Alkanes Flashcards
Order of fractional distillation
C1-3 -gases
C4-12-petrol
C12-16-kerosene
C15-18-diesel oil
C19-35-lubricating oil
C35-70-fuel oil
C70-bitumen
What’s the benefit of cracking
shorter, more useful chains are produced, especially petrol
• some of the products are alkenes, which are more reactive than alkanes.
What are alkanes used as
Alkenes are used as chemical feedstock and are converted into a huge range of other compounds including polymers and a variety of products from paints to drugs. the most important alkene is ethene, which is the starting material for poly(ethene and a wide range of other everyday materials.
Thermal cracking
This reaction involves heating alkanes to a high temperature,
under high pressure, up to 7000 kPa. Carbon-carbon bonds break so that one electron in the covalent bond goes to each carbon atom. So two shorter chains are produced, each ending in a carbon atom with an unpaired electron. These are called free radicals. Free radicals are highly reactive intermediates and react to form a variety of shorter chain molecules.
there are not enough hydrogen atoms to produce two alkanes, one of the new chains must have a double carbon bond, and is therefore an alkene:
More on thermal cracking
Any number of carbon-carbon bonds may break and the chain does not necessarily break in the middle. Hydrogen may also be produced.
Thermal cracking tends to produce a high proportion of alkenes. To avoid too much decomposition the alkanes are kept under the high temp and pressure for a very short time
Catalytic cracking
Catalytic cracking takes place at a lower temperature (
720 K) and lower pressure (but more than atmospheric), using a zeolite catalyst, consisting of silicon dioxide and aluminium oxide). Zeolites have a honeycomb structure with an enormous surface area. They are also acidic. This form of cracking is used mainly to produce motor fuels.
Combustion
Combustion reactions give out heat and have large negative enthalpies of combustion. The more carbons present, the greater the heat output.
For this reason they are important as fuels. Fuels are substances that release heat energy when they undergo combustion. They also store a large amount of energy for a small amount of weight.
Incomplete combustion
In a limited supply of oxygen, the poisonous gas carbon monoxide,CO, is formed. With even less oxygen, carbon (soot) is produced.. Incomplete combustion often happens with longer chain hydrocarbons, which need more oxygen to burn compared with shorter chains.
Carbon monoxide
carbon monoxide, CO, a poisonous gas produced by incomplete combustion
Nitrogen oxide
nitrogen oxides,are produced when there is enough energy for nitrogen and oxygen in the air to combine
This happens in a petrol engine at the high temperatures present, when the sparks ignite the fuel. These oxides may react with water vapour and oxygen in the air to form nitric acid. They are therefore contributors to acid rain and photochemical smog
sulfur dioxide
sulfur dioxide is another contributor to acid rain. It is produced from sulfur-containing impurities are present in crude oil.when combusted this oxide combines with water vapour and oxygen in the air to form sulfuric acid
Carbon particulates
carbon particles, called particulates, which can exacerbate asthma and cause cancer
Unburnt hydrocarbons
unburnt hydrocarbons may also enter the atmosphere and these are significant greenhouse gases. They contribute to photochemical smog which can cause a variety of health problems
Flue gas desulfurisation
The gases given out by power stations are called flue gases so the process of removing the sulfur dioxide is called flue gas desulfurisation. In one method, a slurry of calcium oxide) and water is sprayed into the flue gas which reacts with the calcium oxide and water to form calcium sulfite, which can be further oxidised to calcium sulfate, also called gypsum.
Catalytic converters
These reduce the output of carbon monoxide, nitrogen oxides, and unburnt hydrocarbons in the exhaust gas mixture.
The catalytic converter is a honeycomb made of a ceramic material coated with platinum and rhodium metals. These are the catalysts.
The honeycomb shape provides an enormous surface area, so a little of these expensive metals goes a long way.
Global warming and the green house effect
Carbon dioxide behaves rather like glass. It traps infrared radiation so that the Earth’s atmosphere heats up.. Other greenhouse gases are water vapour and methane. These are more effective than carbon dioxide, but there has not been much change in the level of these gases in the atmosphere in recent years. However, since the industrial revolution fossil fuels level of carbon dioxide has been rising.Gradually, the Earth’s temperature has been rising too and the scientists believe that the increasing level of carbon dioxide is the cause of global warming.
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Water vapour in the environment
. The concentration of water vapour, the most abundant greenhouse gas, in the atmosphere tends to stay roughly the same because of the equilibrium that exists between water vapour and liquid water. However, if the temperature of the atmosphere rises, there will be more water vapour in the air and more greenhouse warming. This may be offset by greater cloud formation and clouds reflect solar radiation.
