Fuel cells

The overall equation for a hydrogen–oxygen fuel cell is:

The half-equation at the negative electrode is:

What is the half-equation at the positive electrode?

This question is about two metal hydrides, lithium aluminium hydride, LiAl H₄, and sodium borohydride, NaBH₄. Both hydrides are used as reducing agents in organic chemistry.

What is the systematic name of the alcohol C₆H₅CH₂OH?

Calculate the mass of LiAl H₄ that the student should use in Step 2 and the percentage yield of pure C₆H₅CH₂OH obtained in Step 4.

Give the mass of LiAl H₄ and the percentage yield of pure C₆H₅CH₂OH to 3 significant figures.

Percentage yield of pure C₆H₅CH₂OH =........ %

In the preparation, LiAl H₄ behaves as a nucleophile.

What is meant by the term nucleophile ?

The mechanism for Step 2 in the reduction of C₆H₅COOC₂H₅ is shown below. In the mechanism, LiAl H₄ can be represented as H^–.

Complete the mechanism by adding curly arrows to the three boxes.

Suggest why the student heated the mixture from Step 2 with dilute aqueous acid in Step 3.

NaBH₄ contains Na⁺ and BH₄^– ions.

Draw a ‘dot‑and‑cross’ diagram for NaBH₄.

Use a different symbol for the electrons from each element.

Show outer electrons only.

NaBH₄/ O₂ cells are being investigated as potential fuel cells.

• The positive electrode contains O₂.

• The negative electrode contains NaBH₄.

The reaction at the positive electrode and the overall cell reaction for a NaBH₄/ O₂ fuel cell are shown below.

Overall cell reaction: NaBH₄ + 2O₂ NaBO₂ + 2H₂O E = 1.64 V

What is the standard electrode potential of the negative electrode in this cell?

Construct the equation for the reaction at the negative electrode of this cell.

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........

Storage cells and fuels cells are types of electrochemical cell.

The electrode potentials for five redox systems are shown in Table 19.1.

A student sets up an electrochemical cell based on redox systems 1 and 4 in Table 19.1.

Draw a labelled diagram to show how this cell could be set up in the laboratory.

Construct the equation for the overall cell reaction.

In acid conditions, MnO₄^2–(aq) disproportionates to form MnO₂(s) and MnO₄^–(aq).

Explain, in terms of oxidation numbers, why disproportionation has taken place.

Explain, in terms of electrode potentials and equilibrium shifts why MnO₄^2– disproportionates in acid conditions. Use the information in Table 19.1.

An alkaline hydrogen-oxygen fuel cell is set up.

The overall equation for the cell reaction is shown below. 2H₂(g) + O₂(g) → 2H₂O(l) Redox system 2 in Table 19.1 is the positive electrode of this cell.

Write the half-equation at the negative electrode.

The cell potential is 1.23 V.

Calculate the electrode potential of the negative electrode.

State one important feature of a fuel cell that is different from a conventional storage cell.

The redox equilibria for a hydrogen–oxygen fuel cell in alkaline solution are shown below.

What is the equation for the overall cell reaction?

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