Syllabus Edition

First teaching 2023

First exams 2025

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

Exam Questions

2 hours40 questions
11 mark

Which row correctly describes oxidation and reduction in terms of the transfer of electrons and changes in oxidation state?

 

Transfer of electrons

Change in oxidation state

 

Oxidation

Reduction

Oxidation

Reduction

A

B

C

D

Gain

Loss

Loss

Gain

Loss

Gain

Loss

Gain

Increase

Increase

Decrease

Decrease

Decrease

Decrease

Increase

Increase

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    21 mark

    The heptahydrate mineral of magnesium sulfate is used in Epsom Salts for preparing therapeutic baths. Magnesium sulfate is also used in some medical applications.

    What is the oxidation state of MgSO4?

    • -2

    • +2

    • +7

    • 0

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    31 mark

    Which row describes the most common oxidation numbers of fluorine, oxygen and hydrogen in compounds?

     

    F

    H

    A

    B

    C

    D

    +7

    +1

    -7

    -1

    -1

    +2

    -1

    -2

    -1

    +1

    -1

    +1

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      41 mark

      Solutions containing chlorate(I) ions decompose on heating as shown:

      3ClO- → ClO3- + 2Cl-

      Which row is correct for the oxidation state of the chlorine in each ion?

       

      ClO-

      ClO3-

      Cl-

      A

      B

      C

      D

      +1

      +1

      -1

      -1

      +5

      +3

      +5

      +3

      -1

      +1

      -1

      +1

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        51 mark

        A student added a solution of iodine to a solution of sodium bromide. Which statement correctly describes what happens?

        • No reaction occurs.

        • The bromide ions are oxidised.

        • The iodine ions are oxidised.

        • Both the bromide and iodide ions undergo changes in their oxidation state.

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        61 mark

        In which of the following reactions is the change in oxidation number the smallest for nitrogen?

        • N2 + 3H2 → 2NH3

        • 3Mg + N2 → Mg3N2

        • 4NH3 + 5O2 → 4NO + 6H2O

        • 2NO + O2 → 2NO2

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        71 mark

        In the compound [ICl2]+[SbCl6], the oxidation number of chlorine is −1.

        What are the oxidation numbers of I and Sb in the compound?

         

        I

        Sb

        A

        B

        C

        D

        +1

        +1

        +3

        +3

        +5

        +7

        +5

        +7

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          81 mark

          Titanium dioxide is obtained from the ore ilmenite, Fe(II)TiO3

          FeTiO3 → TiO2

          What is the change in the oxidation number of titanium in the reaction?

          • +4 to +5

          • +3 to +4

          • No change in oxidation number occurs

          • +6 to +4

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          91 mark

          In which of the following compounds is the oxidation state of nitrogen the highest?

          • NaNO2

          • N2O4

          • N2O

          • NO

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          101 mark

          In which reaction does an element have the largest change in oxidation number?

          • Cr2O72-  +  6Fe2+  +  14H+  →  2Cr3+  +  6Fe3+  +  7H2O

          • 3ClO  → ClO3  +  2Cl

          • 5Fe2+  +  MnO4  +  8H+  →  5Fe3+  +  Mn2+  +  4H2O

          • PbO2  +  Sn2+ +  4H+  →  Sn4+  +  Pb2+  +  2H2O

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          111 mark

          Sulfur dioxide and oxygen difluoride react together. The oxidation number of fluorine in OF2 is -1.

          SO2 + OF2 → SO3 + F2

          Which row is correct?

           

          Sulfur

          Fluorine

          Oxygen in OF2

          A

          B

          C

          D

          Oxidised

          Reduced

          Oxidised

          Reduced

          Reduced

          Oxidised

          Oxidised

          Oxidised

          Reduced

          Reduced

          Reduced

          Oxidised

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            11 mark

            Oxidation numbers can be used to balance equations. Chlorine and hot aqueous sodium hydroxide react to produce chloride ions, chlorate ions and water.

            What are the values of the coefficients p, r and s in the equation?

            pCl2 (g) + qOH- (aq) → rCl- (aq) + sClO3- (aq) + tH2O (l)

             

            p

            r

            s

            A

            B

            C

            D

            3

            3

            2

            2

            5

            6

            5

            4

            1

            2

            1

            2

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              21 mark

              The chemistry of the Group VII elements often involves redox processes. 

              Which statement is correct?

              • Bromine can oxidise chloride ions.

              • Iodide ions are the weakest reducing agent of the first four Group VII ions.

              • In reactions with water, chlorine is oxidised and reduced.

              • Fluorine is a weaker oxidising agent than chlorine.

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              31 mark

              Four reactions are shown. In which reaction is the species in bold acting as an oxidising agent?

              • Cr2O72- + 8H+ + 3SO32- → 2Cr3+ + 4H2O + 3SO42-

              • Mg + Fe2+ → Mg2+ + Fe

              • Cl2 + 2Br- → 2Cl- + Br2

              • Fe2O3 + 3CO → 2Fe + 3CO2

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              41 mark

              In which compound are there two different elements with the same oxidation number?

              • Mg(OH)2

              • Na2SO4

              • HClO

              • NH4Cl

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              51 mark

              When sulfuric acid and sodium iodide react one of the reactions that take place is shown by the equation below

              8NaI + 9H2SO4 → 8NaHSO4 + 4I2 + H2S + 4H2O

              Which species has been oxidised in this reaction?

              • I-

              • SO42-

              • Na+

              • H+

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              61 mark

              The following reaction can be used to determine the mass of titanium dioxide in an ore.

              3TiO2 + 4BrF3 → 3TiF4 + 2Br2 + 3O2

              Which element increases in oxidation number in this reaction?

              • Fluorine

              • Bromine

              • Titanium

              • Oxygen

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              71 mark

              In which reaction does hydrogen behave as an oxidising agent?

              • 2Na  +  H2  →  2NaH

              • N2  +  3H2  →  2NH3

              • H2  +  Cl2  →  2HCl

              • C2H4  +  H2  →  C2H6

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              81 mark

              Chlorine gas reacts with both hot and cold aqueous sodium hydroxide, each resulting in a different set of products.

              What are the oxidation numbers of chlorine in the products of these reactions?

               

              Hot aqueous sodium hydroxide

              Cold aqueous sodium hydroxide

              A

              B

              C

              D

              -1, +5

              -1, +5

              +1, +6

              +1, +6

              -1, +1

              -1, +2

              -1, +1

              -1, +2

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                91 mark

                When solid potassium halides are added to concentrated sulfuric acid, the following reactions take place:

                Reaction 1

                2KBr  +  2H2SO4  →  K2SO4  +  SO2  +  Br2  +  2H2

                Reaction 2

                8KI  +  5H2SO4  →  4K2SO4  +  H2S  +  4I2  +  4H2O

                Reaction 3

                2KCl  +  H2SO4  →  K2SO4  +  2HCl

                In each reaction, what is the largest change in the oxidation number of sulfur?

                 

                Reaction 1

                Reaction 2

                Reaction 3

                A

                B

                C

                D

                1

                2

                2

                4

                4

                4

                8

                8

                1

                0

                0

                1

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                  11 mark

                  When heated ammonium nitrate, NH4NO3, can decompose explosively.

                  NH4NO3 → N2O + 2H2O

                  The nitrogen atoms in NH4NO3 have different oxidation numbers.

                  What are the changes in the oxidation numbers when this reaction proceeds?

                  • +4, -4

                  • -2, -4

                  • +4, -6

                  • +2, +6

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                  21 mark

                  The acid HIO disproportionate when in aqueous solution, this is shown in the equation, where q, r, s and t are simple whole numbers in their lowest ratios.

                  q HIO → r I2 + s HIO3 + t H2O

                  The equation can be balanced using oxidation numbers.

                  What are the values for r and s?

                   

                  r

                  s

                  A

                  4

                  2

                  B

                  4

                  1

                  C

                  2

                  1

                  D

                  1

                  2

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                    31 mark

                    In winemaking to prevent oxidation of ethanol by air, sulfur dioxide (SO2) is added. In order to calculate the amount of SO2, a sample is titrated with iodine (I2). The reaction is a one to one ratio for SO2 and I2.

                    What is the change in the oxidation number of sulfur in this reaction?

                    • +2 to +6

                    • +4 to +6

                    • +2 to +4

                    • +4 to +5

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                    41 mark

                    The table shows the redox potentials of a number of species.

                    Half reaction bold italic E to the power of bold empty set /  V

                    V3+ (aq) + e- rightwards harpoon over leftwards harpoon V2+ (aq)

                    - 0.26

                    VO2+ (aq) + 2H+ (aq) + e- rightwards harpoon over leftwards harpoon V3+ (aq) + H2O (l)

                    + 0.34

                    S4O62- (aq) + 2e- rightwards harpoon over leftwards harpoon 2S2O32- (aq)

                    + 0.47

                    2VO2+ (aq) + 2H+ (aq) + e- rightwards harpoon over leftwards harpoon VO2+ (aq) + H2O (l)

                    + 1.00

                    What is the lowest possible oxidation number that Vanadium (V) can be reduced to by thiosulphate ions?

                    • +2

                    • +3

                    • +4

                    • +5

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                    51 mark

                    The oxidation state of chlorine can range from –1 to +7 in its compounds.

                    Which reagent(s) and conditions are necessary to oxidise elemental chlorine into a compound containing chlorine in the +5 oxidation state?

                    • Concentrated H2SO4 at room temperature.

                    • AgNO3 (aq) followed by NH3 (aq) at room temperature.

                    • Cold dilute NaOH (aq).

                    • Hot concentrated NaOH (aq).

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                    61 mark

                    A student set up experiments to observe the outcome of adding different sodium halides to concentrated phosphoric acid and concentrated sulfuric acid. She recorded her findings in the table below.

                     

                    sodium chloride

                    sodium iodide

                    Conc. H3PO4

                    Conc. H2SO4

                    Colourless acidic gas formed

                    Colourless acidic gas formed

                    Colourless acidic gas formed

                    Purple vapour formed

                    Which deduction can be made from these observations?

                    • Concentrated sulfuric acid is a stronger oxidising agent than chlorine.

                    • Concentrated sulfuric acid is a stronger oxidising agent than iodine.

                    • Concentrated phosphoric acid is a stronger oxidising agent than iodine.

                    • Concentrated phosphoric acid is a stronger oxidising agent than concentrated sulfuric acid.

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                    71 mark

                    If a solution contains both bromine and chlorine, BrO3 ions are produced.

                    The reactions leading to the production of BrO3 ions are shown below: 

                    Reaction 1:     Br2  +  H2O  →  HBr  +  HBrO

                    Reaction 2:     3HBrO  +  Cl2  →  2Cl   +  BrO3  +  Br2  +  3H+

                    1

                    Chlorine is reduced in reaction 2.

                    2

                    Bromine is reduced in both reaction 1 and reaction 2.

                    3

                    Bromine is oxidised in both reaction 1 and reaction 2.

                    Which statements about these reactions are correct?

                    • 1 only

                    • 1 and 2

                    • 2 and 3

                    • 1, 2 and 3

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                    81 mark

                    If a dilute acid is added to an aqueous solution containing nitrite ions, NO2, two different nitrogen compounds are released as gases.

                    2H+ (aq)  +  2NO2 (aq)  →  H2O (l)  +  NO (g)  +  NO2 (g)

                    Which of the three statements below correctly describe the process?

                    1

                    The H+ (aq) ion is oxidised by NO2 (aq).

                    2

                    Some nitrogen atoms are oxidised, and some nitrogen atoms are reduced.

                    3

                    The H+ (aq) ion acts as a catalyst.

                    • 1 and 2

                    • 2 only

                    • 2 and 3

                    • 1, 2 and 3

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                    91 mark

                    Which pair of reactants will result in a redox reaction?

                    1

                    CH3COCH3  +  Tollens’ reagent

                    2

                    CH3CH2CHO  +  Fehling’s reagent

                    3

                    C2H4  +  Br2

                    • 1 only

                    • 1 and 2

                    • 2 and 3

                    • 3 only

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                    101 mark

                    How does concentrated sulfuric acid behave when it reacts with sodium chloride?

                    • As a reducing agent only.

                    • As an oxidising agent only.

                    • As an acid only.

                    • As an acid and an oxidizing agent.

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