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A2 Redox & SEP Part 2 Applications

Candidates should be able to:

(l) appreciate the very wide range of occurrence of redox processes in chemistry;

Redox processes occur widely in chemistry. In the organic chemistry of Unit 4 a number of reactions

involving oxidation or reduction have already been encountered. Candidates should be familiar with

these and be able to quote them under the more general heading of redox.

Very few metals occur uncombined or as native! metals "gold and silver# and to some extent copper#

occur as the metal$.

%ercury can be obtained by heating its oxide. %ost metals are obtained from their ores by reduction.

%etals such as iron and copper are obtained by chemical reduction whereas very reactive metals such

as sodium# potassium and aluminium are extracted by electrolytic reduction.

&lectrolysis is important in other industrial processes# e.g. in the extraction of the halogens fluorine#

chlorine and bromine in which the halide ions are oxidised to the halogen by electrolysis.

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The Hydrogen Fuel Cell

Candidates should be able to:

"m$ explain the principles underlying the operation of the hydrogen fuel cell in terms

of the electrode half reactions 2H++ 2eH2and O2+ 4H++ 4e2H2O inthe presence of a platinum catalyst its potential use for storing energy and generatingelectricity and heat and the !enefits and draw!ac"s of its use (details of cell constructionnot re#uired)$

)*+,& containing platinumcatalyst

air "oxygen$

hydrogen gas

proton exchangemembrane

) simplified diagram of a hydrogenfuel cell.

water

unused hydrogen gas

C)-+,&

This diagra is !or in!oration only and its recall "ill not be re#uired !or the exaination

ydrogen gas is fed into the anode on one side of the cell while oxygen from the air is fed to the

cathode on the other side.

Under the influence of the platinum catalyst# the hydrogen gas is oxidised.

/"g$/00 /e

-he polymer1electrolyte membrane only allows the passage of positive ions to the cathode and the

electrons are forced to the cathode via an external circuit. -his means that the electrons power the

external electrical device.

)t the cathode oxygen# protons and electrons form the waste product of the device which is water.

+/"g$ 0 /00 4e2//+"l$

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3 ) typical cell gives an emf of about .5 V but they can be connected in series to give a

higher voltage or in parallel to give a higher current density in a !uel cell stac$.

Ad%antages o! hydrogen !uel cells

Clean technology 6 water is the only waste product

igh efficiency 6 fuel cells convert chemical energy directly to electrical

energy with no combustion process

7uiet operation

Relatively simple construction

In recent years there has been a large increase in power density

isad%antages o! hydrogen !uel cells

-hey are expensive

8torage of hydrogen poses problems as hydrogen is a very flammable and

explosive gas 9resent cells are not economically viable because they last

for a short time.

-here is a net energy loss since the energy required to produce the hydrogen is

more than that produced in its use