Redox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State) (Cambridge (CIE) AS Chemistry)

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.

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

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

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.

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

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

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

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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?

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

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

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i) Suggest the identity of compound B.

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

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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)  ..........................................................................................................

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ii) Acidified KMnO₄ would also react with solution A.

What is the systematic name of KMnO₄?

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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⁺  +  ....... SO₄^2-  +  ....... H⁺ oxidation numbers:   .......     .......           .......     .......       [3]

ii)   ....... Cr₂O₇^2-  +  ....... NO₂  +  ....... H⁺  →  ....... Cr³⁺  +  ....... NO₃^-  +  ....... H₂O oxidation numbers:   .......     .......           .......     .......       [3]

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

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.

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

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.

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ii) Give the systematic name of NaClO.

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

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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.

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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.

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.

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

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

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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.

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This question is about the tetrathionate ion.

The tetrathionate ion is shown in Fig 1.1.

 Fig 1.1

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

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. 

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iii) Describe the expected observation to show that this reaction had gone to completion. 

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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.

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.

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.

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.

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. 

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

 Table 3.1

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

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.

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) + 2SO₄^2– (aq) + I₂ (aq) + H₂O (l)

 i) Explain why sodium does not appear in the equation.

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 ii) Explain the role of the hydrogen sulfite ions in the reaction. 

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