Redox titrations

This question is about reactions of compounds of iron and copper.

Complete the electron configuration of

an Fe atom: 1s²........

a Cu²⁺ ion: 1s²........

Solutions of iron(II) sulfate, FeSO₄, and copper(II) sulfate, CuSO₄, can be distinguished by adding aqueous sodium hydroxide.

Describe what would be observed when NaOH(aq) is added to each solution.

Observation with FeSO₄(aq)........

Observation with CuSO₄(aq)........

The cyclic organic compound pyridine, C₅H₅N, can act as a monodentate ligand.

The structure of pyridine is shown below.

What is meant by a monodentate ligand ?

Aqueous copper(II) sulfate reacts with salicylic acid to form a complex ion which has two stereoisomers.

One isomer of the complex ion is shown:

Draw the other stereoisomer of the complex.

Identify the type of stereoisomerism shown by the complexes.

Type of stereoisomerism........

Calculate the percentage by mass of CuSO₄•5H₂O in Mixture A.

Give your answer to 3 significant figures.

Percentage of CuSO₄•5H₂O in Mixture A =........ %

The student repeats their method, but after carrying out Step 1 they leave the solution exposed to the air for a few days. Some of the iron(II) sulfate is oxidised to iron(III) sulfate.

The student then continues with Step 2, Step 3 and Step 4 of the method.

This mean titre is greater than 31.15 cm³.

Suggest why this mean titre is greater than 31.15 cm³.

This question is about redox reactions and electrode potentials.

The flowchart shows some reactions of barium and its compounds.

• Write balanced equations for Reaction 1 and Reaction 2.

• Identify the type of reaction in Reaction 3.

Reaction 1: equation........

Reaction 2: equation........

Reaction 3: type of reaction........

Potassium iodate tablets prevent the uptake of radioactive iodine in the human body following a nuclear accident.

The mass of potassium iodate(V), KIO₃, in a tablet can be determined by reaction with an aqueous solution of potassium iodide, K I, in the presence of acid.

A chemist finds that two KIO₃ tablets react with exactly 26.2 cm³ 0.150 mol dm^–3 KI.

Calculate the mass, in mg, of KIO₃ in one tablet.

Give your answer to the nearest whole number.

Standard electrode potentials are measured by comparison with a reference half-cell.

Draw a labelled diagram to show how the standard electrode potential could be measured for the redox system below.

Include details of the apparatus, solutions and the standard conditions needed when measuring this standard electrode potential.

Standard conditions........

Many electric vehicles are powered by lithium-ion cells.

Hydrogen-oxygen fuel cells can also be used to power vehicles.

Six redox systems are shown in the table. State symbols have been omitted.

Redox system	Half-equation	E / V
1	Li+ + e– Li	–3.04
2	2H2O + 2e– H2 + 2OH–	–0.83
3	2H+ + 2e– H2	0.00
4	O2 + 2H2O + 4e– 4OH–	+0.40
5	Li+ + CoO2 + e– LiCoO2	+1.16
6	O2 + 4H+ + 4e– 2H2O	+1.23

A lithium-ion cell involves redox systems 1 and 5.

Construct the overall cell equation for a lithium-ion cell.

Hydrogen-oxygen fuel cells can operate in acidic or in alkaline conditions.

Show that for acidic and alkaline hydrogen–oxygen fuel cells, the standard cell potentials, and the overall cell equations, are the same.

Acidic........

Alkaline........

This question is about compounds and ions of iron(II) and iron(III) that contain ethanedioate ions, C₂O₄^2–.

The C₂O₄^2– ion, shown below, is an example of a bidentate ligand.

Explain what is meant by the term bidentate ligand.

A complex ion E contains three C₂O₄^2– ions bonded to an iron(III) ion in an octahedral shape.

Complex ion E exists as a mixture of two optical isomers.

Draw 3D diagrams to show the structures of the optical isomers of E.

Include any overall charge.

A student plans an investigation to find the number of waters of crystallisation, x, in a sample of hydrated iron(II) ethanedioate, FeC₂O₄•xH₂O.

The student decides to carry out a redox titration between solutions of iron(II) ethanedioate and potassium manganate(VII) in acidic conditions.

In the titration, both iron(II) ions and ethanedioate, C₂O₄^2–, ions are oxidised.

Construct half-equations for the oxidation of iron(II) and ethanedioate ions.

Oxidation of iron(II) ions

Oxidation of ethanedioate ions

The uncertainty in each burette reading is ±0.05 cm³.

Calculate the percentage uncertainty for the titre in Titration 1.

* In the titration, 5 mol of iron(II) ethanedioate reacts with 3 mol of manganate(VII) ions.

Analyse the student’s results to find the number of waters of crystallisation, x, in the hydrated

iron(II) ethanedioate, FeC₂O₄•xH₂O.

Some grass fertilisers contain compounds of iron.

During heavy rain, a fertiliser is washed into a nearby river causing the water to be polluted with a mixture of iron(II) and iron(III) ions.

A student determines the concentration of iron(II) ions in a sample of river water by titration with potassium manganate(VII).

25.0 cm³ portions of river water are acidified with dilute sulfuric acid. Each portion is titrated with 0.00250 mol dm –3 potassium manganate(VII) until a colour change is seen.

State the colour change seen at the end point of the titration.

The student’s titration results are shown in the table below. The trial titre has been omitted.

Complete the table above and calculate the mean titre that the student should use to determine the concentration of iron(II) ions in the river water.

Determine the concentration, in mol dm –3, of iron(II) ions in the river water.

concentration =........ mol dm^–3

Hydrogen peroxide, H₂O₂, can be oxidised by manganate(VII) ions under acid conditions as shown below.

In a titration, 25.00 cm³ of a disinfectant containing hydrogen peroxide reacts with 22.00 cm³ of 0.125 mol dm^–3 KMnO₄(aq).

What is the concentration of H₂O₂, in mol dm^–3, in the disinfectant? Assume that KMnO₄ only reacts with H₂O₂ in the disinfectant.

A 0.0440

B 0.110

C 0.275

D 0.550

Your answer

A student carries out an investigation on vitamin C, C₆H₈O₆.

Vitamin C is extremely soluble in water. This means that vitamin C is removed rapidly from the body. ‘Vitamin C ester’ is available in tablet form as a less soluble source of vitamin C which stays in the body for longer.

Suggest why vitamin C is extremely soluble in water.

A ‘vitamin C ester’ tablet contains an ester with the molecular formula C₂₂H₃₈O₇.

This ester can be prepared by reacting vitamin C with a long chain carboxylic acid, C_xH_yCOOH, in the presence of an acid catalyst.

Vitamin C and the long chain carboxylic acid react in a 1:1 molar ratio.

Determine x and y in the formula of this carboxylic acid.

Vitamin C, C₆H₈O₆, is a weak acid (K_a = 7.94 x 10^–5 (mol dm^–3)), which is often referred to as ascorbic acid.

The label on a carton of orange juice lists the mass of vitamin C, in mg, in a typical serving of 150 cm³.

The student carries out an investigation to check the vitamin C content in the orange juice.

Vitamin C can be oxidised by iodine:

The student dilutes 150 cm³ of the orange juice with water to 250.0 cm³ in a volumetric flask.

The student then titrates 25.0 cm³ volume of this solution with 9.60 × 10^–4 mol dm^–3 iodine solution, I₂(aq).

The mean titre of I₂(aq) is 22.50 cm³.

Determine the mass, in mg, of vitamin C in a 150 cm³ serving of the orange juice.

In Step 1, the student observed that bubbles of gas were produced.

Suggest the formula of the copper(II) compound which reacted with HNO₃ to form the gas, and write a full equation for the reaction.

Formula:........

Equation:........

Write an ionic equation, including state symbols, for the reaction in Step 3.

Suggest a suitable indicator for this titration and state the colour change at the end point in Step 4.

Indicator:........

Colour from........ to........

Determine the percentage, by mass, of copper in the copper ore. Give your answer to an appropriate number of significant figures.

Explain whether the calculated percentage by mass of copper would be higher, lower or the same if the following changes were made to the method.

The potassium iodide was not in excess, in Step 3.

The burette readings were read from the top of the meniscus, in Step 4.

The student then modifies the method in order to obtain a more accurate value for the percentage by mass of copper in the ore. The student decides to use 25.00 g of the copper ore in Step 1.

What further modifications should the student make to produce a more accurate value for the percentage by mass of copper in the ore?

This question is about reactions of ions and compounds of transition elements.

A student carries out two experiments on a solution containing [Cr(H₂O)₆]³⁺(aq).

Experiment 1 The student adds an excess of aqueous ammonia to a solution containing [Cr(H₂O)₆]³⁺ until a purple solution is formed.

Experiment 2 The student carries out the following reaction sequence.

Step 1 NaOH(aq) is added slowly to a solution containing [Cr(H₂O)₆]³⁺(aq) in a boiling tube. A grey–green precipitate forms.

Step 2 An excess of NaOH(aq) is added to the boiling tube. The precipitate dissolves and a green solution forms containing a 6 coordinate complex ion.

Step 3 H₂O₂ is added to the mixture and the boiling tube is heated. A yellow solution forms.

Step 4 The solution in the boiling tube is acidified. The solution now contains Cr₂O₇²⁻(aq).

What is the formula of the complex ion in the purple solution that forms in Experiment 1

Suggest an equation for the reaction in Experiment 2, Step 1. Include state symbols.

Draw a 3-D diagram for the shape of the complex ion that forms in Experiment 2, Step 2 Include the charge of the ion.

What is the formula of the ion that causes the yellow colour in Experiment 2, Step 3

State the colour of the solution that forms in Experiment 2, Step 4.

Vanadium ions have four common oxidation states. Table 18.1 shows the colours of the ions in aqueous solution.

Complete the electron configuration of a V³⁺ ion.

The student adds excess iron to a solution containing VO²⁺(aq) ions, and observes that the colour of the solution changes from blue to green and then to violet.

Use the relevant standard electrode potentials shown in Table 18.2 to explain these observations.

Construct an equation for the first colour change from blue to green.

Iron(II) gluconate, C₁₂H₂₂FeO₁₄, is the active ingredient in some brands of iron supplements.

A student carries out an experiment to determine the mass of iron(II) gluconate in one tablet of an iron supplement, using the method below.

Stage 1 The student crushes two tablets and dissolves the powdered tablets in dilute sulfuric acid.

Stage 2 The student makes up the solution from Stage 1 to 250.0 cm³ in a volumetric flask.

Stage 3 The student then titrates 25.0 cm³ portions of the solution obtained in Stage 2 with 0.00200 mol dm⁻³ potassium manganate(VII).

The student obtains a mean titre of 13.50 cm³.

In this titration, 1 mol of manganate(VII) ions reacts with 5 mol of iron(II) ions.

Explain why the student used 0.00200 mol dm⁻³ potassium manganate(VII) solution for this titration, rather than the more usual concentration of 0.0200 mol dm⁻³ used in manganate(VII) titrations.

Use the student’s results to determine the mass, in mg, of iron(II) gluconate in one tablet.

Give your answer to 3 significant figures.

This question is about the halogen group of elements and some of their compounds.

The halogens show trends in their properties down the group.

The boiling points of three halogens are shown below.

Explain why the halogens show this trend in boiling points.

Hydrogen iodide, H I, is decomposed by heat into its elements:

The decomposition is much faster in the presence of a platinum catalyst.

Complete the enthalpy profile diagram for this reaction using formulae for the reactants and products.

• Use E_a to label the activation energy without a catalyst.

• Use E_c to label the activation energy with a catalyst.

• Use H to label the enthalpy change of reaction.

Compound A is an oxide of chlorine that is a liquid at room temperature and pressure and has a boiling point of 83 °C.

When 0.4485 g of A is heated to 100 °C at 1.00 × 10⁵ Pa, 76.0 cm³ of gas is produced.

Determine the molecular formula of compound A.

Show all your working.

Complete Table 20.1 and calculate the mean titre that the student should use for analysing the results.

The uncertainty in each burette reading is ±0.05 cm³.

Calculate the percentage uncertainty in the titre obtained from titration 1.

Give your answer to two decimal places.

Describe and explain how the student should determine the end point of this titration accurately.

Determine the relative formula mass and formula of the Group 1 iodate(V), B.

Show your working.

Ethanedioic acid, (COOH)₂, is present in rhubarb leaves.

A student carries out a redox titration using aqueous cerium(IV) sulfate, Ce(SO₄)₂(aq), to determine the percentage, by mass, of ethanedioic acid in rhubarb leaves.

In the titration, Ce⁴⁺(aq) ions oxidise ethanedioic acid in hot acid conditions:

Ce⁴⁺(aq) ions have a yellow colour. Ce³⁺(aq) ions are colourless.

The student weighs 82.68 g of rhubarb leaves and extracts ethanedioic acid from the leaves.

The ethanedioic acid is added to dilute sulfuric acid to form a colourless solution which is made up to 250.0 cm³ with distilled water.

The student heats 25.00 cm³ of this solution to 70 °C and titrates this volume with 0.0500 mol dm^–3 Ce(SO₄)₂ from the burette.

The student repeats the titration to obtain concordant (consistent) titres.

Titration results The trial titre has been omitted.

This titration is self-indicating and the student does not need to add an indicator.

What colour change would the student observe at the end point?

Colour change from........ to........

Calculate the percentage, by mass, of ethanedioic acid in the rhubarb leaves.

Give your answer to an appropriate number of significant figures.