Oxidation states

This question is about the halogens and some of their compounds.

Explain why chlorine has a lower boiling point than bromine.

Bromine reacts with fluorine to form BrF₃

Determine the oxidation state of bromine and of fluorine in BrF₃

Draw the shape of a BrF₃ molecule.

Include any lone pairs of electrons that influence the shape.

Explain why the BrF₃ molecule has the shape you have drawn.

Shape

Explanation of shape

Solid sodium bromide reacts with concentrated sulfuric acid.

Two reactions occur

• an acid-base reaction

• a redox reaction.

Give an equation and an observation for each reaction.

Equation for acid-base reaction

Observation for acid-base reaction

Equation for redox reaction

Observation for redox reaction

Iodide ions reduce copper(II) ions in aqueous solution to form a precipitate of copper(I) iodide.

Give an equation for this redox reaction.

This question is about vanadium and some of its compounds.

State the block in the Periodic Table that contains vanadium.

Suggest why vanadium can be pulled into thin wires.

Vanadium(V) oxide is a catalyst in the Contact process.

Give two equations to show how vanadium(V) oxide acts as a catalyst in this process.

Equation 1

Equation 2

Table 6 shows some electrode half-equations and their standard electrode potentials.

Sn reduces VO₂⁺ in acidic solution.

Use the data in Table 6 to deduce an overall equation for the reaction that occurs.

Vanadium(III) sulfate is a pale yellow solid at room temperature.

Explain why vanadium(III) sulfate has a high melting point.

When vanadium(III) sulfate dissolves in water, the vanadium-containing species formed is acidic.

Give an equation to show how vanadium(III) sulfate dissolves in water to form the acidic vanadium-containing species.

Explain why the vanadium-containing species is acidic.

Equation

Compound Q is found in crude oil and has the molecular formula C₃₃H₃₄N₄VO₄

A sample of Q is ionised by electrospray ionisation in a time of flight (TOF) mass spectrometer. The spectrum produced shows that the

Give an equation to show how Q is ionised using electrospray ionisation.

Determine the m / z value of the molecular ion formed in this ionisation process.

Equation

A sample of vanadium contains only⁴⁹V and⁵¹V

The relative atomic mass of vanadium in this sample is 50.97

Calculate the percentage abundance of⁵¹V in this sample.

Hexanedioic acid is an important compound used in organic synthesis.

Hexanedioic acid can be prepared from cyclohexanol.

The equation for the reaction is

C₆H₁₁OH(l) + 2 HNO₃(aq) HOOC(CH₂)₄COOH(s) + N₂O(g) + 2 H₂O(l)

Method

Step 1 Add 5 cm³ of concentrated nitric acid to a boiling tube.

Step 2 Place the boiling tube in a beaker of water at 85 °C

Step 3 Add 2 cm³ of cyclohexanol dropwise to the boiling tube.

Step 4 Leave the reaction mixture for 10 minutes.

Step 5 Cool the boiling tube in a beaker of cold water for a further 10 minutes.

Step 6 Filter the contents of the boiling tube and collect the impure solid.

Step 7 Recrystallise the impure solid using methanol.

Step 8 Wash the purified crystalline product with some ice-cold distilled water, and dry.

Step 9 Measure the melting point of the hexanedioic acid.

Suggest why the apparatus should be set up in a fume cupboard.

Deduce the role of the concentrated nitric acid in the reaction.

Outline how Steps 7 and 8 should be done to obtain the purified crystalline product.

In Step 8, the distilled water must be ice-cold.

Suggest why using water at room temperature lowers the recorded yield of product.

Figure 4 shows how a student set up the apparatus for measuring the melting point of hexanedioc acid in Step 9 (data book value 152 °C).

The apparatus is supported by clamps but these are not shown.

Identify two mistakes the student made in setting up this apparatus.

State the problem caused by each mistake.

Figure 5 shows an alternative process to make hexanedioic acid.

Scientists disagree about whether this alternative process is more sustainable or less sustainable than the process starting from cyclohexanol.

Use the information given in Figure 5 to provide a justification for each point of view.

A student does an experiment to measure the standard electrode potential of an

Figure 1 shows the apparatus the student uses.

Figure 1 shows three mistakes made by the student.

Identify each mistake and the change needed to correct it.

What feature of the voltmeter ensures that the voltmeter measures the cell EMF?

Tick (✓) one box.

The Fe³⁺/Fe²⁺ electrode is the positive electrode when the cell is set up correctly at 298 K

Give the redox equation for the overall reaction that occurs in the cell when the voltmeter is replaced with a lamp.

Table 4 shows some standard electrode potentials.

Finely powdered iron filings are added to a solution of copper(II) sulfate and the mixture is stirred.

The solution gradually changes from blue to pale green and a brown solid forms.

Use the data in Table 4 to explain these observations.

Give an ionic equation for the reaction that occurs.

Give the conventional representation of the cell that has the same overall reaction as in your equation.

Calculate the EMF, in volts, of this cell.

Equation

Conventional representation

This question is about chlorine.

Give an equation to show how chlorine forms an acidic solution in water.

Give an equation for the reaction between chlorine and cold, dilute aqueous sodium hydroxide.

In acidic conditions, ClO₃^– ions oxidise Cl^– ions to form Cl₂

Deduce a half-equation for the oxidation of Cl^– to Cl₂

Deduce a half-equation for the reduction of ClO₃^– to Cl₂

Deduce the overall equation for this reaction.

Half-equation for the oxidation of Cl^– to Cl₂

Half-equation for the reduction of ClO₃^– to Cl₂

Overall equation

Give the equation for the reaction of solid sodium chloride with concentrated sulfuric acid.

State the role of the chloride ions in this reaction.

Equation

Draw the shape of the Cl₃^– ion. Include any lone pairs of electrons that influence the shape.

Chlorine forms an ion with the Group 3 element thallium (T l).

State and explain the bond angle in TlCl₂⁺

This question is about complexes of the transition metal chromium.

State the meaning of the term transition metal complex.

Cr(PF₃)₆ is a complex of chromium that contains molecules of PF₃

The electron pair repulsion theory can be used to predict the shape of a PF₃ molecule.

Draw the shape of a PF₃ molecule. Include any lone pairs of electrons that influence the shape.

Name the shape.

Shape

Suggest why the oxidation state of chromium is zero in Cr(PF₃)₆

The compound [Cr(NH₃)₄Cl₂]Cl contains ammonia molecules.

Deduce the oxidation state of chromium in [Cr(NH₃)₄Cl₂]Cl

Name the type of bond between N and H in ammonia.

The compound [Cr(NH₃)₄Cl₂]Cl contains a complex ion that shows isomerism.

Draw the two isomers of the complex ion.

State the type of isomerism shown.

Isomer 1
Isomer 2

Type of isomerism

Complete the equation to show the formation of one complex that contains chromium in its +3 oxidation state.

This question is about fuel cells.

In a methanol–oxygen fuel cell, the overall reaction is

At the positive electrode, oxygen reacts with hydrogen ions to form water.

Give a half-equation for this reaction.

At the negative electrode, methanol reacts with water to produce carbon dioxide and hydrogen ions.

Give a half-equation for this reaction.

The standard electrode potential for the CO₂ / CH₃OH electrode is +0.03 V

Calculate the standard electrode potential for the O₂ / H₂O electrode.

State why a fuel cell does not need to be electrically recharged.

Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for use in cars.

In which oxide is the named element in its highest oxidation state?

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.

This question is about cells.

The half-equations for two electrodes that combine to make a non-rechargeable cell are

Identify the oxidising agent in this cell.

Figure 1 shows a cross-section through a rechargeable silver–zinc cell.

Suggest the function of the porous separator in Figure 1.

The standard electrode potentials for two half-equations for the silver–zinc cell are

Give an equation for the overall reaction that occurs when the cell is recharging.

The EMF of an alkaline hydrogen–oxygen fuel cell is +1.23 V The standard electrode potential for one of the electrodes in the alkaline hydrogen–oxygen fuel cell is

Give the half-equation for the other electrode and calculate its standard electrode potential.

Equation

Suggest why the EMF values of the acidic and alkaline hydrogen–oxygen fuel cells are the same.

Which of these oxidation states is correct?

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 the development of lithium cells. The value of for lithium suggests that a lithium cell could have a large EMF.

Table 9 shows some electrode potential data.

Use data in Table 9 to explain why an aqueous electrolyte is not used for a lithium cell.

In the 1970s lithium-iodine cells became a common power source for

Use the data in Table 9 to calculate the cell EMF of a standard lithium-iodine cell.

An EMF value for a commercial lithium-iodine cell is 2.80 V

Suggest why this value is different from the value calculated in Question 09.2.

In some lithium cells, lithium perchlorate (LiClO₄) is used as the electrolyte.

Deduce the oxidation state of chlorine in LiClO₄

In other lithium cells, lithium cobalt oxide electrodes and lithium electrodes are used.

Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode.

Give an equation for the reaction that occurs at the negative lithium electrode.

Which equation does not show the reduction of a transition metal?

This question is about a glucose–oxygen fuel cell.

When the cell operates, the glucose (C₆H₁₂O₆) molecules react with water at the negative electrode to form carbon dioxide and hydrogen ions.

Oxygen gas reacts with hydrogen ions to form water at the positive electrode.

Deduce the half-equation for the reaction at the negative electrode.

Deduce the half-equation for the reaction at the positive electrode.

Give the equation for the overall reaction that occurs in the Glucose–oxygen fuel cell.

The negative electrode is made of carbon and the positive electrode is made of platinum.

Give the conventional representation for the glucose–oxygen fuel cell.

State what must be done to maintain the EMF of this fuel cell when in use.

This question is about emissions of oxides of nitrogen from petrol and diesel engines.

Explain how oxides of nitrogen are formed in engines.

State why it is desirable to decrease emissions of oxides of nitrogen from vehicles.

Modern diesel vehicles use diesel exhaust fluids, such as AdBlue, to decrease emissions of oxides of nitrogen.

AdBlue reacts with water in the hot exhaust gases to form ammonia. In the presence of a catalyst the ammonia reacts with oxides of nitrogen to form nitrogen and water.

Give the oxidation state of nitrogen in each of NO₂, NH₃ and N₂

Complete the equation for the reaction between NO₂ and NH₃

Oxidation state of nitrogen in

Equation

Petrol vehicles have a catalytic converter which decreases emissions of oxides of nitrogen. Platinum in the catalytic converter acts as a heterogeneous catalyst.

State the meaning of the term heterogeneous catalyst.

Some carbon particulates are also formed in both diesel and petrol vehicles.

Explain why carbon particulates are formed.

Standard electrode potentials are measured by comparison with the standard hydrogen electrode.

State the substances and conditions needed in a standard hydrogen electrode.

It is difficult to ensure consistency with the setup of a standard hydrogen electrode. A Cu²⁺(aq) /Cu(s) electrode ( = +0.34 V) can be used as a secondary standard.

A student does an experiment to measure the standard electrode potential for the TiO²⁺(aq)/Ti(s) electrode using the Cu²⁺(aq)/ Cu(s) electrode as a secondary standard.

A suitable solution containing the acidified TiO²⁺(aq) ion is formed when titanium(IV) oxysulfate (TiOSO₄) is dissolved in 0.50 mol dm³ sulfuric acid to make 50 cm³ of solution.

Describe an experiment the student does to show that the standard electrode potential for the TiO²⁺(aq) /Ti(s) electrode is 0.88 V

The student is provided with:

• the Cu²⁺(aq)/ Cu(s) electrode set up ready to use

• solid titanium(IV) oxysulfate (M_r = 159.9)

• 0.50 mol dm³ sulfuric acid

• a strip of titanium

• laboratory apparatus and chemicals.

Your answer should include details of:

• how to prepare the solution of acidified TiO²⁺(aq)

• how to connect the electrodes

• measurements taken

• how the measurements should be used to calculate the standard electrode potential for the TiO²⁺(aq)/Ti(s) electrode.

Give the half-equation for the electrode reaction in the TiO²⁺(aq)/Ti(s) electrode in acidic conditions.

Table 2 shows some electrode potential data.

Use the data in Table 2 to explain why copper does not react with most acids but does react with nitric acid.

Give an equation for the reaction between copper and nitric acid.

Equation

In which conversion is the metal reduced?

Which shows the correct oxidation state and co-ordination number of cobalt in

This question is about some Group 7 compounds.

Solid sodium chloride reacts with concentrated sulfuric acid.

Give an equation for this reaction. State the role of the sulfuric acid in this reaction.

Equation

Fumes of sulfur dioxide are formed when sodium bromide reacts with concentrated sulfuric acid.

For this reaction

• give an equation

• give one other observation

• state the role of the sulfuric acid.

Equation

Chlorine reacts with hot aqueous sodium hydroxide as shown in the equation.

3Cl₂ + 6NaOH → NaClO₃ + 5NaCl + 3H₂O

Give the oxidation state of chlorine in NaClO₃ and in NaCl

State, in terms of redox, what happens to chlorine in the reaction in Question 05.3.

Solution Y contains two different negative ions.

To a sample of solution Y in a test tube a student adds

• silver nitrate solution

• then an excess of dilute nitric acid

• finally an excess of concentrated ammonia solution.

The observations after each addition are recorded in Table 3.

Give an equation to show the conversion of D into G.

Ionic equation to form E

Equation to show the conversion of D into G

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 equation for the reaction between ammonia and oxygen is shown.

Some standard entropies are given in Table 3.

Calculate the entropy change for the reaction between ammonia and oxygen.

Calculate a value for the Gibbs free-energy change (ΔG), in kJ mol^–1, for the reaction between ammonia and oxygen at 600 °C

(If you were unable to obtain an answer to Question 03.1, you should assume that the entropy change is 211 J K^–1 mol^–1. This is not the correct answer.)

The reaction between ammonia and oxygen was carried out at a higher temperature.

Explain how this change affects the value of Δ G for the reaction.

Platinum acts as a heterogeneous catalyst in the reaction between ammonia and oxygen. It provides an alternative reaction route with a lower activation energy.

Deduce the change in oxidation state of nitrogen, when NH₃ is oxidised to NO

When ammonia reacts with oxygen, nitrous oxide (N₂O) can be produced instead of NO

Give an equation for this reaction.

This question is about vanadium compounds and ions.

Use data from Table 4 to identify the species that can be used to reduce VO₂⁺ ions to VO²⁺ in aqueous solution and no further. Explain your answer.

Give the oxidation state of vanadium in [VO(H₂O)₅]²⁺

The [V(H₂O)₄Cl₂]⁺ ion exists as two isomers. One isomer is shown.

Draw the structure of the other isomer and state the type of isomerism.

Type of isomerism

Heating NH₄VO₃ produces vanadium(V) oxide, water and one other product.

Give an equation for the reaction.

Vanadium(V) oxide is the catalyst used in the manufacture of sulfur trioxide.

Give two equations to show how the catalyst is used and regenerated.