CH2 Electrolysis

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  • 7/30/2019 CH2 Electrolysis


    Electrolysis uses an electric current to break down compounds which containions, producing elements.

    The basic idea is a simple one:

    when atoms lose electrons they become positive ions (this happens to metals,

    and to hydrogen). During electrolysis the positive ions may be given electrons,

    turning them back into atoms.

    when atoms gain electrons they become negative ions (this happens to non-

    metals). During electrolysis, the negative ions may give up their electrons,

    turning them back into atoms also.

    Remember: this only works for ionicsubstances not giant covalent or

    simple molecular.

    In addition, the ions must be free to

    moveso it doesnt work with solidionic substances because the ionsare fixed in the giant ionic lattice.The substance must be molten or

    dissolved in solution.

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    The ionic substance we want to break down is called the electrolyte.

    We dip two electrodes into the molten or dissolved electrolyte, and these areconnected to a source of electric current. It is these electrodes which will supply or

    remove electrons from the ions in the electrolyte.

    Like the ions, the electrodes also have a charge. The electrode connected to thepositive side of the power supply is positively charged, and called the ANODE.

    The electrode connected to the negative supply is called the CATHODE and isnegatively charged.

    Remember that like charges repel, and opposite charges attract ?

    The positive ions in the electrolyte are attracted to the negative

    electrode (CATHODE) where they can gain electrons.

    The negative ions in the electrolyte are attracted to the positiveelectrode (ANODE) where they can give up electrons.

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    Electrolysis experiments:

    + -+ -anode


    molten electrolyteelectrolyte solution

    source of electric current

    Electrolysis in industry used to obtain

    aluminium from aluminium ore (bauxite)

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    We will need to be able to write equations which show what is going on at the

    electrodes. To do this, we are going to need to be able to show not only what the

    different chemical substances are (using their formulae) but also what state they are


    We use state symbols, written in brackets, subscript, at the end of a formula.

    We use (s) for solid,

    (l) for a pure liquid (i.e. not a solution)

    (aq) for a substance dissolved in water

    (g) for gas

    e.g. PbBr2(s) solid lead bromidea giant ionic lattice

    PbBr2(l) liquid lead bromide after heating and melting (molten)

    PbBr2(aq) lead bromide dissolved in water, making a solution

    Br2(g) bromine gas

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    Electrolysis of ionic compounds after melting

    The simplest situation to consider is what happens when an ionic compound is

    melted and electrolysis is carried out. The only ions present are those in the ionic

    compound. Well consider copper chloride as our example.At the negative electrode (cathode):The positively charged copper ions (Cu2+) are attracted.

    Here they can receive the electrons they need to turn them back into atoms in this

    case two electrons. We show this with a half-equation:

    Cu2+ + 2e- Cu What we would SEE is a pinkish layer ofcopper metal being deposited on thecathode.

    At the positive electrode (anode):Negatively charged chloride ions (Cl-)are attracted

    At the electrode they each give up one electron to form chlorine atoms

    Chlorine atoms on their own have unfilled outer shells, so pairs of chlorine atoms

    form a covalent bond and making Cl2 molecules. The half-equation is:


    Cl2 + 2e- What we would SEE is a stream of bubbles of pale

    green chlorine gas being produced at the anode.

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    When electrons are gained, we call this REDUCTION. Since ions receive electronsat the cathode, this is where they are being reduced to elements

    e.g. Pb2+(l) + 2e- Pb(l) lead ions gain two electrons and are reduced to lead

    When electrons are lost, we call this OXIDATION. Since ions give up electrons atthe anode, this is where they are being oxidised to form elements

    e.g. 2Br-(l) Br2(g) + 2e- bromide ions lose electrons and are oxidised to bromine

    gas. Its a gas because the electrolyte is hot !




    Loss (of electrons)



    Gain (of electrons)reduction of lead

    ions to lead metal

    at the cathode

    oxidation of bromide

    ions to bromine gas

    at the cathodeHEAT

    molten lead bromide

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    More about half-equations:A half equation is used to show the reduction (gain of electrons) which is takingplace at the cathode.

    A second half-equation is used to show the oxidation (loss of electrons) which istaking place at the anode.

    They always balance:- there are the same number and type of atoms/ions on each side

    - the charges always add to the same on each side

    For metals they show the number of electrons gained to turn the ion into an atom:

    Na+(l) + e- Na(l) Cu

    2+(aq) + 2e

    - Cu(s) Al3+

    (l) + 3e- Al(l)

    For non-metals they show the number of electrons needed to turn the ions into

    atoms. They also show the non-metals bonding to form molecules:

    2Cl-(l) Cl2(g) + 2e- 2H+(aq) + 2e- H2(g) 2O-(l) O2(g) + 2e-

    Hydroxide ions give up electrons forming oxygen and water:

    4OH-(aq) 4e- + O2(g) + 2H2O(l)

    The data sheet you get in your exam shows you what charge different ions have.

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    Electrolysis of ionic compounds dissolved in water:

    When an ionic substance is dissolved in water the ions are free to move around, rather

    than being locked up in the giant ionic lattice, so they can move to the electrodes.

    In a solution, however, the ions from the ionic compound are NOT the only ions

    present. There are also hydrogen ions, H+(aq) and hydroxide ions, OH-(aq), from the


    There is therefore a competition at the negative cathode between the positively

    charged metal ions from the ionic compound and the hydrogen ions.

    There is also a competition at the positive anode between the negatively charged non-

    metal ions from the ionic compound and the hydroxide ions.

    + -





    attraction of ions in a copper

    chloride solution

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    At the cathode:

    The least reactive of the two positive ions is the one which is forced to leave thesolution, either as hydrogen gas, H2(g) , or by being deposited as the metal.

    The reactivity series of the metals, including the position of hydrogen isgiven in your data sheet in the exam.

    Calcium is more reactive than

    hydrogen, so remains in the solution.

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    At the anode:

    A similar competition occurs. The basic rule is that ifchloride, bromide or iodideions are present they will be forced to leave the solution as the halogen(chlorine, bromide or iodine).

    Other ions are able to stay in the solution, forcing the hydroxide ions to be oxidisedto oxygen gas and water.

    4OH-(aq) 4e- + O2(g) + 2H2O(l)

    e.g. 2Cl-(aq) 2e- + Cl2(g)

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    Industrial uses of electrolysis 1: Electrolysis of brine

    Brine is a concentrated solution of sodium chloride in water. There are therefore

    four ions present in a brine solution. We can use the rules from the last section to

    decide what will happen:

    Positive ionsNa+(aq) more reactive - will remain in solution

    H+(aq) less reactive - given off as H2 at the cathode 2H+

    (aq) + 2e- H2(g)

    Negative ionsCl-(aq) given off as the halogen Cl2 at the anode

    OH-(aq) will remain in solution 2Cl-(aq) Cl2(g) + 2e


    The ions remaining, Na+ and

    OH- form a sodium hydroxide


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    All three products of electrolysis are highly useful, so this is a

    very large scale industrial process.

    HYDROGEN is used:- in the Haber process for making ammonia- forhydrogenation of vegetable oils to make spreads etc.- as a fuel

    CHLORINE is used:- for making bleach

    - for sterilising (killing bacteria) in drinking water- for purifying swimming pool water

    SODIUM HYDROXIDE is used:- in soap and paper manufacturing- to increase the pH of waste water (in order to remove

    heavy metal ions by precipitating them as metal hydroxides)- for making bleach

    Environmental:Lots of electrical energy is needed to do electrolysis. This has to be produced by

    burning fossil fuels, so CO2 is released into the environment (global warming) and

    finite reserves of crude oil are used up.

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    Industrial uses of electrolysis 2: Electroplating

    Electrolysis can be used to coat a conductive object in a layer

    of another metal.

    This might be done to provide a protective layer e.g. to

    prevent corrosion, or may be to change the properties or

    appearance of the metal.

    The metal object to be coated is used as the negative

    electrode, and the electrolyte is a solution containing ions ofthe metal which is to form the coating.


    chrome-plating of steel parts on automobiles. to

    make them more corrosion-res