Variable oxidation states of transition metals

This question is about the chemistry of copper.

Deduce the number of unpaired electrons in a Cu²⁺ ion.

Describe how water acts as a ligand in the formation of [Cu(H₂O)₆]²⁺

A solution of sodium carbonate is added to a blue solution containing

State what is observed.

Give an equation for the reaction.

Equation

When a solution containing [Cu(H₂O)₆]²⁺ ions reacts with an excess of concentrated hydrochloric acid, a ligand exchange reaction occurs.

Blue Yellow

The shape and the colour of the [CuCl₄]^2– ion are different from the shape and the

Explain

• the shape of each complex ion and why these shapes are different

• how colour arises in complex ions

• why the [CuCl₄]^2– ion has a yellow colour, different from the blue colour of the

An excess of water, followed by an excess of concentrated aqueous ammonia, is added to a solution containing [CuCl₄]^2– ions.

State what is observed.

Give an equation for the overall change that occurs.

Equation

A solution contains M₂[Cu(C₂O₄)₂(H₂O)₂] where M is a metal.

A student does a titration to determine the identity of M.

• 2.52 g of solid M₂[Cu(C₂O₄)₂(H₂O)₂] are dissolved in dilute sulfuric acid.

• This solution is transferred to a volumetric flask and made up to 250 cm³ with dilute sulfuric acid.

• A 25.0 cm³ portion is pipetted into a conical flask.

• The mixture in the conical flask is heated to 60 °C and then titrated with 0.0200 mol dm^–3 KMnO₄ solution.

• Several titrations are done and the student’s results are shown in Table 3.

In this titration, only the C₂O₄^2– ions react with the MnO₄^– ions. 2 mol of MnO₄^– ions react with 5 mol of C₂O₄^2– ions.

Give an equation for the reaction between C₂O₄^2– ions and MnO₄^– ions in acidic solution.

Calculate the mean titre from the student’s results.

Calculate the M_r of M₂[Cu(C₂O₄)₂(H₂O)₂]

Identify M.

Draw the structure of two stereoisomers of the complex ion in M₂[Cu(C₂O₄)₂(H₂O)₂] and include the charge on each structure.

Structure 1
Structure 2

Which electron configuration represents an element that can have a maximum oxidation state of +5 in its compounds?

This question is about vanadium ions.

Table 3 shows some standard electrode potential values.

Use the data in Table 3 to explain why Zn reduces an aqueous solution of VO₂⁺ ions to V²⁺ ions, but does not reduce it any further.

Identify the species in Table 3 that can reduce an aqueous solution of VO₂⁺ to V

Two half-cells Fe²⁺(aq) / Fe(s) and VO²⁺(aq) / V³⁺(aq) are connected.

Calculate the EMF of this cell.

Give the conventional representation for this cell.

Give a half-equation for the reaction that occurs at the negative electrode.

Cell representation

Half-equation

0.151 g of impure NH₄VO₃ is added to dilute sulfuric acid to form a solution containing

All the VO₃^– ions are converted to VO₂⁺ ions.

These VO₂⁺ ions are reduced to aqueous V²⁺ ions by reaction with an excess of zinc.

The excess of zinc is removed by filtration and washed.

The filtrate, containing the V²⁺ ions, is titrated with a 0.0200 mol dm^–3 solution of

29.43 cm³ of KMnO₄ solution are needed to oxidise all the V²⁺ ions to VO₂⁺ ions.

The ionic equation for the reaction of MnO₄^– ions with V²⁺ ions is

Calculate the percentage purity of the NH₄VO₃ Give your answer to 3 significant figures.

Percentage purity

Which compound can decolourise acidified potassium manganate(VII) solution?

This question is about hydrogen peroxide, H₂O₂

The half-equation for the oxidation of hydrogen peroxide is

Hair bleach solution contains hydrogen peroxide.

A sample of hair bleach solution is diluted with water. The concentration of hydrogen peroxide in the diluted solution is 5.00% of that in the original solution. A 25.0 cm³ sample of the diluted hair bleach solution is acidified with dilute sulfuric acid. This acidified sample is titrated with 0.0200 mol dm⁻³ potassium manganate(VII) solution. The reaction is complete when 35.85 cm³ of the potassium manganate(VII) solution are added.

Give an ionic equation for the reaction between potassium manganate(VII) and acidified hydrogen peroxide.

Calculate the concentration, in mol dm^–3, of hydrogen peroxide in the original hair bleach solution.

(If you were unable to write an equation for the reaction you may assume that the mole ratio of potassium manganate(VII) to hydrogen peroxide is 3:4 This is not the correct mole ratio.)

State why an indicator is not added in this titration.

Give the oxidation state of oxygen in hydrogen peroxide.

Hydrogen peroxide decomposes to form water and oxygen.

Give an equation for this reaction.

Calculate the amount, in moles, of hydrogen peroxide that would be needed to produce 185 cm³ of oxygen gas at 100 kPa and 298 K

The gas constant, R = 8.31 J K⁻¹ mol⁻¹

Equation

Hydrazine (N₂H₄) is used as a rocket fuel that is oxidised by hydrogen peroxide. The equation for this reaction in the gas phase is

The enthalpy change for this reaction, ∆ H = – 789 kJ mol

Table 3 shows some mean bond enthalpy values.

Define the term mean bond enthalpy.

Use the equation and the data in Table 3 to calculate a value for the O–O bond enthalpy in hydrogen peroxide.

The reaction between vanadium(IV) ions and manganate(VII) ions in acidic solution can be represented by the equation

What volume, in dm³, of 0.020 mol dm³ KMnO₄ is needed to oxidise 0.10 mol of vanadium(IV) ions completely?

This question is about electrode potentials and electrochemical cells.

State the meaning of the term electrochemical series.

Table 8 shows some electrode potentials.

State two conditions needed for the following half-cell to have = 0.00 V

Identify the weakest reducing agent in Table 8.

Use half-equations from Table 8 to deduce an equation for the reduction of VO₂⁺ to form VO²⁺ in aqueous solution by iron.

Use data from Table 8 to explain why [Co(H₂O)₆]³⁺(aq) will undergo a redox reaction

Give an equation for this reaction.

Equation

Suggest why the two cobalt(III) complex ions in Table 8 have different electrode potentials.

This question is about ethanedioic acid (HOOCCOOH) and the ethanedioate ion (⁻OOCCOO⁻).

Ethanedioic acid reacts with propane-1,3-diol (HOCH₂CH₂CH₂OH) to form a polyester.

Draw the repeating unit of this polyester.

Explain why polyesters are biodegradable but polyalkenes are not biodegradable.

Sodium ethanedioate is used to find the concentration of solutions of potassium manganate(VII) by titration. The equation for this reaction is

A standard solution is made by dissolving 162 mg of Na₂C₂O₄ (M_r = 134.0) in water and making up to 250 cm³ in a volumetric flask.

25.0 cm³ of this solution and an excess of sulfuric acid are added to a conical flask. The mixture is warmed and titrated with potassium manganate(VII) solution. The titration is repeated until concordant results are obtained.

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

Figure 1 shows the 25.0 cm³ pipette used to measure the sodium ethanedioate solution.

On Figure 1, draw the meniscus of the solution when the pipette is ready to transfer 25.0 cm³ of the sodium ethanedioate solution.

Potassium manganate(VII) is oxidising and harmful. Sodium ethanedioate is toxic.

Suggest safety precautions, other than eye protection, that should be taken when:

• filling the burette with potassium manganate(VII) solution

• dissolving the solid sodium ethanedioate in water.

Filling the burette

Dissolving the solid

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

Figure 2 shows the burette containing potassium manganate(VII) solution.

Give two practical steps needed before recording the initial burette reading.

When Na₂C₂O₄(aq) is added to a solution containing [Fe(H₂O)₆]³⁺ ions, a reaction occurs in which all six water ligands are replaced by ethanedioate ions.

Explain why the replacement of the water ligands by ethanedioate ions is favourable. In your answer refer to:

• the enthalpy and entropy changes for the reaction

• how the enthalpy and entropy changes influence the free-energy change for the reaction.

A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaC l O, in a sample of bleach solution.

Method:

• Dilute a 10.0 cm³ sample of bleach solution to 100 cm³ with distilled water.

• Transfer 25.0 cm³ of the diluted bleach solution to a conical flask and acidify using sulfuric acid.

• Add excess potassium iodide to the conical flask to form a brown solution

• Add 0.100 mol dm^–3 sodium thiosulfate solution (Na₂S₂O₃) to the conical flask from a burette until the brown solution containing l₂(aq) becomes a colourless solution

The student uses 33.50 cm³ of sodium thiosulfate solution.

The density of the original bleach solution is 1.20 g cm^–3

The equations for the reactions in this experiment are

Use all the information given to calculate the percentage by mass of NaC l O in the original bleach solution.

Give your answer to 3 significant figures.

The total uncertainty from two readings and an end point error in using a

What is the total percentage uncertainty in using the burette in this experiment?

Tick (✓) one box.

Which compound decolourises acidified potassium manganate(VII) solution?

What is the minimum volume, in cm³, of 0.02 mol dm^–3 KMnO₄ solution needed to

A student does an experiment to determine the percentage of copper in an alloy.

The student

• reacts 985 mg of the alloy with concentrated nitric acid to form a solution (all of the copper in the alloy reacts to form aqueous copper(II) ions)

• pours the solution into a volumetric flask and makes the volume up to 250 cm³ with distilled water

• shakes the flask thoroughly

• transfers 25.0 cm³ of the solution into a conical flask and adds an excess of potassium iodide

• uses exactly 9.00 cm³ of 0.0800 mol dm^–3 sodium thiosulfate (Na₂S₂O₃) solution to react with all the iodine produced.

The equations for the reactions are

Calculate the percentage of copper by mass in the alloy.

Give your answer to the appropriate number of significant figures.

Suggest two ways that the student could reduce the percentage uncertainty in the measurement of the volume of sodium thiosulfate solution, using the same apparatus as this experiment.

State the role of iodine in the reaction with sodium thiosulfate.

Give the full electron configuration of a copper(II) ion.

Copper(I) iodide is a white solid.

Explain why copper(I) iodide is white.

Iodine vaporises easily.

Calculate the volume, in cm³, that 5.00 g of iodine vapour occupies at 185 °C and 100 kPa

The gas constant R = 8.31 J K^–1 mol^–1

Give your answer to 3 significant figures.

The percentage by mass of iron in a steel wire is determined by a student.

The student

• reacts 680 mg of the wire with an excess of sulfuric acid, so that all of the iron in the

Give the equation for the reaction between iron and sulfuric acid.

The titration results are shown in Table 3.

Calculate the mean titre.

Give the overall ionic equation for the oxidation of Fe² by manganate(VII) ions, in acidic conditions.

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

Name the piece of apparatus used for these stages of the method.

Adding the potassium manganate(VII) solution

The balance used to weigh the 680 mg of iron wire has an uncertainty of ±0.005 g

A container was weighed and its mass was subtracted from the total mass of the container and wire.

Calculate the percentage uncertainty in using the balance.