Edexcel A Level Chemistry

Topic Questions

A LevelChemistryEdexcelTopic Questions6. Advanced Inorganic Chemistry (A Level Only)6.1 Redox II

6.1 Redox II

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1a4 marks

This question is about the Ag+(aq)|Ag(s) half-cell.

A student was asked to plan an experiment to measure the standard electrode potential of the Ag+ (aq)|Ag(s) half-cell.

i)
State the conditions of temperature and pressure under which standard electrode potentials are measured.

(1)

ii)
The student drew the diagram shown.

q2aii-9cho-al-3-june-2019-qp-edexcel-a-level-chem

Identify three mistakes in this diagram and the modifications that should be made to correct them.

(3)

Mistake in diagram Modification needed to correct mistake
   
   
   

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1b
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The standard electrode potential, E to the power of ⦵, of the Ag+ (aq) | Ag (s) half-cell is +0.80 V.

The effect of changing the concentration of the ions on the value of the electrode potential, E, in this half-cell is calculated using the equation.

E equals E to the power of ⦵ plus fraction numerator R T over denominator 96500 end fraction space cross times space In open square brackets Ag to the power of plus space end exponent open parentheses aq close parentheses close square brackets

where T is the temperature in kelvin and R is the gas constant.

The electrode potential of a Ag+ (aq) | Ag (s) half-cell was measured at 20 °C and found to be +0.72 V.

Calculate the concentration of silver ions, in mol dm-3, in this half-cell.

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2a1 mark

Tablets containing potassium manganate(VII), KMnO4 , are dissolved in water forming an antiseptic solution to treat skin conditions. The manufacturers claim that each tablet contains 400 mg of KMnO4 .

To check the claim, the titration procedure outlined was carried out.

  • Five tablets were dissolved in distilled water to make 100.0 cm3 of solution.
  • Some of the KMnO4 solution was used to fill a burette.
  • 25.0 cm3 of sodium ethanedioate solution, Na2C2O4 (aq), of concentration 0.200 mol dm−3, was added to a conical flask and warmed.
  • Sulfuric acid, of concentration 2 mol dm−3 , was also added to the conical flask.
  • The KMnO4 solution was added to the flask from the burette, until the end-point.

The equation for the reaction between MnO4− ions from the KMnO4 and C2O42− ions from the sodium ethanedioate solution is shown.


16H+ (aq) + 2MnO4− (aq) + 5C2O42− (aq) → 2Mn2+ (aq) + 10CO2 (g) + 8H2O (l)

Give the colour change at the end-point of the titration.

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2b3 marks
i)
Complete the dot-and-cross diagram for the ethanedioate ion.
Show the outer electrons only.

(2)

q8bi-9cho-al-1-nov-2020-qp-edexcel-a-level-chem

ii)
Determine the oxidation number of carbon in the ethanedioate ion, C2O42−.
(1)

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2c1 mark

Give the reason why sulfuric acid was also added to the conical flask.

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2d2 marks

This redox reaction could be used in an electrochemical cell.
The cell half-equations are

2CO2(g) + 2e−⇌ C2O42− (aq)

8H+(aq) + MnO4−(aq) + 5e− ⇌ Mn2+(aq) + 4H2O(l)

Write a cell diagram for this cell using the conventional representation.

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2e
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The results of the titration are shown.

Run Trial 1 2 3
Final volume / cm3 17.50 34.10 17.20 34.10
Initial volume / cm3 0.00 17.30 0.00 17.20
Titre / cm3 17.50   17.20  
Concordant titres (✓)        
Mean titre / cm3        

i)
Complete the table.

(2)

ii)
The equation for the reaction between MnO4- ions from the KMnO4 and C2O42− ions from the sodium ethanedioate solution is shown.

16H+(aq) + 2MnO4−(aq) + 5C2O42−(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l)


Use this equation and your mean titre from (e)(i) to calculate the mass, in mg, of KMnO4 in one tablet.

Give your answer to an appropriate number of significant figures.

(5)

iii)
A textbook suggested the conical flask should be heated during the titration, as the reaction between the MnO4− ions and the C2O42− ions is slow.

Use these electrode potentials and your knowledge of homogeneous catalysis to deduce why the heating is very important at the start of the titration, but less important as the titration proceeds. Justify your answer.
You may include equations in your justification.

Electrode system Eθ/ V
2CO2 (g) + 2e− ⇌ C2O42− (aq) +0.64
Mn3+ (aq) + e–  ⇌ Mn2+ (aq) +1.49
MnO4− (aq) + 8H+ (aq) + 5e− ⇌ Mn2+ (aq) + 4H2O (l) +1.51

(4)

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3a1 mark

Yellow gold is used to make jewellery. It is an alloy of copper, gold and silver.
The purity of gold is measured in carats. The higher the carat, the higher the percentage of gold in the alloy. Pure gold is 24 carat.
A sample of yellow gold is analysed using the steps below.

Step 1

Excess concentrated nitric acid is reacted with 1.250 g of the alloy. The gold does not react but the copper and silver do react. The half-equations are

Cu (s) → Cu2+ (aq) + 2e−
Ag (s) → Ag+ (aq) + e−
2HNO3 (aq) + e− → Nbegin mathsize 16px style O subscript 3 to the power of minus sign end style (aq) + NO2 (g) + H2O (I)
Step 2 The mixture is diluted with distilled water and the gold is filtered off.
Step 3

Excess hydrochloric acid is added to the filtrate. It reacts with the silver ions to
form a precipitate of silver chloride.

Ag+ (aq) + Cl− (aq) → AgCl (s)
Step 4 The silver chloride precipitate is filtered off, washed, dried and weighed.
The mass of silver chloride formed is 0.706 g.
Step 5

Excess potassium iodide is added to the remaining solution.
A precipitate of copper(I) iodide and a solution of iodine forms.

2Cu2+ (aq) + 4I− (aq) → 2Cul (s) + I2 (aq)
Step 6

The resulting mixture is titrated with 0.100 mol dm−3 sodium thiosulfate solution.

I2 (aq) + 2S2begin mathsize 16px style O subscript 3 to the power of 2 minus sign end style (aq) → 2I− (aq) + S4straight O subscript 6 superscript 2 minus end superscript (aq)

The titre is 39.40 cm3.

Write the equation for the reaction of copper with concentrated nitric acid, using the half-equations given in Step 1. State symbols are not required.

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3b2 marks

State the indicator used and its colour change at the end-point in the titration in Step 6.

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3c
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The table shows the percentage by mass of gold in four different carats of yellow gold.

Carat Percentage by mass of gold
9 37.5
10 41.7
14 58.3
18 75.0

Determine, using the experimental data, the carat of the sample of yellow gold that was analysed.

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1a3 marks

An electrochemical cell is made from the electrode systems represented by these half-equations.

Cu2+ (aq) + 2e− ⇌ Cu (s)
        Mn3+ (aq) + e− ⇌ Mn2+ (aq)

The E subscript cell superscript ⦵ value is measured using the apparatus shown.

q5-9cho-al-1-june-2018-qp-edexcel-a-level-chem

Complete the diagram by adding labels on the dotted lines provided.

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1b2 marks

A salt bridge is used to connect the two half-cells.

i)
State what chemical is contained in the salt bridge.

(1)

ii)
Give a possible reason why the salt bridge cannot be replaced by an unreactive metal wire.

(1)

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1c
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In this cell, the copper is oxidised and E subscript cell superscript ⦵ = +1.15V.

Cu2+ (aq) + 2e− ⇌ Cu (s)   E to the power of ⦵= +0.34 V
Mn3+ (aq) + e− ⇌ Mn2+ (aq)

i)

Write the overall ionic equation for the reaction taking place.
State symbols are not required.

(1)

ii)
Calculate the value of the standard electrode potential for the Mn3+(aq) Ι Mn2+(aq) half-cell.

(1)

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