AQA A Level Chemistry

Topic Questions

1.9 Oxidation, Reduction & Redox Equations

1a3 marks

State three definitions of reduction.

1b2 marks

An atom’s oxidation state is the charge on an individual atom, if the bonding were completely ionic.

State two things that oxidation states can be used for.

1c3 marks

Complete Table 1 with the oxidation states of the elements in the following species.

Table 1

Species

S2-

Sn2+

V3+

Si

Sb3+

H-

Oxidation state

 

 

 

 

 

 

1d3 marks

Oxidation states are sometimes visible in the names of chemicals. Complete Table 2 with the oxidation states of the stated elements in the following species.

Table 2

 

Cu in copper (I) oxide

Iron in iron (III) chloride

P in phosphorous (V) oxide

Oxidation state

 

 

 

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2a2 marks

Determine the oxidation state of sulfur in CaSO3. Show your working.

2b1 mark

Chlorine trifluoride contains only the nonmetals chlorine and fluorine. Carbon dioxide contains only the nonmetals carbon and oxygen.

How do you decide which element has the positive oxidation state?

2c2 marks

Determine the oxidation state of chlorine in chlorine trifluoride, ClF3. Show your working.

2d1 mark

A student suggests that the oxidation states of elements in compounds are always whole numbers. Is the student correct? Justify your answer.

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3a1 mark

What is an oxidising agent?

3b1 mark

The dichromate ion, Cr2O72-, reacts with sulfite ions, SO32-, according to the following equation

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

Are the sulfite ions, SO32-, acting as an oxidising or reducing agent. Justify your answer.

3c2 marks

Identify which species is acting as the oxidising agent in the following reaction. Justify your answer.

Cl2 + 2Br →  2Cl + Br2

3d3 marks

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

Explain whether the reaction between sodium hydroxide and hydrochloric acid is a redox reaction or not.

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4a3 marks

Dichromate ions, Cr2O72-, react with Cu+ ions in the presence of acid, H+ to form chromium (III) ions, Cr3+, copper (II) ions, Cu2+, and water.

The unbalanced redox equation for this reaction is:

Cr2O72- + Cu+ + H+ → Cr3+ + Cu2+ + H2O

i)
Determine the change in oxidation state of the copper in each copper species.

ii)
Is the copper (I) ion, Cu+, acting as an oxidising or reducing agent?
4b2 marks

The unbalanced redox equation for the reaction of dichromate ions, Cr2O72-, with Cuions in the presence of acid, H+ to form chromium (III) ions, Cr3+, copper (II) ions, Cu2+and water is:

Cr2O72- + Cu+ + H+ → Cr3+ + Cu2+ + H2O

Determine the change in oxidation state of the chromium from the dichromate (VI) ion to the chromium (III) ion .

4c1 mark

The unbalanced equation for the reaction of manganate ions, MnO4-, with iron (II) ions, Fe2+, ions in the presence of acid, H+ to form manganese (II) ions, Mn2+, iron (III) ions, Fe3+, and water is:

MnO4-- + Fe2+ + H+ → Mn2+ + Fe3 + + H2O

One manganese atom undergoes a change in oxidation state of -5.

One iron atom undergoes a change in oxidation state of +1.

This leads to the first step of balancing the redox equation, as shown in Figure 1.

Figure 1

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By comparing the overall unbalanced equations, suggest why this method of balancing might not work as easily for the dichromate equation from parts (a) and (b), shown below:

Cr2O72- + Cu+ + H+ → Cr3+ + Cu2+ + H2O

4d3 marks

Using the information form parts (a), (b) and (c), balance the redox equation for the reaction of dichromate ions, Cr2O72-, with Cu+ ions in the presence of acid, H+ to form chromium (III) ions, Cr3+, copper (II) ions, Cu2+, and water.

Cr2O72- +  __ Cu+ + __ H+ → __ Cr3+ + __ Cu2+ + __ H2O

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5a2 marks

Define a disproportionation reaction.

5b2 marks

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

Cu2+ + Mg → Mg2+ + Cu

5c2 marks
i)
Write a balanced equation for the disproportionation reaction of sulfuric acid with copper (I) oxide to form copper, copper sulfate and water.

ii)
Identify which element, in this reaction, undergoes disproportionation.
5d2 marks
i)
Write a balanced equation for the reaction of nitrogen dioxide with water to form nitric acid and nitrous acid, HNO2.

ii)
Identify which element undergoes disproportionation.

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1a2 marks

The tetrathionate ion is shown in Figure 1.

Figure 1

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Calculate and explain the oxidation state of sulfur in the tetrathionate ion.

1b4 marks

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

i)
Write a balanced symbol equation for this reaction.

ii)
Identify the oxidant in this reaction.

iii)
Describe the expected observation to show that this reaction had gone to completion.
1c3 marks

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

2 NaHSO3 + S2Cl2 → Na2S4O6 + 2 HCl

i)
Deduce the oxidation state of sulfur in all of the sulfur-containing chemicals.

ii)
Evaluate if this reaction is a disproportionation reaction.
1d3 marks

Two students are asked to comment on the atom economy for the formation of sodium tetrathionate using two different methods.

Method 1:        2 Na2S2O3 + Cl2 → Na2S4O6 + 2 NaCl

Method 2:        2 NaHSO3 + S2Cl2 → Na2S4O6 + 2 HCl

Student 1 concludes that method 1 has the higher atom economy because the second reactant is smaller.

Student 2 concludes that method 2 has the higher atom economy because the waste product is smaller.

i)
Deduce which student has identified the method with the highest atom economy.
Table 1 shows the relative molecular mass of the chemicals required for both methods.
Table 1

Molecule

Mr 

Chlorine

71.0

Disulfur dichloride

135.2

Hydrogen chloride

36.5

Sodium bisulphite

104.1

Sodium chloride

58.5

Sodium tetrathionate 

270.4

Sodium thiosulfate

158.2

ii)
Explain why neither student has given the correct justification for their choice of the highest atom economy.

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2a7 marks

Cyanide-containing compounds are commonly used in organic chemistry to lengthen the carbon chain. They are also used in gold and silver mining because of their ability to dissolve these metals and their ores.

i)
Use your knowledge of oxidation states, explain why a student might mistakenly think that the formula for a cyanide ion is CN+.

ii)
Use your knowledge of oxidation states to identify the correct oxidation state of carbon in the ionic compound, KCN.

iii)
Use your knowledge of chemical bonding to explain how the cyanide ion has the correct formula of CN-.
2b6 marks

Copper (I) thiocyanate, CuSCN, can react with potassium iodate, KIO3, in the presence of hydrochloric acid according to the following unbalanced equation.

CuSCN + KIO3 + HCl → CuSO4 + KCl + HCN + ICl + H2O

i)
Write the half equation involving copper (I) thiocyanate forming copper sulfate and hydrogen cyanide.

ii)

The carbon in copper (I) thiocyanate, CuSCN, has the same oxidation state as the carbon in dichlorodifluoromethane, CCl2F2.
Using your knowledge of oxidation states, state whether each component element in copper (I) thiocyanate, CuSCN, is oxidised or reduced.

2c5 marks

Copper (I) thiocyanate can react with potassium iodate in the presence of hydrochloric acid according to the following unbalanced equation.

CuSCN + KIO3 + HCl → CuSO4 + KCl + HCN + ICl + H2O

i)
Write the half equation involving potassium iodate forming potassium chloride and iodine.

ii)
Using your knowledge of oxidation states, state whether each component element in potassium iodate is oxidised or reduced.

2d1 mark

Use your answers from parts (b) and (c) to balance the symbol equation for the reaction of copper (I) thiocyanate with potassium iodate in the presence of hydrochloric acid.

__ CuSCN + __ KIO3 + __ HCl → __ CuSO4 + __ KCl + __ HCN + __ ICl + __ H2O

 

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3a3 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 the reaction is reduction or oxidation.

3b2 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.

3c2 marks

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

Write two balanced symbol 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.

3d3 marks

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

  1. Warm 0.5 g copper (II) oxide with 5 cm3 concentrated hydrochloric acid for 1 minute.
  2. Add 1.0 g of copper turnings.
  3. Gently boil for 5 minutes.
  4. Filter the solution into 250 cm3 of deionised water.
  5. Allow the copper (I) chloride precipitate to settle. 
  6. Decant the liquid.
  7. Allow the copper (I) chloride to dry.

i)
Write a balanced symbol equation, including state symbols, for the reaction described.

ii)
Explain, using oxidation states, if this is a redox, disproportionation reaction.

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4a3 marks

The reaction of sodium iodide with concentrated sulfuric acid forms a variety of products as shown in Table 1.

Complete Table 1 by identifying which products are formed by oxidation or reduction.

Table 1      

Chemical

Formed by

NaHSO4

 

SO2

 

Na2SO4

 

I2

 

HI

 

S

 

H2O

 

H2S

 

4b2 marks

Sulfur dioxide reacts with a solution of copper (II) chloride according to the following equation.

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

i)
Identify the reducing agent in this reaction.

ii)
Identify the specific oxidising agent in this reaction.
4c1 mark

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.

2 IO3 (aq) + 5 HSO3- (aq) → 3 HSO4- (aq) + 2 SO42- (aq) + I2 (aq) + H2O (l)

Explain why sodium does not appear in the equation.

4d2 marks

Explain the role of the hydrogen sulfite ions in the reaction shown in part (c).

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5a3 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 ion and the oxidation of the Fe2+ ion.

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.

5b2 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.

5c5 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 0.01 mol dm-3 potassium manganate (VII) solution.

The average titre was found to be 26.65 cm3 of potassium manganate (VII) solution.

Calculate the mass of iron, in mg, in the tablet. Give your answer to the appropriate number of significant figures.

5d2 marks

A student wanted to repeat the same titration while reducing the percentage uncertainty of the 50 cm3 burette. 

Use your answer to part (c) to suggest a suitable titre value to reduce uncertainty in then burette. Calculate the concentration of the potassium manganate (VII) solution required. Give your answer in standard form.

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1a5 marks

Household bleach 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 state of chlorine in all of the chlorine-containing reactants and products.

iii)
State what specific type of redox reaction this reaction is. Explain your answer.
1b4 marks

The mixing of household bleach with ammonia during cleaning should be avoided, as a redox reaction between the ammonia and the chlorate 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 states of the nitrogen atom in NH3 and in N2H4?

ii)
What is the oxidising agent in this reaction? Explain your answer.

iii)
Why are the risks of the toxic chlorine gas being produced greater than the risks 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 oxidise hydrogen peroxide to oxygen gas. The reaction is carried out with both species under acidic conditions. 

i)
Identify the oxidising 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 reaction for this reaction. 
1d2 marks

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

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2a4 marks

Halide ions 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 state of silver in AgNO3 and AgX, and deduce the oxidation state of the halide in X- and in AgX.

ii)
Is the precipitation of silver halides a redox reaction? Explain your answer.
2b7 marks

Halide ions can also react with each other 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 oxidised or reduced.

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

iv)
Use the reaction above and your knowledge of the halogens, to explain whether chlorine or bromine is a stronger oxidising agent. 
2c5 marks

Chlorine also oxidises 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.

iii)
Use the two half-equations in part (i) and (ii) to construct the overall redox equation.

iv)
Bromine reacts with sulfur dioxide in a different way to chlorine. Explain why.
2d5 marks

Chlorine is widely used in water purification. Besides killing dangerous germs like bacteria and viruses, chlorine also helps reduce unwanted tastes and odors in water by reacting with organic chemicals in water. 

i)
Write the overall redox reaction of chlorine with water.

ii)
Deduce the oxidation states of chlorine in all of the chlorine-containing compounds.

iii)

The reaction between chlorine and water is a disproportionation reaction. Define the term disproportionation and use oxidation states to explain why the above reaction is an example of this.

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3a6 marks

Thermite is a mixture of finely powdered aluminium and iron oxide. 

In a thermite reaction, the aluminium reacts with iron oxide to produce hot molten iron. This process is often utilised in remote locations for welding railway lines.

Initially, a lot of heat is required to start off the reaction. However, once started, the reaction itself releases a significant amount of heat which is enough to melt the iron.

i)
Write the overall equation for the thermite reaction, including state symbols.

ii)
Determine the oxidation states of each of the metals in the reactants and products.

iii)
Deduce the reduction and oxidation half-equations for this reaction.
3b3 marks

The thermite reaction in part (a) is a special type of redox reaction. 

i)
Explain what type of reaction the thermite reaction is.

ii)
What can be deduced about the chemical properties of the aluminium and iron metals involved in this reaction?
3c4 marks

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

Table 1

Compound/Ion

Oxidation state

Oxygen in Na2O2

 

Hydrogen in MgH2

 

Nitrogen in NO3-

 

Chlorine in ClF

 
3d6 marks

The iron of the 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)

i)
Define the term reduction.

ii)
What is the oxidising agent in this reaction? Explain your answer.

iii)
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

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

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4a5 marks

Fossil fuels contain relatively large amounts of sulfur. When these fossil fuels are burnt, sulfur gets into the atmosphere and acts as a pollutant. 

The sulfur is oxidised in the air and rises very high into the atmosphere, where it mixes and reacts with water, oxygen and other chemicals to form more acidic pollutants known as acid rain.

Acid rain causes corrosion of buildings, statues and can damage aquatic and plant life.

i)
Sulfur can react in different ratios with oxygen to form sulfur dioxide or sulfur trioxide. Write equations to show the oxidation of sulfur to form sulfur dioxide and to form sulfur trioxide. Include state symbols in your equations. 

ii)
In which reaction is the increase in oxidation state the greatest? Explain your answer.
4b4 marks

Acid rain can also be caused by oxides of nitrogen when they dissolve in water.

Deduce the oxidation state of the nitrogen atom in the given ions and compounds to complete Table 1 below.

Table 1

Compound/Ion

Oxidation state

N2

 

NO3-

 

HNO3

 

NO2+ 

 
4c5 marks

Nitrogen is very unreactive, due to the strong triple bonds within the molecule, and will only react with oxygen in air to form nitrogen oxides under extreme conditions (such as lightning).

Nitrogen fixing bacteria in the soil convert the unreactive molecular nitrogen in the atmosphere into ammonia or other related nitrogen compounds. This process is called nitrogen fixation.

i)
Write an ionic equation for the conversion of nitrogen to ammonia and hydrogen gas by nitrogen fixation.

ii)
Is the reaction in part (i) a reduction or oxidation reaction? Explain your answer.

iii)
The ammonia produced by nitrogen fixation can be further converted to nitrite ions (NO2-). Nitrite ions in turn are oxidised to nitrate ions, which are mainly used by plants for protein synthesis.


Write an equation for the conversion of nitrite ions to nitrate ions and deduce the oxidation states of nitrogen in the nitrogen-containing compounds.

4d4 marks

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

i)
Explain the role of the oxidising agent in a redox reaction in terms of electron transfer.

ii)
State the most common oxidation state of an oxygen atom when in a compound.

iii)
Which compounds are an exception to your answer in part (ii)? Explain your answer.

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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 (III) oxide. Electrolysis is then carried out, to extract the aluminium from the aluminium oxide. Oxygen gas is also formed as a byproduct of this part of the process. 

i)
Write down the overall equation for the extraction of aluminium from aluminium (III) oxide by electrolysis.

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

Other metals can be extracted from their metal ores by reacting the ore with carbon. Iron, for example, is extracted from iron (III) oxide (Fe2O3) in a huge container called a blast furnace, during a redox reaction with carbon.

i)
Write the overall equation for the reaction of iron (III) oxide with carbon.

ii)
What type of redox reaction is the reaction of iron (III) oxide with carbon? Explain your answer.

iii)
Use your answer to part (ii) to explain why aluminium cannot be extracted from aluminium (III) oxide by reacting it with carbon.
5c3 marks

Ferric chloride (FeCl3) is used to treat sewage, industrial waste and to purify water. FeCl3 can be formed from the reaction of iron with chlorine.

i)
Write the reduction half-equation for this reaction. Include state symbols.

ii)
Write the oxidation half-equation for this reaction. Include state symbols.

iii)
Deduce the overall redox equation using your answers to parts (i) and (ii). Include state symbols.
5d2 marks

When ferric chloride is dissolved in water, it undergoes hydrolysis and gives off heat. The resulting acidic and corrosive solution is used to purify water and treat sewage.

The hydrolysis reaction of ferric chloride in water is as follows:

FeCl3 (aq) + 3H2O (l) → Fe(OH)3 (aq) + 3HCl (aq)

State whether the hydrolysis of ferric chloride is a redox reaction. Explain your answer.

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