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A LevelChemistryCIEExam Questions6. Electrochemistry6.1 Redox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State)

Redox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State) (CIE A Level Chemistry)

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6.1 Redox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State)

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6.1 Redox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State)

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1a4 marks

This question is about oxidation numbers.

Complete Table 1.1 giving the oxidation number of sulfur in the following compounds.

Table 1.1

Compound  Oxidation number
SO2  
SO3  
H2SO4  
SF4  

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1b1 mark

State the oxidation number of sulfur in its elemental form of S8. 

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1c3 marks

The reaction between tin chloride and magnesium is classed as a redox reaction.

SnCl4 + 2Mg → 2MgCl2 + Sn 

i)
State the change in oxidation number of magnesium.
[1]

ii)
State the change in oxidation number of tin.
[1]

iii)
Deduce which species is acting as the reducing agent.
[1]

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1d3 marks

Antimony reacts with nitric acid to form antimony oxide, nitrogen monoxide and water as shown the following equation.

Sb + HNO3 → Sb2O3 + NO + H2O

i)
State the change in oxidation number of antimony.
[1]

ii)
State the change in oxidation number of nitrogen.
[1]

iii)
Identify the oxidising agent in this reaction.
[1]

 

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2a2 marks

State two definitions of reduction.

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2b3 marks

Complete Table 2.1 with the oxidation numbers of the elements in the following species.

Table 2.1

Species Oxidation number
S2-  
Pb2+  
V3+  
Si  
Sb3+  
H-  

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2c3 marks

Oxidation number are sometimes visible in the names of chemicals.

 

Complete Table 2.2 with the oxidation numbers of the given elements in the following species.

Table 2.2

Compound  Oxidation number
Copper in copper(I) oxide  
Iron in iron(III) chloride  
Tin in tin(IV) chloride  
Vanadium in vanadium(II) oxide   

 

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3a2 marks

This question is about disproportionation reactions.

Explain what is meant by the term disproportionation reaction.

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3b2 marks

State whether the displacement reaction between copper(II) ions and magnesium is an example of a disproportionation reaction. Explain your answer.

Cu2+ + Mg → Mg2+ + Cu

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3c2 marks
i)
Write a balanced equation for the disproportionation reaction of sulfuric acid with copper(I) oxide to form copper, copper sulfate and water.

[1]

ii)
Identify which element, in this reaction, undergoes disproportionation.

[1]

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3d2 marks
i)
Write a balanced equation for the reaction of nitrogen dioxide with water to form nitric acid and nitrous acid, HNO2.

[1]

ii)
Identify which element undergoes disproportionation.

[1]

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1a4 marks

This question is about redox reactions.

When calcium reacts with Fe(NO3)2, a redox reaction occurs.

Ca + Fe(NO3)2 → Ca(NO3)2 + Fe

i)
Explain in terms of electrons, which species has been oxidised and which has been reduced.

[2]

ii)
Use oxidation numbers to show which species acts as the reducing agent.

[2]

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1b1 mark

What is the full systematic name for Fe(NO3)2?

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1c5 marks

Another redox reaction occurs between cobalt oxide, Co2O3, and hydrochloric acid, HCl.

Co2O3 + 6HCl → 2CoCl2 + 3H2O + Cl2

i)
Explain, in terms of oxidation numbers, what has been reduced.

[2]

ii)
A student describes this reaction as disproportionation. Explain, using oxidation numbers, whether the student is correct or not.

[3]

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1d3 marks

Magnesium oxide, MgO, reacts with nitric acid, HNO3, when heated.

MgO + HNO3 → Mg(NO3)2 + H2O

i)
Identify the oxidation numbers of 

Mg in MgO ........................................................
N in HNO3 ........................................................
Mg in Mg(NO3)2 ........................................................
N in Mg(NO3)2 ........................................................

[2]

ii)
Explain whether the reaction between magnesium oxide and nitric acid is a redox reaction.

[1]

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2a1 mark

Under suitable conditions, SCl2 reacts with water to produce a yellow precipitate of sulfur and a solution A. Solution A contains a mixture of SO2 (aq) and compound B.

What is the oxidation number of sulfur in SCl2?

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2b4 marks
i)
Deduce how the oxidation number of sulfur changes during the reaction of SCl2 with water.

[2]

ii)
What type of reaction is this? Justify your answer.

[2]

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2c2 marks
i)
Suggest the identity of compound B.

[1]

ii)
Construct an equation for the reaction between SCl2 and water.

[1]

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2d3 marks
i)
What would you observe when each of the following reagents is added to separate samples of solution A?

AgNO3 (aq)     ................................................................................................................
K2Cr2O7 (aq) / H+ (aq)  .................................................................................................................

[2]

ii)
Acidified KMnO4 would also react with solution A.

What is the systematic name of KMnO4?

[1]

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3a
Sme Calculator6 marks

This question looks at different redox reactions.

For each of the equations below, write the oxidation number of the element printed in bold  underneath its symbol, and balance the equation by adding appropriate numbers before each species.

i)
  ....... MnO4-  +  ....... SO2  +  ....... H2O  →  ....... Mn2+  +  ....... SO42-  +  ....... H+
oxidation numbers:   .......     .......           .......     .......      

[3]

ii)
  ....... Cr2O72-  +  ....... NO2  +  ....... H+  →  ....... Cr3+  +  ....... NO3-  +  ....... H2O
oxidation numbers:   .......     .......           .......     .......      

[3]

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3b2 marks

What would you see happening in the reaction shown in (a)(ii)?

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3c3 marks

Zinc with displace copper from copper sulfate solution.

i)
Write the balanced symbol equation for the reaction.

[1]

ii)
Explain, using oxidation numbers, why this is a redox reaction.

[2]

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3d3 marks

Use a half equation to explain what species is acting as the oxidising reagent in the reaction in (c).

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4a3 marks

Household bleach contains NaClO as its active ingredient.

NaClO can be produced from the reaction between chlorine and water.

i)
State the conditions required for NaClO to be produced from chlorine and sodium hydroxide.

[1]

ii)
Give the systematic name of NaClO.

[1]

iii)
When chlorine and sodium hydroxide react under different conditions, sodium chlorate(V) is produced.
Give the formula for sodium chlorate(V).

[1]

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4b4 marks

The balanced symbol equation showing the reaction of chlorine and sodium hydroxide to form NaClO is shown below.

Cl2 (aq) + 2NaOH (aq) → NaClO (aq) + NaCl (aq) + H2O (l)

i)
Give the ionic equation for this reaction.

[1]

ii)
Explain, using oxidation numbers, why this is an example of a disproportionation reaction.

[3]

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4c1 mark

The concentration of NaClO in bleach can be found by reacting it with hydrogen peroxide, H2O2.

NaClO (aq) + H2O2 (aq) → NaCl (aq) + O2 (g) + H2O (l)

Explain, using oxidation numbers, why chlorine has been reduced in this reaction.

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4d
Sme Calculator5 marks

A student reacted an excess of aqueous hydrogen peroxide to 10.0 cm3 of a sample of bleach. They collected 108 cm3 of oxygen gas.

i)
How many moles of oxygen gas were collected? Assume that under the conditions used, the molar gas volume is 24.0 dm3 mol-1.
Show your working.

[1]

ii)
How many moles of NaClO were in the 10.0 cm3 sample of bleach?
Show your working.

[1]

iii)
What was the concentration of NaClO, in mol dm-3, of NaClO in the sample of bleach?
Show your working.

[1]

iv)
The label on the bottle of bleach gives the concentration of NaClO in g per 100 cm3.
Use your answer from part (iii) to deduce the concentration of NaClO in the bleach in g per 100 cm3.
Show your working.
[2]

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1a
Sme Calculator2 marks

This question is about the tetrathionate ion.

The tetrathionate ion is shown in Fig 1.1.

 
 
tetrathionate-ion

Fig 1.1
 

Calculate the oxidation number of sulfur in the tetrathionate ion. Explain your answer.

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1b4 marks

Sodium tetrathionate can be formed by reacting sodium thiosulfate, Na2S2O3, with iodine.

 
i)
Suggest an equation for the reaction between sodium thiosulfate and iodine.
 
[2]
 
ii)
Identify the oxidising agent in this reaction.
 
[1]
 
iii)
Describe the expected observation to show that this reaction had gone to completion.
 
[1]

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1c3 marks

Sodium tetrathionate can also be made by reacting sodium bisulfite, NaHSO3, with disulfur dichloride, S2Cl2.

2NaHSO3 + S2Cl2 → Na2S4O6 + 2HCl
 

Explain if this reaction can be described as a disproportionation reaction. Include reference to oxidation numbers of sulfur in your answer.

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2a3 marks
Photochromic glass contains an evenly distributed mixture of copper(I) chloride and silver(I) chloride. When sunlight passes through the glass the silver(I) chloride is separated into its ions. The chloride ions are then converted into chlorine atoms and the silver(I) ions into silver atoms. The silver atoms cluster together causing the lenses of photochromic glasses to darken.

Write equations for the processes involved in the darkening of photochromic glasses and explain if each reaction is reduction or oxidation.

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2b2 marks

Prescription photochromic sunglasses react to ultraviolet light and are adjusted to the needs of the wearer.

 

Explain why someone wearing photochromic sunglasses might not receive the full benefit of the glasses when driving a car on a cold, bright day.

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2c2 marks

The darkening process caused by the formation of silver atoms is reversible.

 

Write two equations to show how copper(I) chloride can react with the products from part (a) to remove the silver atoms and cause the lens to lighten.

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2d3 marks

Copper(I) chloride, similar to that used in photochromic glass, can be made in the laboratory by the following method:

  • Warm 0.5 g copper(II) oxide with 5 cm3 concentrated hydrochloric acid for 1 minute.
  • Add 1.0 g of copper turnings.
  • Gently boil for 5 minutes.
  • Filter the solution into 250 cm3 of deionised water.
  • Allow the copper(I) chloride precipitate to settle.
  • Decant the liquid.
  • Allow the copper(I) chloride to dry.
 
i)
Write an equation, including state symbols, for the reaction described.
 
[1]
 
ii)
Explain, using oxidation numbers, if this is a redox, disproportionation reaction.
 
[2]

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3a3 marks
The reaction of sodium iodide with concentrated sulfuric acid forms a variety of products as shown in Table 3.1.
 
Table 3.1
 

Chemical

Formed by

NaHSO4 

  

SO2 

  

Na2SO4 

  

I2 

  

HI

  

S

  

H2O

  

H2S

  
 

Complete Table 3.1 by identifying which products are formed by oxidation or reduction. 

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3b2 marks

Sulfur dioxide reacts with a solution of copper(II) chloride.

 
SO2 (g) + 2H2O (l) + 2CuCl2 (aq) → H2SO4 (aq) + 2HCl (aq) + 2CuCl (s)
 

Identify the reducing agent and oxidising agent in this reaction.

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3c3 marks

Sodium iodate is one of the main sources of iodine in the world. To extract the iodine, sodium iodate is reacted with sodium hydrogen sulfite, NaHSO3, according to the following ionic equation.

 
2IO3 (aq) + 5HSO3– (aq) → 3HSO4– (aq) + 2SO42– (aq) + I2 (aq) + H2O (l)
 
i)
Explain why sodium does not appear in the equation.
 
[1]
 
ii)
Explain the role of the hydrogen sulfite ions in the reaction.
 
[2]

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