Chemical cells and batteries

An alkaline hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity.

The overall equation for the reaction is

Figure 2 represents an alkaline hydrogen-oxygen fuel cell in use to power a motor.

Give a half-equation for the reaction occurring, in alkaline conditions, at each electrode.

At negative electrode

At positive electrode

Suggest why it is not necessary to replace the KOH electrolyte during the operation of this fuel cell.

Describe how the reactions at the electrodes generate an electric current that can power the motor in the external circuit.

This fuel cell uses pure oxygen, rather than air, as an oxidising agent.

Suggest the identity of the gas in air that would react with the KOH and affect the efficient operation of the cell.

When an alkaline hydrogen-oxygen fuel cell is used to power the motor in a car, the hydrogen can be stored as a liquid.

Suggest an advantage of storing hydrogen as a liquid.

Which graph shows the change in EMF, with time, for an alkaline hydrogen-oxygen fuel cell?

Tick (✓) one box.

An acidic hydrogen-oxygen fuel cell has different reactions at the electrodes.

Suggest why the EMF of the acidic hydrogen-oxygen fuel cell is the same as that of the alkaline hydrogen-oxygen fuel cell.

This question is about metals and their compounds.

State why the atomic radius of calcium is greater than the atomic radius of magnesium.

Magnesium reacts with steam.

Give an equation, including state symbols, for this reaction.

Similar-sized pieces of barium and magnesium are added to separate 100 cm³ samples of dilute sulfuric acid. In each case the sulfuric acid is in excess.

The barium reacts quickly at first. After a few minutes the reaction stops, even though there is still some unreacted barium in the flask.

The magnesium reacts more slowly than the barium, but the reaction continues until all the magnesium has reacted.

Explain why

• the barium initially reacts more quickly than the magnesium

• the barium reaction stops before all the barium has reacted.

A metal nitrate X(NO₃)₂ completely decomposes when heated.

A 0.832 g sample of X(NO₃)₂ decomposes on heating to produce a total of

Deduce the identity of metal X.

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

Identity of metal X

Sodium reacts with aluminium and hydrogen to form solid NaAlH₄

Give an equation for this reaction.

Suggest why NaAlH₄ has a high melting point.

Equation

Give the equation for the reaction between H₃PO₄ and an excess of NaOH

Lithium is an important metal used in cells to power mobile phones.

In a lithium cell, a lithium cobalt oxide electrode and a lithium electrode are used.

Give the equation for the reaction that occurs at the positive electrode.

Commercial electrochemical cells can be rechargeable or non-rechargeable.

State why lithium cells can be recharged.

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 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.

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.