Page 41 - Curriculum Visions Dynamic Book

Environmental pollution
The combustion that occurs inside a motor engine is not very efficient. If it were completely efficient then all the fuel would be changed into carbon dioxide and water and two harmless gases would escape from the exhaust pipes of vehicles. However, because the fuel is not completely burned up (oxidised) carbon monoxide is formed and many hydrocarbon particles are ejected from the partly burnt fuel.
When cars idle they are at their least efficient, so idling cars produce more carbon monoxide than cars travelling quickly. This is why carbon monoxide pollution is higher in cities (where many cars are moving slowly) than in the countryside. (See also the discussion of smog on the next page for nitrogen dioxide pollution.)
Catalytic converter
A catalytic converter is a device that looks like an additional silencer set into the exhaust systems of modern petrol vehicles.
The catalytic converter is actually a chemical device, designed to cause a range of reactions with the exhaust gases of the engine (carbon monoxide and nitrogen oxides) and hydrocarbon fragments and convert these potentially harmful pollutants into the harmless, naturally occurring products: nitrogen, carbon dioxide and water.
The catalyst is called an oxidation/reduction catalyst because it causes the oxidation of carbon monoxide and hydrocarbon fragments to carbon dioxide and water, using the oxygen from the nitrogen oxides, thus converting them to inert nitrogen gas (reduction).
The catalyst is a combination of about three grams of the rare elements platinum and palladium, which explains why the catalytic converters are expensive. The catalyst is fixed to the surface of some supporting structure, such as a porous ceramic material (whose total surface area is equivalent to two soccer pitches). This provides the most efficient exposure of the catalyst to the gases.
EQUATION: Oxidation of carbon monoxide
Carbon monoxide + oxygen ➪ carbon dioxide 2CO(g) + 2O (from NOx) ➪ 2CO2(g)
EQUATION: Reduction of nitric oxide
Nitric oxide ➪ nitrogen + oxygen
atomised: broken up into a very fine mist. The term is used in connection with sprays and engine fuel systems.
catalyst: a substance that speeds up a chemical reaction but itself remains unaltered at the end of the reaction.
ceramic: a material based on clay minerals, which has been heated so that it has chemically hardened.
Combustion of powders and mists
Chemical reactions work faster when the surface area of contact between the two reagents is high. Chemical reactions work very quickly when two liquids are mixed because the molecules of the compounds are in complete contact. The fine mist
of fuel in a vehicle engine reacts quickly because the surface area of the fuel is large (see page 40).
A large block of coal burns slowly because there is little surface area with which the oxygen can react. By contrast, a powder of coal dust will combust so quickly that it will create an explosion. Other organic materials will behave in the same way, for example flour dust in a flour mill (flour is a fuel because it
is made of a carbon compound). This is the reason coal mine and grain store managers have to be so careful not to allow the build-up of dust or the use of flames or sparks.
Also...
A car has an engine that creates chemical reactions thousands of times each minute. Each of the reactions is a form of combustion, mixing a fuel and oxygen at a high temperature inside
a closed cylinder.
Because the reaction happens inside the cylinders of motor vehicles, the engines are called internal combustion engines.
The mixture of oxygen and gasoline is brought to a high temperature partly by squeezing it inside the cylinder and partly by igniting a spark in the top
of the cylinder. But diesel engines do not use spark plugs at all. They simply compress the mixture until it gets so hot that the chemical reaction occurs.
Because it is atomised, most of the fuel is able to undergo combustion at the same time.
2NO(g) ➪ N (g) + 2
O (g) 2
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