Selasa, 25 November 2008

ELECTROCHEMISTRY

Electrolytes and non-electrolytes
Electrolyte

•A chemical compound which conducts electric current in a molten state or aqueous solution and undergoes chemical change.
•Contain ion which more about freely to carry electrical charge.
•Can break down into its elements at electrods when an electric current flows through it.
Non-electrolyte
•A chemical compound which cannot conduct electric current in all states.
•No charge on a non-electrolyte when an electric current flows through it.
•Made up from molecules only. There are no ions which can carry electrical charges in non-electrolyte.

Electrolysis Of Molten Compound
Electrolysis process
•The process of breaking down chemical compounds into their constituent elements using electric current.
•An electrolyte in the form of molten compounds or aqueous solution can be broken down into its element.
Electrolytic cell
•A type of cell which uses electrical energy to produce chemical reaction.
•Reaction in electrolytic cell will not occur without an external source of electrical energy.
•Anode (connected to the positive terminal of an electric source).
•Cathode (the electrode which is connected to the negative terminal of an electric source).

Cation and anion in molten compounds
•A molten compound which can be electrolysed is made up of ionic compounds.
•Cation (positively charged ion)
•Anion (negatively charged ion)
In solid state
@ cations and anions in ionic compounds are not free to move. This is
because cations and anions are arrange in a fixed crystal lattice.
@ there are no free ions to move and carry electrical charges, solid ionic
compound cannot conduct electricity.
In molten form
@ cation and anion in ionic compounds move freely to carry electrical
charges. Thus, molten ionic compounds can conduct electricity.
Electrolysis of molten compound
•During electrolysis
@ anions (negative ions) will be pulled towards the anode (positive
electrode). At the anode, cations will be discharged by accepting
electron from the cathode.

@ Cations (positive ions) will be pulled towards the cathode (negative
electrode). At the cathode, cations will be discharged by accepting
electrons from the cathode.

Electrolysis Of Aqueous Solution
Cation and anions in aqueous solutions
•Water molecules in a aqueous solution slightly dissociate to produce hydrogen ions and hydroxide ions
•Thus, aqueous solution contain hydrogen ions and hydroxide ions besides electrolyte ions.
•When there is more than one type of cation attracted to the cathode or more than one type of anion attracted to the anode, the choice of ion to be discharged depends of the following factors:
@ position of the ions in the electrochemical series ( ECS ).
@ concentration of ions in the electrolyte.
@ type of electrode.

Concentration of ions in electrolyte solutions
•Concentration of ions in electrolyte solution, also affects the choice of ions to be discharged.
•An ion that is more concentrated is preferable discharged.
•However, the concentration of ions normally affect the choice of ions to be discharged at the anode only.
•The type of ion chosen for discharged at the cathode is still determined by the position of ion in the ECS.
ELECTROLYSIS IN INDUSTRIES
Extraction Of Metal
•Electrolysis can be used to extract metals from their ores.
•Metals that are more reactive than carbon such as:
@ potassium @ magnesium
@ sodium @ aluminium
@ calcium
cannot be extracted through heating of their metal oxides with carbon.

•Metal that are more reactive need to be extracted from their molten ores using the electrolysis process.
•In the process, the molten ore is the electrolyte and inert electrodes such as carbon are used as positive and negative electrodes.

Purification Of Metal
•Metal that have been extracted from their ores are normally not pure and contain impurities which need to be removed.
•These metal can be purify by electrolysis.
•In the purification process, the impure metal is made the anode while the pure metal plate is made the cathode.
•The electrolyte is the solution which contain the metal ions.
•During electrolysis, impure metal at the anode will release electrons and dissolve into the electrolyte as metal ions.
•These metal ions can receive electrons and form pure metal at the cathode.
•The size of impure metal (the anode) becomes smaller and impurities will settle at the bottom of the container.
•The size of pure metal (the cathode) becomes larger due to the deposition of pure metal. The mass of pure metal plate increase.


Electroplating Of Metals
•Electroplating
@ metals that can be plated with other metals through electrolysis.
•The aim of metal plating through electrolysis includes:
@ making the metal more resistant to corrosion.
@ making the metal appear more attractive.
•Metal that frequently used as plating metal:
@ silver, Ag @ cadmium, Cd
@ gold, Au @ copper, Cu
@ nickel, Ni @ zinc, Zn
@ chromium, Cr @ tin, Sn
•Electroplating process
@ plating metal is used as the anode while metal to be plated is used as a
cathode.
•The electrolyte used
@ an aqueous solution which contains ions of the plating metal.

•During electrolysis
@ plating metal or the anode releases electrons and dissolves to form metal
ions.
@ ions of the plating metal then, move to the cathode, receive electrons there
and form a thin layer of metal on the cathode.

VOLTAIC CELL
Voltaic cell
•A type of cell which produces electrical energy from the chemical reactions occuring inside it.
•Chemical energy is transformed into electrical energy.
•Example of chemical cell
@ simple voltaic cell @ Daniell cell
@ lead-acid accumulator @ dry cell
@ alkaline cell

Simple voltaic cell
A simple voltaic cell
•Has 2 different conductors (metal or carbon) which are immersed in an electrolyte solution and connected to each other with connecting wires.
•The conductors are connected through and external circuit to a bulb, ammeter, voltmeter or galvanometer.
•Metal that located higher up in the ECS (more electropositive metal) will get at the negative terminal and vice versa.
•At the negative terminal, the metal electrode will release electron and dissolve to form metal ions.
•At the positive terminal, cation of a metal will accepts electrons and be discharged to form neutral atoms.

Daniell cell
•Daniell cell
@ made up of two electrodes of different type of metals that immersed in two
separate electrolyte solutions.
@ the electrolyte solution in Daniell cell can be separated either by a porous
pot or salt bridge.
•Porous pot / salt bridge
@ function : to separate two electrolyte solutions but allows ion to pass
through in to complete circuit.
@ can be made from any electrolyte that does not react with the electrodes in
the Daniell cell.
@ example : dilute sulphuric acid
: sodium nitrate solution
: potassium chloride solution

•Chemical reactions
@ basically same as a simple voltaic cell.
@ produce electrical energy.
@ metals that are located at higher position in ECS act as the negative
terminal.
@ metals that are located at lower position in ECS act as the positive terminal.





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