Syllabus Edition

First teaching 2014

Last exams 2024

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Redox Processes (DP IB Chemistry: SL)

Exam Questions

3 hours46 questions
1a3 marks

Chlorine reacts with water to form chlorine water via the following equation.

Cl2 + H2O → HOCl + HCl

State the oxidation number of chlorine in the following species

   Cl2 …………………..

   HOCl ………………….

   HCl …………………..

1b1 mark

Chlorine is an oxidising agent.

Define oxidising agent in terms of electrons. 

1c2 marks

Nitrogen monoxide, NO, is formed when silver metal reduces nitrate ions, NO3- , ions in an acidic solution. State the oxidation numbers of nitrogen in NO and NO3-

1d1 mark

State the half equation for the formation of silver ions, Ag+ (aq), from silver metal.

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2a6 marks

Deduce the oxidation numbers of of the elements in the following species.

S2- …………………

Sn2+ …………………

V3+ …………………

Si …………………

Sb3+ …………………

H- …………………

2b3 marks

Oxidation states are sometimes visible in the names of chemicals. Deduce the oxidation numbers of the stated elements in the following species.

Copper in copper(I) oxide ………………..

Iron in iron(III) oxide ………………..

Phosphorus(V) oxide …………………

2c2 marks

The dichromate(VI) ion, Cr2O72- (aq) , reacts with sulfite ions, SO32- (aq), as follows.

Cr2O72- (aq) + 8H+ (aq)+ 3SO32-  (aq) →  2Cr3+ (aq) + 4H2O (l) + 3SO42- (aq)

i)
State whether the sulfite ions, SO32- (aq), are acting as an oxidising or reducing agent.
ii)
Justify your answer to part (i).
2d3 marks

Redox reactions can be identified by either reduction and oxidation occurring or the presence of a reducing agent and an oxidising agent.

i)
Deduce if the reaction between hydrochloric acid and sodium hydroxide is a redox reaction.

HCl + NaOH → NaCl + H2O
ii)
Justify your answer.

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3a1 mark

An iron tablet, weighing 1.35 g was dissolved in dilute sulfuric acid. The sample was dissolved in sulfuric acid to oxidise all of the iron to Fe2+ ions.

The solution is then titrated with 0.02 mol dm-3 potassium dichromate, K2Cr2O7, producing Fe3+ and Cr3+ ions in acidic solution. The titration requires 31.00 cm3 of K2Cr2O7 for 1.35 g of the sample.

Balance the following half equations:

Fe2+ (aq) → Fe3+ (aq) + ………

Cr2O72-(aq) + 14H(aq) + ……… → …….Cr3+ (aq) + …...H2O (l)
3b
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2 marks

The overall equation for the reaction in part (a) is as follows.

6Fe2+ (aq) + Cr2O72- (aq) + 14H+ (aq) → 6Fe3+ (aq) + 2Cr3+ (aq) + 7H2O (l)

i)
Using the information in part (a), calulate the number of moles of potassium dichromate, K2Cr2O7 used.
ii)
Use your answer to part (b) (i) to determine the number of moles of Fe2+ in the sample.
3c
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2 marks

Using the information in part (a) calculate the mass, in grams, of iron in the original sample.

3d
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1 mark

Using the information in part (a) and your answer to part (c) calculate the percentage of iron in the original 1.35 g iron tablet.

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4a2 marks

Zinc metal will react with copper sulfate solution. State the equation for this reaction.

4b1 mark

Predict the products, if any, of the reaction between lead(IV) oxide and zinc.

4c5 marks

A voltaic cell is made from a half-cell containing a zinc electrode in a solution of zinc nitrate and a half-cell containing a silver electrode in a solution of silver nitrate. Using section 25 in the data booklet, label the following diagram.


9-1-ib-sl-sq-easy-voltaic-cell-q4c

4d3 marks

State three differences between a voltaic cell and an electrolytic cells. 

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5a1 mark

A 500 cm3 sample of lake water was analysed using the Winkler Method. 0.0600 mol dm-3 sodium thiosulfate solution, Na2S2O(aq), was titrated against liberated iodine. It was determined that the average titre of Na2S2Owas 19.30 cm3.

Calculate the amount, in moles, of sodium thiosulfate used in the titration. 

5b1 mark

In the Winkler method 1 mol of O2 is equivalent to 4 mol of Na2S2O3 Deduce the amount, in moles, of oxygen that has reacted.

5c
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2 marks

Use your answer to part (b) and section 6 in your data booklet to calculate the following

  • The concentration, in mol dm-3, of oxygen in the water ………………..
  • The concentration, in g dm-3, of oxygen in the water …………………

5d3 marks

Define the biological oxygen demand (BOD).

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1a3 marks

Common household bleach is a cleaning product which smells like chlorine gas and is therefore, also called chlorine bleach.
It contains a mixture of sodium chlorate (NaOCl), sodium chloride and water and can be made by dissolving chlorine gas in a solution of sodium hydroxide.

i)
Write a balanced equation with state symbols for this reaction.
ii)
Deduce the oxidation number of chlorine in all of the chlorine-containing reactants and products
1b4 marks

The mixing of household bleach with ammonia during cleaning should be avoided, as a
redox reaction between the ammonia and the chlorate(I) ions in bleach will generate toxic chlorine gas and hydrazine (N2H4).
 

The overall redox reaction for this reaction is shown below. 

            2NH3 (aq) + 2ClO- (aq) → N2H4 (aq) + Cl2 (g) + 2OH- (aq)

i)
What are the oxidation numbers of the nitrogen atom in NH3 and in N2H4?
ii)
What is the oxidizing agent in this reaction? Explain your answer.
iii)
Why is the hazard of the toxic chlorine gas being produced greater than the hazard of hydrazine?
1c5 marks

Due to the risks associated with chlorine-based bleach, alternative bleaches are often used instead. These bleaches are based on peroxides such as hydrogen peroxide.  

Manganate(VII) ions oxidize hydrogen peroxide to oxygen gas. The reaction is carried out with both species under acidic conditions. 

i)

Identify the oxidizing and reducing agents in this reaction.

ii)
Write the half-equation for the oxidation of hydrogen peroxide to oxygen gas.
iii)

The manganate(VII) ions themselves get reduced to manganese(II) ions. Write down the half-equation for the reduction of manganate(VII) ions.

iv)

Deduce the overall redox equation for this reaction. 

1d2 marks

Explain how the oxidation number of the oxygen atom in H2O2 is different from its oxidation state in other compounds.

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2a2 marks

Metals can often be seen written as a list, from the most reactive metal to the least reactive metal. This list is known as the reactivity series of metals and can be used to predict the feasibility of a reaction. 

Below is a section of the reactivity series of metals, ordered from most to least reactive: 

            Calcium

            Magnesium

            Aluminium

            Zinc

            Iron

            Tin

            Lead 

A piece of zinc was placed into a solution of iron(II) sulfate and a solution of magnesium sulfate.

Predict, giving a reason, whether a reaction would occur in each solution. 

2b2 marks

Copper is below lead on the reactivity series shown in part (a). A piece of zinc was placed into a solution of copper(II) sulfate. Write the half equation for the zinc and identify the type of reaction taking place.

2c4 marks

Many chemical reactions are redox reactions as they involve the transfer of electrons.

i)
Explain the role of the oxidizing agent in a redox reaction in terms of electron transfer.
ii)
State the most common oxidation number of an oxygen atom when in a compound.
iii)
Which oxygen compounds are an exception to your answer in part (ii)? Explain your answer.
2d3 marks

The following reaction is an example of a common redox reaction:

      5Fe2+ (aq) + MnO4- (aq) +8H+ (aq) → 5Fe3+ (aq) + Mn2+ (aq) + 4H2O (l) 

Deduce the oxidation numbers of iron and manganese in the above reaction, both as reactants and as products.           

State which substance is reduced.

 

2e4 marks

The amount of iron in some dietary iron supplements was analyzed by redox titration. Four tablets were crushed and dissolved in 50.0 cm3 of 2.00 mol dm-3 sulfuric acid. The solution was then transferred to a 250 cm3 volumetric flask and made up to 250 cm3 with distilled water. 

A 25.0 cm3 sample of the iron tablets solution was titrated against 0.00500 mol dm-3 potassium manganate(VII) and 25.8 cm3 was needed for complete reaction. 

Determine the amount of iron, in mol, in one tablet.

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3a4 marks

Halide ions, such as chloride, Cl-, can be identified using chemical tests. If an unknown compound is dissolved in dilute nitric acid, and then silver nitrate solution is added, a precipitate will form if the unknown solution contains halide ions. The precipitate formed will be a silver halide. 

The general equation for the precipitation reaction of halide ions with silver nitrate solution is:

            AgNO3 (aq) + X- (aq) → AgX (s) + NO3- (aq)

i)

Deduce the oxidation number of silver in AgNO3 and AgX and deduce the oxidation number of the halide in X- and in AgX.

ii)
Is the precipitation of silver halides a redox reaction? Explain your answer.
3b7 marks

Halide ions can also react with other halogens in aqueous solutions. Chlorine reacts in a redox reaction with an aqueous solution of sodium bromide, to form sodium chloride and bromine.

         Cl2 (aq) + NaBr (aq) → NaCl (aq) + Br2 (aq)

i)
State what type of redox reaction this is.
ii)
Using the overall redox reaction above, deduce the half-equation for chlorine. State whether chlorine is oxidized or reduced.
iii)

Using the overall redox reaction above, deduce the half-equation for bromine. State whether bromine is oxidized or reduced.

iv)
Use the reaction above and your knowledge of the halogens, to explain whether chlorine or bromine is a stronger oxidizing agent.
3c2 marks

Chlorine also oxidizes sulfur dioxide (SO2) in aqueous solutions to sulfate ions (SO42-) under acidic conditions.

i)
Deduce the half-equation for the reduction of chlorine in aqueous solution.
ii)
Deduce the half-equation for the oxidation of sulfur dioxide in aqueous solution.
3d1 mark

Use the two half-equations from part (c) to construct the overall redox equation for this reaction.  

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4a1 mark

The iron of railway lines rusts when it comes into contact with water and oxygen. The overall redox equation for the rusting of iron is as follows: 

         4Fe (s) + 3O2 (g) + 6H2O (g) → 4Fe(OH)3 (s) 

Define the term reduction.

4b2 marks

State, with a reason, the oxidizing agent in this reaction in part (a).

4c3 marks

A student investigates the rate of rusting of a piece of iron under different conditions.           

Figure 1 shows the set-up of the students’ experiment.

Figure 1

9-1-sq-q4c-medium-ib-chemistry

Predict in which test tube(s) the iron metal will not rust. Explain your answer.

4d4 marks

Deduce the oxidation number of each of the stated elements in the ions and compounds to complete Table 1 below. 

                      Table 1 

Species

Oxidation number

Oxygen in Na2O2

 

Hydrogen in MgH2

 

Nitrogen in NO3-

 

Chlorine in ClF

 

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5a3 marks

Aluminium is present in the Earth’s crust in aluminium ore, called bauxite. A number of processes are done to this ore, to extract the aluminium from it. The bauxite is initially purified to produce aluminium oxide, Al2O3. Electrolysis is then carried out on the molten Al2O3, to extract the aluminium.

i)

Write down the overall equation for the extraction of aluminium from aluminium oxide by electrolysis.

ii)
State whether the aluminium oxide is oxidized or reduced in the electrolysis reaction. Explain your answer.
5b4 marks

Another ionic compound which can undergo electrolysis is molten lead bromide.

i)
Explain, in terms of ions and electrons, what would happen in an electrolytic cell during the electrolysis of lead bromide, using carbon electrodes.  
ii)

State two different ways in which electrical charge flows in the electrolysis apparatus.

5c3 marks

State the products formed at each electrode during the electrolysis of molten lead bromide, giving the equations at each electrode with state symbols.  

5d3 marks

Draw a labelled diagram of the apparatus suitable to carry out the electrolysis of molten lead bromide. Include the direction of electron flow, the negative electrode (cathode), the positive electrode (anode) and the electrolyte.

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6a1 mark

The list below shows three metals from the activity series in order of reactivity.

9-1-sq-q6a-medium-ib-chemistry

Deduce which of the three metals is the strongest reducing agent.

6b2 marks

A voltaic cell can be made by joining two half-cells together, such as  Zn/Zn2+ and Ni/Ni2+.

Write a balanced equation for the overall reaction taking place when the two half-cells are connected together, and state which species is undergoing oxidation.

6c1 mark

Cell diagrams are a way to represent the redox reactions taking place in voltaic cells.

Write a cell diagram for the overall cell reaction taking place in part (b).

6d3 marks

Complete the partially labelled diagram in Figure 1, of the apparatus used in the voltaic cell in part (b). Show the direction of the movement of the electrons and ions in the cell.                                                      

Figure 1

9-1-sq-q6d-medium-ib-chemistry

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1a3 marks

A student sets up a titration to determine the amount of iron(II) sulfate in an iron tablet. They titrate the iron(II) sulfate solution with potassium manganate(VII) solution.

i)
Write the balanced, ionic half equations to show the reduction of the manganate(VII) ion and the oxidation of the Fe2+.
[2]

ii)
Use your answers to part (i) to write an overall redox equation for the titration of iron(II) sulfate with potassium manganate(VII) solution.
[1]
1b2 marks

The iron(II) sulfate solution is acidified before titration to stop the manganate ion forming unwanted manganese dioxide. 

Explain the effect that not acidifying the iron(II) sulfate would have on the final 
calculation of the estimated mass of iron.

1c
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5 marks

The student dissolved the iron tablet in excess sulfuric acid and made the solution up to 250 cm3 in a volumetric flask. 25.0 cm3 of this solution was titrated with 0.0100 mol dm-3 potassium manganate(VII) solution. The average titre was found to be 26.65 cm3 of potassium manganate(VII) solution. 

i)
Calculate the amount, in moles, of iron(II) ions in the 250 cm3 solution.
[3]
ii)
Calculate the mass of iron, in mg, in the tablet. 
[2]
1d1 mark

Iron sulfate reacts with chromium to produce chromium(III) sulfate, Cr2(SO4)3 , and iron

Deduce the overall ionic equation for the reaction occurring 

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2a5 marks

Molten potassium bromide can be electrolysed using graphite electrodes.

i)
Draw the essential components of this electrolytic cell.
[3]
ii)
Identify the products at each electrode.
[2]

2b3 marks

State the half equations for the oxidation and reduction processes and deduce the overall cell reaction, including state symbols. 

Oxidation half equation …………………

Reduction half equation ……………….

Overall equation ……………………………
2c1 mark

Explain why solid potassium bromide does not conduct electricity. 

2d3 marks

A voltaic cell is made from a half-cell containing a magnesium electrode in a solution of magnesium nitrate and a half-cell containing a silver electrode in a solution of silver(I) nitrate.

9-1-ib-sl-sq-hard-q2d-mg-and-ag-voltaic-cell
i)
Use section 25 of the data booklet to determine which electrode is positive and to write the equation for the reaction at the positive electrode, including state symbols. 
[1]
ii)
Compare the processes at the positive electrodes in voltaic and electrolytic cells. 
[2]

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3a
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3 marks

State the oxidation state of phosphorus in the following compounds.

H2PO4………………………………………………………..

HPO………………………………………………………..

H3PO………………………………………………………..
3b
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2 marks

The tetrathionate ion is shown below:

1-20

i)

Determine the oxidation state of sulfur in the ion.
[1]

 

ii)
Justify your answer to part ii).
[1]
3c3 marks

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

i)
State the balanced symbol equation for this reaction.
[2]
ii)
Identify the oxidising agent in this reaction.
[1]
3d1 mark

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

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4a3 marks

A 150.0 cm3 sample of pond water was analysed using the Winkler method to determine its biological oxygen demand (BOD). Initially it took 29.40 cm3 of 0.010 mol dm-3 Na2S2O3 to react with iodine.


After five days it required 13.70 cmof 0.010 mol dm-3 Na2S2O3 to react with iodine.


The unbalanced equations for the Winker method are shown below.

......Mn2+ (aq) + ......OH- (aq) + O2 (aq) → ......MnO2 (s) + ......H2O (l) 

MnO2 (s) + ......I(aq) + ......H(aq) → Mn2+ (aq) + I2 (aq) + ......H2O (l) 

......S2O32- (aq) + I2 (aq) → S4O62- + ......I- (aq)  


Balance the equations for the Winkler method. 

4b3 marks

Deduce the reducing agent in the reaction between S2O32- and I2. Justify your answer. 

4c
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3 marks

Use the information in part a) and section 6 in your data booklet to determine the initial concentration, in ppm, of oxygen. 

4d
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3 marks

Use the information in part a) and section 6 in the data booklet to determine the concentration, in g dm-3, of oxygen after five days. 

4e
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2 marks

Determine the BOD of the pond water in ppm. 

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5a
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3 marks

15.00 cm3 of ethanedioic acid, H2C2O4 (aq), requires 10.30 cm3 of a 0.250 mol dm-3 solution of sodium hydroxide, NaOH (aq), for complete neutralisation using a phenolphthalein indicator for the first permanent colour change.

15.00 cm3 of the same H2C2O4 solution required 12.35 cm3 of potassium permanganate solution, KMnO4 (aq), solution for complete oxidation to carbon dioxide and water in the presence of dilute sulfuric acid to further acidify the H2C2O4 solution for the first permanent colour change. 

i)
Using the following equation, calculate the amount, in moles, of H2C2O4 (aq).

H2C2O4 (aq) + 2NaOH (aq) → Na2C2O4 (aq)+ 2H2O (l) 

[2]

ii)
Calculate the concentration, in mol dm-3, of H2C2O4 (aq). 
[1]
5b3 marks

Deduce the following half equations and overall redox equation for the reaction outlined in part a).

MnO4- (aq) to Mn2+ (aq) .........................

H2C2O4 (aq) to CO2 (g) ...........................

Overall equation ......................................

5c
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2 marks

Calculate the concentration, in mol dm-3of the potassium manganate(VII), KMnO4, solution.

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