Electrode potentials & electrochemical cells

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.

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 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

A cell with EMF = +2.15 V is made from two electrodes.

The half-equations for the two electrodes are shown.

positive electrode:

PbO₂(s) + 3 H⁺(aq) + HSO₄^–(aq) + 2 e^– PbSO₄(s) + 2 H₂O(l)

negative electrode:

What is the standard electrode potential of the PbO₂ / PbSO₄ electrode?

Figure 1 shows a cell used to measure the standard electrode potential for the half-cell

State the purpose of the salt bridge.

Identify an ionic compound that could be used in the salt bridge.

State how, if at all, the EMF of this cell will change if the surface area of the platinum electrode is increased.

The standard electrode potential, for the half-cell is shown.

Water is added to the beaker containing the magnesium chloride solution.

What is the effect on the magnitude of the EMF of the cell?

Tick (✓) one box.

The voltmeter V shown in Figure 1 is replaced by a bulb.

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

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.

This question is about catalysis.

Zeolites are used as heterogeneous catalysts in the catalytic cracking of alkanes.

Tetradecane (C₁₄H₃₀) can be cracked to form octane and a cycloalkane.

Give an equation for this reaction.

State the meaning of the term heterogeneous.

Equation

A student determines the concentration of ethanedioate ions in an acidified solution by titration with potassium manganate(VII) solution.

The mixture is warmed before the addition of potassium manganate(VII) solution because the reaction is slow at first. When more potassium manganate(VII) solution is added, the mixture goes colourless quickly due to the presence of an autocatalyst.

Explain the meaning of the term autocatalyst.

Explain, using equations where appropriate, why the reaction is slow at first and then goes quickly.

The reaction between peroxodisulfate ions and iodide ions in aqueous solution can be

Table 6 gives relevant standard electrode potentials.

Use the electrode potential data to suggest how Co²⁺ catalyses the reaction.

Which change to a hydrogen electrode has no effect on the electrode potential?

Some electrode potential data are shown.

Which is a correct statement about this cell?

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.

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.

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

Figure 1 represents the cell used to measure the standard electrode potential for the

Name the piece of apparatus labelled A.

State the purpose of A.

Name the substance used as electrode B in Figure 1.

Complete Table 1 to identify C, D and E from Figure 1. Include the essential conditions for each.

The standard electrode potential, , for the Fe³/Fe² electrode is 0.77 V

Give the ionic equation for the overall reaction in the cell in Figure 1.

State the change that needs to be made to the apparatus in Figure 1 to allow the cell reaction to go to completion.

A student sets up a cell as shown in the cell representation.

The student measures the cell EMF, E_cell, with several different concentrations of

The results are shown in Table 2.

Complete Table 2 to show the value missing from experiment 4.

Plot a graph of E_cell against ln ([Zn²]/[Cu²]) on the grid.

This equation shows how E_cell varies with concentration for this reaction.

This equation is in the form of the equation for a straight line, y = mx + c

Calculate the gradient of your plotted line on the graph in question 03.6. You must show your working.

Use your gradient to calculate the temperature, T, at which the measurements of E_cell were taken. (If you were unable to calculate a gradient you should use the value 0.016 V This is not the correct value.)

In experiment 2 in Table 2 the electrode potential of the Cu²/Cu electrode is +0.33 V

Use data from Table 2 in question 03.6 to calculate the electrode potential for the

Give one reason why your calculated value is different from the standard electrode potential for Zn²/Zn electrode.

The values for two electrodes are shown.

What is the EMF of the cell Fe(s)|Fe²⁺(aq)||Cu²⁺(aq)|Cu(s)?

A student set up the cell shown in Figure 2.

The student recorded an initial voltage of +0.16 V at 25 °C

Explain how the salt bridge provides an electrical connection between the two solutions.

The standard electrode potential for the Cu²⁺/Cu electrode is

Calculate the electrode potential of the left-hand electrode in Figure 2.

Electrode potential
V

Both electrodes contain a strip of copper metal in a solution of aqueous Cu²⁺ ions.

State why the left-hand electrode does not have an electrode potential of +0.34 V

Give the conventional representation for the cell in Figure 2. Include all state symbols.

When the voltmeter is replaced by a bulb, the EMF of the cell in Figure 2 decreases over time to 0 V

Suggest how the concentration of copper(II) ions in the left-hand electrode changes when the bulb is alight. Give one reason why the EMF of the cell decreases to 0 V

Change in concentration of copper(II) ions in the left-hand electrode

Reason why the EMF decreases to 0 V

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.

Which ion cannot catalyse the reaction between iodide (I^–) and peroxodisulfate (S₂O₈^2–)?

Use the data below to help you answer this question.