(1)
  ☐ A oxidation only
  ☐ B reduction only
  ☐ C redox
  ☐ D disproportionation

ii)
Cyclohexane was added to the resulting solution from (a)(i). The mixture was shaken and then allowed to stand for a few minutes. Two layers were formed.

[Density: aqueous layer solution = 1.10 gcm–3, cyclohexane layer = 0.78 gcm–3]

The colour of the lower layer was
(1)

  ☐ A pale yellow
  ☐ B purple
  ☐ C red
  ☐ D pale green

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1b4 marks

Concentrated sulfuric acid was added to a small quantity of solid potassium iodide in a test tube.

i)
In this exothermic reaction, which of the following mixtures of gases would be produced?

(1)

  ☐ A hydrogen iodide and sulfur dioxide only
  ☐ B hydrogen iodide and hydrogen sulfide only
  ☐ C hydrogen iodide, sulfur dioxide and hydrogen sulfide
  ☐ D hydrogen iodide, hydrogen sulfide and iodine

ii)
Hydrogen iodide is a gas which reacts in a similar way to hydrogen chloride.

State the observation when the hydrogen iodide gas is passed over the mouth of an open bottle of concentrated ammonia solution.
Write an equation, including state symbols, for the reaction.
(3)

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1c5 marks

Potassium iodate(V) can be prepared by adding solid iodine to a hot aqueous solution of potassium hydroxide.

The equation for the reaction is

3I2 + 6KOH → KIO3 + 5KI + 3H2O

Potassium iodate(V) can be separated from the other reaction product using their differing solubilities in water.

  Solubility in water at 25 °C
/ mol dm–3
KI 8.92
KIO3 0.43

i)
Outline a procedure that you could use to obtain a sample of dry, solid potassium iodate(V) from the reaction mixture.

(3)

ii)
Describe how you would show that iodide ions are present in an aqueous solution of potassium iodide.

(2)

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1d1 mark

Fluorine is an element in Group 7.

Group 7 includes the elements chlorine, bromine and iodine.

Some information about the melting and boiling temperatures of Group 7 elements is shown in the table.


Element Melting temperature / K Boiling temperature / K
chlorine 172 238
bromine 266 332
iodine 387 457

Which is the expected boiling temperature of fluorine, in kelvin, K?

  ☐ A 4
  ☐ B 85
  ☐ C 575
  ☐ D 610

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2a4 marks

This question is about the reactions of the halogens and their salts.

The potassium halides react with concentrated sulfuric acid to form hydrogen halides.

i)

The equation for this reaction for potassium chloride can be written

KCl + H2SO4 → HCl + KHSO4

The hydrogen chloride does not react further.

State why this reaction is not a redox reaction.

(1)

ii)

On descending Group 7, the hydrogen halides become better reducing agents.

Explain how the reactions of potassium chloride, potassium bromide and potassium iodide with concentrated sulfuric acid provide evidence for this statement.

No explanation of the trend is required.

(3)

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2b4 marks

The reaction that occurs between chlorine and sodium hydroxide depends on the temperature.

i)

At room temperature the reaction that occurs is

Cl2 + NaOH → NaClO + NaCl

Explain, with reference to oxidation numbers, why this is a disproportionation reaction.

(2)

ii)

With hot sodium hydroxide solution, a different disproportionation reaction occurs. Sodium chlorate(V) is one of the products.

Complete the equation for this reaction. State symbols are not required.

(2)

......................Cl2 + ......................NaOH →

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2c
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Chlorine is used as a bleach in the textiles industry. Any excess chlorine can be removed by reduction to chloride ions.

The half-equation for the reaction of chlorine is

Cl2 + 2e– → 2Cl–

In one reaction, 768 cm3 of chlorine gas was reduced.

i)
Calculate the number of moles of electrons gained by chlorine molecules during this reaction.
[Under these conditions one mole of gas occupies 24 dm3]

(2)

ii)

The reducing agent was a solution containing thiosulfate ions, S2O32-.

The chlorine reacted with 40 cm3 of a 0.20 mol dm–3 solution of these ions.

Deduce the number of moles of electrons lost by each atom of sulfur in the thiosulfate ion, and hence the final oxidation state of the sulfur in the product.

(3)

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3a5 marks

This question is about chlorine and its compounds.

Potassium chlorate(V) can be produced by passing chlorine gas into hot, concentrated potassium hydroxide solution.

3Cl2 + 6KOH → 5KCl + KClO3 + 3H2O

i)
This reaction is an example of
(1)
  ☐ A oxidation only
  ☐ B reduction only
  ☐ C disproportionation
  ☐ D decomposition

ii)

A dot-and-cross diagram for the chlorate(V) ion (ClO3) is shown.

q9aii-8cho-as-1-june-2019-qp-edexcel-a-level-chem

Predict the shape and bond angle (O—Cl—O) of the chlorate(V) ion.

Justify your answer.

(4)

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3b4 marks
i)
The following reaction occurs when potassium chlorate(V) is heated at a suitable temperature.
Complete the equation by balancing it.
State symbols are not required.

.............. KClO3 → ................KCl +............KClO4

(1)

ii)

The table shows some properties of potassium chloride and potassium chlorate(VII).

  Potassium chloride KCl Potassium chlorate(VII) KClO4
Solubility in water (mol/100 g) 4.81 × 10–1 1.29 × 10–2
Solubility in ethanol (mol/100 g) 2.9 × 10–4 8.7 × 10–6

Devise a brief method to show how the compounds produced in the decomposition of potassium chlorate(V) could most effectively be separated.
Use information from the table.

(3)

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3c
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Chlorine gas, Cl2 , can be dissolved in swimming pool water to disinfect it.

An Olympic-sized swimming pool contains about 2500 m3 of water.
The chlorine content is 2 ppm (parts per million) by mass.
Calculate the number of moles of chlorine, Cl2 , in the swimming pool.

[One ppm is equivalent to 1 g of chlorine dissolved in 1 × 106 g of water.
Density of water = 1 g cm–3]

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3d4 marks

When chlorine gas is dissolved in water, it reacts according to the equation

Cl2 + H2O → HCl + HClO

The chloric(I) acid (HClO) produced is much more effective as a disinfectant than dissolved chlorine.
Chloric(I) acid is a weak acid and has little effect on the pH of the water.

Swimming pools usually have a chlorine content of 1 – 3 ppm.
Use the equation to explain one disadvantage of a chlorine content that is much lower than 1 ppm and one disadvantage of a chlorine content that is much higher than 3 ppm.

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3e4 marks

In many swimming pools, sodium chlorate(I) has replaced chlorine gas as a disinfectant.
Sodium chlorate(I) is an ionic compound. It is very soluble in water.

NaClO (aq) → Na+ (aq) + ClO– (aq)

i)
Describe, using diagrams to illustrate your answer, the interactions between each of the ions and the solvent when sodium chlorate(I) dissolves in water.

(2)

ii)

The displayed formulae of ethanol and chloroethane are shown.

q9eii-8cho-as-1-june-2019-qp-edexcel-a-level-chem

Ethanol is very soluble in water whereas chloroethane is almost insoluble in water.

Explain this observation by comparing the types of intermolecular forces formed by each of these molecules with water.

(2)

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3f
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Calcium chlorate(I), Ca(ClO)2 , can be used to disinfect drinking water.

The concentration of chlorate(I) ions required to disinfect water is about 5.6 × 10–6 mol dm–3.

Calculate the mass of calcium chlorate(I), in g, that should be added to 1000 dm3 of water to produce a chlorate(I) ion concentration of 5.6 × 10–6 mol dm–3.

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