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    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.1 PART VII: REDOX REACTIONS, CHEMICAL CELLS

     AND ELECTROLYSIS

I. Chemical Cells in Daily Life

    A. Types of chemical cells

     There are two classes of chemical cells:

1. Primary cells

     They are not rechargeable. Once their chemicals are used up, they have to be thrown away.

     Common primary cells are:

    ; Zinc-carbon cells

    ; Alkaline manganese cells

    ; Silver oxide cells

a. Zinc-carbon cells

b. Alkaline manganese cell

     Alkaline manganese has a higher capacity than zinc-carbon cells.

     Alkaline manganese cells are commonly used in Discmans and motorized toys.

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.2 c. Silver oxide cell

     Silver oxide cell is a type of button cell.

     Button cells are small, long-life cells, which are commonly used in calculators, heating aids, pacemakers

    and watches.

2. Secondary cells

     They are rechargeable.

     The common rechargeable dry cells are:

    ; Nickel metal hydride (NiMH) cells

    ; Lithium ion cells

    ; Lead-acid accumulator

a. Nickel metal hydride (NiMH) cells

    (i) NiMH cells can provide a large amount of energy per unit weight or per unit volume of a

     cell (high energy density).

    (ii) NiMH cells are environmental friendly.

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.3

    b. Lithium ion cell

    (i) Lithium ion cells provide the greatest amount of energy for a given size or weight of all

     commercial rechargeable cells.

    (ii) Lithium ion cells are comonly used in mobile phones and other portable electronic

     devices because of their light weight.

c. Lead-acid accumulator

    Lead-acid accumulator is commonly used in cars. A 12 V accumulator for cars normally

     consists of six cells joined in series.

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.4 B. Basic Terms related to Dry Cells

    (i) Discharge

     It is the conversion of chemical energy of a cell into electrical energy.

(ii) Discharge curve

     When a cell is being discharged, a plot of voltage against time will give a discharge curve.

(iii) Charge capacity

     It is the quantity of electricity which a cell can deliver under certain specified conditions. It is usually

    expressed as mAh (read as " milliampere-hour").

(iv) Service life

     It refers to the period of useful service of a cell under specified conditions, before it falls to a certain

    voltage (usually 0.8V).

(iv) Shelf life

    o It refers to the duration of storage (at 21C) at the end of which a cell retains only 90% capacity.

    o If a cell has a shelf life of 3 years, 90% of its capacity still remains after 3 years of storage (at 21C).

(v) Cycle life

     It is the number of times that a secondary cell can be charged and discharged before it can no longer

    deliver a sufficient amount of energy.

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.5

    C. Characteristics of the chemical cells

    a. Primary Cells

     Zinc-carbon cell Alkaline manganese cell Silver oxide cell (+): carbon (+): manganese(IV) oxide (+): silver oxide Electrodes (-): zinc (-): zinc (-): zinc

    Ammonium chloride Potassium hydroxide Potassium hydroxide Electrolyte

    1.5V 1.5V 1.5V Max. voltage

    Constant voltage over Voltage over Falls rapidly Falls slowly discharge discharge

    No Yes Yes Steady current?

    No Yes No Large current?

    1.5 3 2 Shelf life (years)

    Short Long Long Service life

    Cheap Medium Expensive Price

    Leakage of electrolyte Leak proof; small internal Light weight; small size Other

    occurs in unsealed types resistance characteristics

    Torches, small radios, Cassette players; discmans, Quartz watches, remote control units motorized toys, flash guns, calculators, hearing aids,

    other appliances where pacemakers Usage

    there is heavy continuous

    use

    b. Secondary cells

     Lithium ion cell NiMH cell Lead-acid accumulator (+): lithium metal oxide (+): nickel(II) hydroxide (+): lead plates coated with

    (-): lithium (-): hydrogen absorbing lead(IV) oxide Electrodes

    alloys (-): lead plates Lithium salt in organic Potassium hydroxide Sulphuric acid Electrolyte solvent

    3.7 V 1.2 V 2 V Max. voltage

    Light Moderate Very heavy Weight

    Very high High Lowest among the three Energy density

    1 1 0.5 Shelf life (years)

    500+ 500+ 180+ Cycle life

    Expensive Medium Cheap Price

    Offer a relatively low Can deliver high discharge Generally too big and eavy Other discharge current, but high currents, but a heavy lead for higher power characteristics low may overheat the pack reduces the cells cycle life applications Electric razors, electric Electric razors, electric Automotive applications, Usage

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.6

    toothbrushes, medical toothbrushes, video wheelchairs, golf carts

    equipment, cameras, mobile phones,

    communications emergency backup

    equipment, portable DVD lighting, medical

    players, PDAs, laptop equipment, electric

    computers vehicles

    D. Choosing a chemical cell for a particular use

     When deciding which type of dry cell to use for a particular purpose, consider the following points:

    ; Price

    ; Size

    ; Voltage

    ; Charge capacity of the cell

    ; Ability to supply a steady current

    ; Whether a small or large current is required

    ; Whether the cell is used continuously or intermittently

    ; Whether the cell is rechargeable or not

    ; Risk of leakage

    ; Shelf life

    ; Whether it is environmental safe or not

Classwork

    Suggest which type of cell you would use for each of the following devices. Give reasons for your choice in

    each case.

    a. Torch

    b. Hearing-aid

    c. Walkman

    d. Remote control toy

    e. Watch

    f. Heavy duty motor-driven appliance

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.7

    E. Comparing Life Of Two Common Types of Cells

     A student is asked to carry out the following experiment to study the discharge of dry cells.

    (a) A zinc-carbon dry cell of size AAA is discharges by three 2.5 V light bulbs arranged in parallel. (b) The voltage is recorded every 4 minutes until the voltage drops to 0.8V.

    (c) The data obtained is plotted on a graph.

    (d) The above procedures are repeated by replacing the zinc-carbon with an alkaline manganese cell of the

     same size.

    (e) Result:

    0 4 8 12 16 20 24 28 30 36 40 Time (min)

    Voltage (V) 1.5 1.43 1.3 1.1 0.7 - - - - - - Zinc-carbon cell

    Voltage (V) 1.5 1.48 1.44 1.40 1.34 1.26 1.16 1.06 0.94 0.81 0.66 Alkaline manganese cell

Questions:

    1. What is the average lifetime of the two cells?

    2. If the price of one zinc-carbon cell is HK$1.50 and the price for one alkaline manganese cell is HK$2.50.

    What is the cost for discharging each cell for one minute?

    3. Decide which cell is the better buy?

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.8

    F. The use of chemical cells and pollution

     Many cells are useless when they become flat.

     Disposal of these flat cells cause pollution problems. The materials inside the cells do not decompose

    even after a long time. These materials may combine with other compounds and form harmful

    substances which pollute the environment.

     Rechargeable cells are becoming more popular now. These cells can be recharged over 500 times.

    Although they are more expensive and a special charger is needed for recharging, the cost is much lower

    in the long run.

     Remedy:

     a. Use mains electricity whenever possible.

     b. Choose and use dry cells wisely, e.g. by using rechargeable cells instead of non-rechargeable ones.

     c. Some manufacturers produce alkaline manganese cells with little or no mercury.

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.9 II. Simple Chemical Cells (Electrochemical Cells)

     Chemical cells are the devices which change the chemical energy to electrical energy.

    ; The electrical energy is in the form of electricity (or electric current). Electricity or electric current

    is a flow of electric charges.

; In metals or graphite, it is the flow of electrons. In electrolytes, it is the movement of ions.

A. Simple Chemical Cells

    a. A simple chemical cell can be set up by dipping two different metals (metal couple) in an electrolyte.

    The two metals are connected by an external wire.

    b. A simple chemical cell contains all the essential parts of any electric cell.

     (i) negative electrode (anode 陽極) from which electrons are given out.

     (ii) positive terminal (cathode 陰極) receives electrons flow from the anode.

     (iii) electrolyte 電解質in which ions are free to move to complete the circuit.

    Part VII Redox Reactions, Chemical Cells & Electrolysis/P.10

    +-e

    +

    negative terminalpositive terminalsymbol(cathode)(anode)

c. Some examples of chemical cell

     (1) Lemon cell

     (i) Two different metal strips (metal couple) are inserts into a lemon.

     (ii) A digital multimeter or a high resistance voltmeter is connected to the metal strips.

     (iii) Magnesium is the negative pole (anode) because it gives out electrons.

     Copper is the positive pole (cathode) because it receives electrons.

     How does a lemon cell work?

     a. The lemon juice (citric acid) inside the lemon acts as the electrolyte. This allows electricity to flow

     through it to complete the circuit.

     b. Magnesium has higher reactivity than copper, therefore, magnesium has higher tendency to lose

     electrons than copper. Thus the electrons flow from magnesium to copper through the external

     circuit.

     c. Electrons flow from negative pole to positive pole.

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