Battery Questions

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March 2013

10 Nickel-zinc cells are rechargeable. The half-reactions that take place in such cells are:



10.1 Write down the general name given to cells that are rechargeable. (1)
10.1 Secondary (cells)

10.2 Write down the balanced net (overall) cell reaction that takes place in this cell. (3)
10.2

10.3 The emf of one such cell is 1,65 V and its capacity is 1 500 mA.h.

10.3.1 Define the term cell capacity. (2)
10.3.1 The ability (of a cell) to store/deliver charge.

10.3.2 Calculate the maximum electrical work that can be done by this cell. (4)
10.3.2





March 2012

11 Mercury(II) oxide batteries are sometimes used in watches and cameras.
The two half-reactions involved in this battery and their respective reduction potentials are given below.



11.1 Which half-reaction (A or B) takes place at the cathode of this battery?
Refer to the given reduction potentials and give a reason for the answer. (2)
11.1 A
More positive reduction potential. Larger reduction potential.

11.2 Write down the net (overall) reaction that takes place in this battery. (3)
11.2

11.3 Write down the SYMBOL or FORMULA or NAME of the substance that acts as reducing agent in this battery.
Use oxidation numbers to explain the answer. (2)
11.3 Zn
Oxidation number increases from 0 to +2 and is thus oxidised.

11.4 State ONE safety concern regarding the disposal of these batteries. (2)
11.4 Mercury is poisonous / corrosive when in contact with skin
May contaminate ground water / water resources / soil / crops.

Nov 2012

10 The following half-reactions take place when a non-rechargeable alkaline cell is in use:



10.1 Write down the general name used for non-rechargeable cells. (1)
10.1 Primary cells

10.2 Which ONE of the above equations (1 or 2) represents the half-reaction that takes place at the cathode? Give a reason for your answer. (2)
10.2 Equation 2
• This is a reduction equation
• Reduction takes place (at the cathode)

10.3 Give a reason why the cell 'dies' after delivering current for a while. (1)
10.3 ANY ONE:
• The cell reaction reaches equilibrium.
• The rates of the forward and reverse reactions become equal.
• Substances reach their equilibrium concentrations.

10.4 The emf of the alkaline cell is 1,5 V. The maximum electrical work that can be done by this cell is 3 x 104J.
Calculate the:

10.4.1 Cell capacity of this cell in A.h (4)
10.4.1

10.4.2 Maximum constant current that this cell can deliver for 20 hours (3)
10.4.2


March 2011

10 A lead-acid battery (car battery) consists of six cells and has a battery capacity of 20 A.h.
The half-reactions that take place in each cell and their respective standard reduction potentials are represented below:



10.1 Are car batteries primary or secondary batteries? (1)
10.1 Secondary

10.2 Write down the equation for the net (overall) cell reaction that takes place in each cell of this battery. (3)
10.2


OR


10.3 Calculate the emf of the BATTERY, consisting of six cells, under standard conditions. (5)
10.3

10.4 Calculate the maximum time that this battery will be able to supply a constant current of 5 A to an appliance connected to it. Assume that the capacity of the battery remains constant. (4)
10.4

10.5 State TWO environmental risks associated with the irresponsible disposal of lead-acid batteries. (2)
10.5 ANY TWO:
• Acid used as electrolyte can contaminate groundwater.
• Plastic casings are non-degradable and can pollute the environment.
• Lead (in electrodes) is a heavy metal and can harm crops / plants.


Nov 2010

10 Lead-acid batteries have been used in cars for the past 85 years. The equations of the half-reactions that take place in each cell of such batteries are shown below.



10.1 Write down the oxidation number of lead (Pb) in PbSO4(s).
10.1 +2

10.2 Write down the balanced equation for the net (overall) cell reaction.
10.2

10.3 Which ONE of the reactants is the reducing agent in this cell reaction? Give a reason for the answer. (2)
10.3 Pb / lead
• Pb is oxidised / loses electrons.
• Highest reducing ability / stronger reducing agent / smaller reduction potential (Eo) /causes reduction
• The oxidation number of Pb increases (from 0 to 2)

One of the safety concerns related to the lead-acid battery is the dangers associated with recharging (that is reversing the net reaction) of a flat battery. Water in the battery can be electrolysed to produce hydrogen and oxygen gas during recharging.

10.4 Use the Table of Standard Reduction Potentials and write down the halfreaction which explains the formation of oxygen gas. (2)
10.4

10.5 Why is the recharging of flat batteries a safety concern? (1)
10.5 The gases produced during recharging (hydrogen and oxygen) may explode if sparked.

10.6 If the cell capacity of such a cell is 3,5 A.h, calculate the number of electrons that flow through the cell in 30 minutes. Assume the cell discharges completely during the 30 minutes.
(The charge on one electron is -1,6 x 10-19 C.) (5)
10.6


Nov 2009

12 Some cells, such as the nickel-cadmium cell used in calculators and electric shavers, can be recharged. Others, such as those used in watches and torches, cannot be recharged.

12.1 Are rechargeable cells primary or secondary cells? (1)
12.1 Secondary cells

12.2 The half-reactions occurring in a nickel-cadmium cell are shown below:



The emf of the nickel-cadmium cell is 1,4 V.

12.2.1 Which ONE of these half-reactions occurs at the cathode? Give a reason for your answer. (2)
12.2.1 Equation II


Reduction (takes place at cathode).
Electrons gained (at cathode)

12.2.2 Write down the balanced equation for the overall cell reaction. (3)
12.2.2

12.2.3 Calculate the maximum work done by the cell under standard conditions as 1 mol of Cd is used up.
(NOTE: 1 mole of electrons has a charge of 9,65 x 104 C.) (4)
12.2.3


Nov 2009 Unused

12 A sketch of a dry cell, such as the one used in a torch or radio, is shown below.



In some types of dry cells, the electrolyte paste is made of ammonium chloride which is acidic. In other types of cells the paste is alkaline, due to an alkali such as potassium hydroxide. The alkaline cells are known to last longer than those containing ammonium chloride.

12.1 Why is the above cell referred to as a dry cell? (1)
12.1 Electrolyte is a paste/not a liquid

12.2 Give a reason why alkaline cells last longer than cells that contain acid. (2)
12.2 The acid in the battery attacks (reacts with) the zinc, causing it to corrode faster than the alkaline-based battery.

12.3 A certain alkaline battery made to operate over a 20-hour period is
marked 1,5 V; 1000 mA.h.

12.3.1 Calculate the maximum work (in joule) that can be done by this battery. (4)
12.3.1

12.3.2 Calculate the maximum constant current that the battery can supply for 20 hours. (3)
12.3.2

12.4 Give ONE reason why rechargeable batteries have less impact on global warming than non-rechargeable batteries. (1)
12.4 Rechargeable batteries can be re-used and less is manufactured/transported - less greenhouse gases emitted

OR

More primary batteries need to be manufactured as they cannot be re-used and transportation / manufacturing leads to emission of greenhouse gases.


Additional Exemplar 2008

11.2 A cell such as the one described above is not much useful. However, the principle is used in batteries for cars, torches, computers, et cetera. These batteries are called secondary cells. One such battery is the mercury cell. The half reactions occurring in this cell are shown below.



11.2.1 Write down the overall cell reaction. (2)
11.1.4

11.2.2 Why does the use of this battery pose an environmental hazard? (1)
11.2.2 Mercury is poisonous


Exemplar 2008

12 A dry cell, as shown in the diagram below, does not contain a liquid electrolyte. The electrolyte in a typical zinc-carbon cell is a moist paste of ammonium chloride and zinc chloride.



The paste of ammonium chloride reacts according to the following half-reaction:



Manganese(IV) oxide is included in the cell to remove the hydrogen produced during half-reaction (i), according to the following reaction:



The combined result of these two half-reactions can be represented by the following half reaction:



12.1 Explain why it is important that the hydrogen produced in half-reaction (i) is removed by the manganese(IV) oxide. (2)
12.1 The build-up of H2(g) in the cell could cause the cell to burst or explode if ignited.

In a zinc-carbon cell, such as the one above, half-reaction (iii) and the half-reaction that takes place in the Zn/Zn2+ half-cell, produce an emf of 1,5 V under standard conditions.

12.2 Write down the half-reaction occurring at the anode. (2)
12.2

12.3 Write down the net ionic equation occurring in the zinc-carbon cell. (2)
12.3

12.4 Calculate the reduction potential for the cathode half-reaction. (4)
12.4.

12.5 When in use the zinc casing of the dry cell becomes thinner, because it is oxidised. When not in use, it still corrodes. Give a reason for the latter observation. (2)
12.5 NH4+ is an acid and reacts with the zinc casing.

12.6 Dry cells are generally discarded when 'flat'. Why is the carbon rod the most useful part of the cell, even when the cell is flat? (2)
12.6 Can be reused as an electrode in other situations.






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