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3.1 - Coggle Diagram
3.1
Reduction potentials
When the cell EMF is measured using the standard hydrogen electrode, this value is called the reduction potential of that particular half-cell. They can be compared because they ae gathered in the same way
Any half-cell that has a positive value when connected to the SHE undergoes reduction and contains a species which is an oxidising agent towards H+ ions
Any half-cell that has a negative value when connected to the SHE undergoes oxidation and so contains a species which is a reducing agent towards H2 molecules
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Three types of half-cell, depending on the chemical states
- The metal/metal ion half-cell
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Used when an element is an aqueous ion is oxidised or reduced to form a different aqueous ion. Consists of a solution made up of both the oxidised and reduced form, both of concentration 1 moldm-3 and an electron made of platinum and coated with very fine platinum powder (known as platinum black)
When either reduced or oxidised species is a gas and the other is an aqueous ion. Made up of a 1moldm-3 solution of the aqueous ions associated with the gas, a platinum black electrode dipped into the solution and glass cover over the Platinum electrode that surrounds the electrode in a steady stream of the gas kept at a pressure of 1 atmosphere
A salt bridge is used to connect the two solutions which allows ions to move between the two solutions and carry electrical energy
The salt bridge at this level is often a piece of filter paper soaked in saturated potassium nitrate solution. This is used because almost all potassium and nitrate compounds are soluble in alter and so will not form a solid that could block conduction of electrical energy
Fuel Cells
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HYDROGEN FUEL CELL
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At the anode, hydrogen gas is oxidised to from H+ ions
H2 -> 2H+ + 2e-
The H+ ions then pass through a semi-permeable membrane and reach the cathode, where they accept electrons and react with oxygen to form water:
O2 + 4H+ + 2e-
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Advantages:
- water is the only water product = no contribution to global warming
- Far less energy lost as heat = more efficient
- H2 can be produced by the electrolysis of water making it a renewable energy source
Disadvantages:
- Hydrogen is highly flammable making it difficult and expensive to store
- The most common H2 source is the reformation of methane from fossil fuels therefore crude oil is still needed and CO2 is still being produced just at an earlier production stage
- A lot of electricity is needed is needed to undertake electrolysis therefore through making hydrogen CO2 is still produced unless electricity is made through renewable sources
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