Fuels. Reaction Rate. Electrolysis.

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Lecture 22

• Fuels. Reaction Rate. Electrolysis.

Chapter 11.6 ? 11.15

• Liquid, Solid, and Gaseous Fuels
• Reaction Rates
• Oxidation and Reduction

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Fuels
Fuel is a substance whose oxidation reaction is
strongly exothermic.
Other requirements for good fuels low price,
abundance, easy transportation, non-toxic
combustion products.
Carbon and some of its compounds fulfill these
requirements. These are petroleum, natural gas,
coal. Their combustion products are CO2 and H2O.
Petroleum products give off 44?49 kJ/g of
heat. Each gram of gasoline requires 15 g of air
for complete combustion.
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Fuel Combustion
Complete combustion reaction 2C8H18 25O2 ? 16
CO2 18H2O
Incomplete combustion reaction 2 C8H18 20 O2 ?
11 CO2 15 H2O 3 CO C2H6
? Ethane
? Octane
CO combines with the hemoglobin, replacing O2,
and is lethal even in small concentrations (1).
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Pollution Problem
Incomplete combustion occurs due to a non-perfect
burning process and causes air pollution.
In 1920s it was found that adding a lead compound
(tetraethyllead) to gasoline improves burning in
engines. This improves engine performance, but
adds lead compounds to engine emission.
One way to reduce pollutions is to add a
catalytic converter to the exhaust system
(platinum or rhodium) to change polluting gases
into harmless. Another method is to change
gasoline (add ethanol or produce a different
mixture of hydrocarbons).
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Gaseous Fuels

• Gaseous and solid fuel produce mostly the same
  substances as a result of combustion (CO2 and
  H2O).

The most common gaseous fuel is natural gas (CH4).
A gas fuel can be produced synthetically by
passing very hot steam over coal C H20 ? CO
H2, C 2H2 ? CH4
Hydrogen is a very good fuel as it gives off 143
kJ/g. However, it is still too expensive.
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Solid Fuels
The main solid fuels in use are coal (32 kJ/g)
and wood (19 kJ/g).
Natural coal contains additional substances
(e.g., sulfur). Additionally, oxidation of
nitrogen produces acid rains. pH of acid rains
can be as low as 2?3 (pure water pH7).
Poisonous products can be converted into less
harmful. 2SO2 O2 2CaCO3 ? 2CaSO4 2CO2
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Reaction Rates
Some reactions, especially with ionic substances,
are very fast.
The most important factors that affect reaction
rates are Temperature the higher it is, the
faster the reaction Concentration the same as
above Surface area the greater it is, the faster
the reaction Catalysts can increase or decrease
the reaction rate
Catalysts are substances that either speed up or
slow down a reaction being unaffected.
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Oxidation and Reduction
Oxidation is the chemical combination of a
substance with oxygen. Reduction is the removal
of oxygen from a compound.
A more general definition Oxidation is a loss of
electrons by the atoms of an element. Reduction
is a gain of electrons.
The two processes must take place together.
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Electrolysis
An electrode is a conductor through which
electric current enters or leaves a solution.
Negative electrode is called cathode, positive ?
anode.
2NaCl ? 2Na Cl2 Cl? ? Cl e? Na e? ? Na
Electroplating is depositing of a metal on an
object made of another metal.
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Electrochemical Cells
Oxidation-reduction reactions can produce
electric currents. Currents can be used by adding
conducting wires.
All kinds of batteries based on
oxidation-reduction reactions are electrochemical
cells.
Fuel cell use reactants continuously. Hydrogen-oxy
gen cell combines 1 kg of H with 8 kg of O and
produces 200 MJ of electric energy.
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Summary
The most efficient currently used fuels are
pollutant and need to be replaced by safer
ones. Reaction rates depend on the temperature,
concentration, surface area, and the presence of
a catalyst. Fuel cells are the most promising
energy suppliers of the future.