[Edu.joshuatly.com] Pahang JUJ SPM 2011 Chemistry

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    CHAPTER 6: ELECTROCHEMISTRY

    A. Electrolytes and Non Electrolytes

    1) Electrolytes

    Electrolytes are substancesthat can conduct electricity when they are in molten states or aqueoussolutionand undergo chemical changes.

    Examples of electrolytes are:

    a) dilute acids and alkalisb) molten zinc bromidec) sodium chloride solution

    Electrolytes are able to conduct electricity because there are free ions presentin the moltenor aqueous

    state.

    2) Non-electrolytes

    Non electrolytes are substances thatcannotconduct electricity either in molten state or aqueous solution.

    Examples of non-electrolytes are:

    a) naphthalene,b) benzenec) alcohold) tetrachloromethane

    In other words, all non-electrolytesare covalent compounds which do not contain ions and thus, they arenot able to conduct electricity.

    Battery bulb is lighted

    Diagram 6.1:An electrolyte can conduct electricity because of the presence of free moving ions.

    1. What is an electrolyte?Substances that can conduct electricity when they are in molten state oraqueous solution and undergo chemical changes.

    2. Explain why a sugar solution does not conduct electricity while a common

    salt solution conducts electricity.Sugar solution does not have freely moving ions while salt solution do have freely movingions.

    MoltenPbBr2

    NaCl

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    Experiment: To Investigate the electrolysis of molten lead (II) bromide, PbBr2.

    1. Lead (II) bromide, PbBr2is an ionic compound.2. Hence, it consists of the positive lead (II) ions, Pb

    2+and

    the negative bromide ions,Br-.

    3. In solid lead (II) bromide, PbBr2, these ions do not move freely but are held in afixed positions in a lattice. When it melts, the ions are free to move.

    Battery mentol switch

    Diagram 6.2: Electrolysis of molten lead (II) bromide, PbBr2

    4. During themolten lead(II) bromide, PbBr2,

    i) the negative bromide ions,Br-are attracted to the anode,

    ii) the positive lead(II) ions,Pb2+

    are attracted to the cathode.

    In the molten state, heat energy that is provided breaks down theelectrostatic forces so that ions are able to move freely. Thus we have:

    PbBr2(s) Pb2+ +2Br-

    What happens to the cathode and anode?

    5. At the anode, two bromine atoms combine to form a bromine gas.The half equation that we have are as below:

    2Br-(l) Br2 (g) + 2e-

    At the cathode, lead(II) ions, Pb2+

    undergo discharge whereby each of the ionsaccepts two electrons to form a lead atom. The half equation are as follows:

    Pb

    2+

    (l) + 2e

    -

    Pb(s)

    7. Combining the two half equations, we get the overall equation that represents the electrolysis ofmolten lead (II) bromide.

    Pb2+

    (l) + 2Br-(l) Pb (s) + Br2 (g)

    The process of discharge can either be donating electrons or accepting electronsBromine gas, Br2is red-dish brown in color and is poisonous.

    Anode

    Lead(II)ion, Pb

    2+

    Cathode

    molten lead(II) bromide,PbBr2

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    Test :

    1. State the meaning of the following terms.

    a) anode

    An electrode which is connected to the positive terminal of an electric source.

    b) cathode

    An electrode which is connected to the negative terminal of an electric source.

    c) electrolysis

    Aprocess whereby compounds in either molten or aqueous state are brokendown into their constituent by passing electricity through them.

    2. A molten oxide, Q2O3 is electrolysed using carbon electrodes.

    a) Draw a labeled diagram to show the set-up of apparatus for the electrolysis.

    Battery mentol switch

    b) Write the formulae for the ions present in the electrolyte.

    Formulae ions are Q3+

    and O2-

    c) State the ions that move to each of the electrodes during the electrolysis.

    i) the negative ions, O2-

    are attracted to the anode,ii) the positive ions, Q

    3+are attracted to the cathode.

    d) Write the half equation of the reaction at each of the electrodes.

    Half equations: anode terminal: O2-

    O2 + 2e-

    Half equations: cathode terminal: Q3+

    + 3e- 3Q

    d) Named the substances formed at each of the electrodes.

    Positive electrodes: an atom of substance Q is performed.Negative electrodes: an oxygen gas is released.

    Anode

    Carbonelectrodes

    Cathode

    molten oxide,Q2O3

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    CHAPTER 6 :ELECTROCHEMISTRY

    Voltaic Cell

    Example of simple voltaic cell

    1. Voltaic cell also known as galvanic cell.2. Denial cell is another example of a voltaiccell.3. In Denial cell, the two solutions are connected through a salt bridgeor a

    porous pot.4. The solution used in the salt bridge is usually a diluted acid or a solution of

    sodiumor potassiumsalts.5. The function of the salt bridge or porous pot is to stop the two electrolytes from mixing.

    But allow the movementof the ions in order to complete the electric circuit.

    Electrolytic cellElectrical energy is supplied to produced

    chemical reaction

    Chemical cell Chemical reaction produced the electricalenergy

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    How does a voltaic cell work?

    The electrochemical series

    Li

    Ca

    MgAlZn

    SnPb

    CuHgAg

    Increasing

    tendency

    of metals

    to lose

    electron

    K

    Na

    Fe

    H

    Reaction in the voltaic cell using magnesium ribbon and copper plate as electrodes

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    The apparatus set-up of a Daniell Cell

    using a salt bridgeThe apparatus set-up of a Daniell Cell

    using a porous pot

    Zinc is higher than copper in the electrochemical series. Thus it releases electronsmoreeasily than copper. Each zinc atom donatestwo electrons to form a zinc ion.The zinc ions

    are released into the electrolyte. The zinc platedissolves gradually.Since the Zinc is higher than copper in the electrochemical series. It acts as the cathode ofthe cell(negative terminal).The ionic equation of cathode:Zn(s)Zn2+ (aq) + 2eCopper as the anodeof the cell (positive terminal).The electrons are acceptedby thecopper(II) ions to form copper metal. The copper plate becomes thickergradually.The ionic equation of anode: :Cu2+ (aq) + 2eCu(s)The intensity of the blue colour of the copper(II) sulphate solution decreasesas theconcentration of copper(II) ions decreases gradually.

    Decreases donates ion electrons zinc plate anode cathode thicker accepted

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    Cells Used in Advantages Disadvantages

    Torches

    Radio

    Electrical toys

    Cassetteplayers

    Nospillage

    Small in size

    Easily carried about

    Produces regular current andvoltage

    Obtained in different sizes

    Cannot be recharged

    Not long lasting

    Electrolyte tends to leak

    Low voltage is produced

    Motor vehicles Can be recharged

    Produces a high voltage and usefor a long period of time

    Produces high current(up to 75A)for heavy duty purposes

    Big in size

    Expensive

    Leak charges when not in used

    Spillage of acid can occurr

    Heavy and difficult to carriedabout

    Cassette players

    Electrical toys

    Appliances which are inuse for long periods

    Last longer than dry cell

    Produces constant current

    No liquid electrolyte

    Expensive

    Cannot be recharged

    Electrolyte leakage can occur

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    Cells Used in Advantages Disadvantages

    Cameras

    Watches

    Calculators

    Small in size

    Last a longer time

    Very mobile and easily used

    Very expensive

    Cannot be recharged

    Mercury is not environmentfriendly

    Electrical toys

    Cassette players

    Radios

    Can be recharged

    Smaller size than accumulator

    No spillage

    Can be recharged up to 500times

    Expensive

    Transformer is needed forrecharging

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    Differences between electrolytic and voltaic cell

    Characteristic Electrolytic cells Chemical cells

    EnergyElectricalenergy produces chemical

    reactions

    Chemicalreactions produce electricalenergy.

    Current Current supplyinto the cell. Current producedby the cell

    Anode/CathodeCathode: terminal negative

    Anode : terminal positive

    Cathode: terminal positive

    Anode : terminalnegative

    Positiveterminal

    Anion gives out electrons to anode Electronsreceive by positive terminal.

    NegativeTerminal

    Cationaccept electron from negativeterminal.

    Electronsgiven out from negative terminal.

    Type ofelectrode

    Pairs of graphite, platinumor suitablemetals.

    Pair of different metals.

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    CHAPTER 8 : SALTS

    8.1 Definitions of salts

    A salts is a compo