Just as coal has formed by the action of heat and pressure on the remains of trees and plants on land over millions of years, so oil and natural gas have formed by the action of heat and pressure on the remains of sea plants and animals over millions of years.
The remains were buried in sediments which excluded the air (kept out oxygen) and stopped them decaying.
More sediment buried the remains deeper and deeper until pressure and heat eventually turned them into coal, oil and natural gas.
They are called fossil fuels because they are buried underground (from Latin fossilis - dug up).
Fossil fuels are a finite resource and non-renewable.
The oil deposits are formed in porous rock sediments. Porous rock has pores in it. Pores are small holes (see for example sandstone). The small holes allow the oil and natural gas to pass through the rock and rise until they are stopped by a layer of non-porous rock.
Non-porous rock (for example shale) has no holes, and acts as a barrier to prevent the oil and natural gas rising.
The oil and natural gas become trapped underground.
The oil is called crude oil (or petroleum, from Latin - rock oil), and has natural gas in it or in a pocket above it trapped by non-porous rock.
Drilling through the rock allows the oil and gas to escape to the surface. Natural gas is mostly methane (CH4).
Crude oil is a mixture of substances (mostly hydrocarbons).
Crude oil is a mixture of substances which are mostly hydrocarbons.
A hydrocarbon is a compound containing hydrogen and carbon only.
Since crude oil is a mixture of different hydrocarbon compounds, the different hydrocarbons will have different boiling points. A sample of crude oil will therefore have a range of boiling points, and the mixture can be separated by fractional distillation.
Organic compounds belong to different families, though all are based on carbon C, hydrogen H, and other elements such as oxygen O and nitrogen N etc.
The compounds in each family have a similar chemical structure and a similar chemical formula. Each family of organic compounds forms what is called a homologous series.
Different families arise because carbon atoms readily join together in chains (catenation) and strongly bond with other atoms such as hydrogen, oxygen and nitrogen.
The result is a huge variety of 'organic compounds'. The name comes from the fact that most of the original organic compounds studied by chemists came from plants or animals.
A homologous series is a family of compounds which have a general formula and have similar chemical properties because they have the same functional group of atoms (e.g. C=C alkene, C-OH alcohol or -COOH carboxylic acid)
Members of a homologous series have similar physical properties such as appearance, melting/boiling points, solubility etc. but show trends in them e.g. steady increase in melting/boiling point with increase in carbon number or molecular mass.
The molecular formula represents a summary of all the atoms in the molecule.
The structural or displayed formula shows the full structure of the molecule with all the individual bonds and atoms shown (though there are different 'sub-styles' of varying detail, see below).
Alkanes are the simplest homologous series of compounds and their names follow this pattern, CH4 - methane C2H6 - ethane C3H8 - propane C4H10 - butane C5H12 - pentane
I.e. they have a prefix (meth-, eth-, prop-, but-, etc.), which depends on the number of carbon atoms in the molecule and a common suffix (-ane).
The general chemical form la for an alkane is CnH2n+2
As well as using a normal type of molecular formula to describe an organic molecule, they can be represented by drawing out their structure i.e. by showing how the atoms are connected, or bonded to each other.
In order to do this a few rules have to be followed;
Carbon atoms must be bonded four times
Hydrogen atoms must bond only once.
Isomerism occurs when two or more compounds have the same chemical formula but have different structures, e.g. for the molecular formula C4H10 there are two possibilities - one 'linear' and one with carbon chain 'branching’.
Butane is linear while its branched isomer is methyl propane.
As the number of carbon atoms increases, the number of possible isomers increases rapidly. All families or homologous series exhibit isomerism.
Number of structural isomers
Physical properties of alkanes
Lower molecular weight alkanes are gases. Methane, ethane, propane and butane are gases at ordinary room temperature. Higher alkanes up to those having 17 carbon atoms are liquids; higher alkanes are solids at room temperature.
Melting and Boiling Points
Homologous alkanes show increase in melting and boiling points. Similar to the behavior of elements in the same group in a periodic table.
Alkanes, like all other organic chemicals are insoluble in water. They are however soluble in organic liquids. Alkanes are non-polar and are hence soluble in other non-polar liquids and not in water, as water is a polar molecule.
Chemical Reactions of Alkanes
Substitutional reactions of alkanes
Alkanes are most inert of all homologous series.
They are not very reactive unless burned. But they will react with strong oxidising chemicals like chlorine when heated or subjected to u.v light.
A substitution reaction occurs and a chloro-alkane is formed e.g. a hydrogen atom is swapped for a chlorine atom and the hydrogen combines with a chlorine atom forming hydrogen chloride. This process is called halogenation.
The UV light causes the formation of free radical halogen atoms by providing enough energy for the bond between the two halogen atoms to break. A halogen atom attacks the alkane, substituting itself for a hydrogen atom. This substitution may occur many times in an alkane before the reaction is finished.
Alkanes, along with all other types of hydrocarbon, will burn in an excess of oxygen to give carbon dioxide and water only as the products, e.g.CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
in general, CnH2n+2(g) + (1.5n+0.5)O2(g) → nCO2(g) + (n+1)H2O(g)
If there is not enough oxygen present then instead of carbon dioxide, carbon monoxide, CO, is produced.
Carbon monoxide is particularly toxic and absorbed into blood, through respiration, very easily.
For domestic heating systems it is particularly important that enough air can get to the flame to avoid carbon monoxide being generated in the home.
Car engines also require a lot of air and there is a lot of research going on to make the internal combustion engine more efficient, and so put out less carbon monoxide.
Alkanes are saturated hydrocarbons.
Molecules of saturated hydrocarbons contain only single bonds between all carbon atoms in the series.
Hence their reactivity with other chemicals is relatively low.
Hydrocarbons, which contain two hydrogen atoms less than the corresponding alkanes, are called alkenes.
They have one double bond and are unsaturated carbon compounds. Alkenes cannot be obtained directly from crude oil.
They can only be obtained by cracking of alkanes.
In industry the fractions obtained from the fractional distillation of crude oil are heated at high pressure in the presence of a catalyst to produce shorter chain alkanes and alkenes. E.g. C10H22(s) → C5H12(s) + C5H10(s)
They are unsaturated hydrocarbons with a general formula CnH2n. Unsaturated means the molecule has a C=C double bond to which atoms or groups can add after breaking the double bond.
All alkenes have a C=C double bond in their structure and their names follow this pattern. Their names end in ...ene C2H4 - ethene C3H6 - propene C4H8 - butene C5H10 - pentene
The general chemical formula for an alkene is CnH2n
Addition Reactions of Akenes
The double bond of an alkene will undergo an addition reaction with aqueous bromine to give a dibromo compound.
The orange bromine water is decolourised in the process. E.g. ethene reacts with bromine water to give 1,2-dibromoethane,
Alkenes may be turned into alkanes by reacting the alkene with hydrogen gas at a high temperature and high pressure.
A nickel catalyst is also needed to accomplish this addition reaction. E.g. ethene reacts with hydrogen to give ethane,
This reaction is also called saturation of the double bond.
In ethene the carbon atoms are said to be unsaturated.
In ethane the carbon atoms have the maximum number of hydrogen atoms bonded to them, and are said to be saturated.
The carbon-carbon double bond may also be oxidised i.e. have oxygen added to it.
This is accomplished by using acidified potassium manganate (VII) solution at room temperature and pressure.
The purple manganate (VII) solution is decolourised during the reaction. E.g. ethene reacts with acidified potassium manganate (VII)(aq) to give ethan-1,2-diol,
All alkenes will react with free radical initiators to form polymers by a free radical addition reaction.
Some definitions -
monomer - a single unit e.g. an alkene.
The alkene monomer has the general formula:
Where R is any group of atoms, e.g. R=CH3 for propene.
The reaction progresses by the separate units joining up to form giant, long chains -
Polymer- a material produced from many separate single monomer units joined up together.
An addition polymer is simply named after the monomer alkene that it is prepared from.
The structure above shows just 4 separate monomer units joined together. In a real polymer, however, there could be 1000's of units joined up to form the chains. This would be extremely difficult to draw out and so the structure is often shortened to a repeat unit.
There are 3 stages to think about when drawing a repeat unit for a polymer -
Draw the structure of the desired monomer.
Change the double bond into a single bond and draw bonds going left and right from the carbon atoms.
Place large brackets around the structure and a subscript n and there is the repeat unit.
Platic bags, bowls, packaging
Insulation and pipes
Crates, boxes, plastic ropes
Non-stick frying pans
Laboratory preparation of ethene gas
In the lab ethene is prepared by cracking kerosene or candle wax.
Kerosene is poured over sand and this is kept at the bottom of a hard glass test tube.
A few pieces of pumice stone or porcelain is kept a little distance away. The sand is slowly heated.
After a while the porcelain portion of the test tube is heated. This is done alternately.
The heated kerosene first vaporizes and then cracks.
When the vapours pass over the hot porcelain, they crack again into smaller and smaller molecules.
The gases are then passed over water. Ethene is collected by downward displacement of water.
It can be understood that this method for collecting ethene gas does not give pure ethene gas.
This is because from cracking, we get many types of molecules. All those, which are lighter than water and insoluble in water, will be collected.
Ethene by dehydration of alcohols
To obtain pure ethene gas, another method is followed.
This is from a chemical reaction with ethanol and concentrated sulphuric acid.
The temperature of the mixture of ethanol or ethyl alcohol and concentrated sulphuric acid is increased to 160°C.
The acid acts as a dehydrating agent and picks up a water molecule from the ethanol molecule, leaving the reaction product as ethene gas.
The laboratory equipment to produce ethene gas is shown below. About 20 to 25 ml of ethanol is taken in a round bottomed flask.
Concentrated sulphuric acid is added to it from a thistle funnel slowly. Heat is supplied from a Bunsen burner and the temperature of the flask is raised to 160°C.
Ethene gas starts evolving and it can be collected over water by downward displacement of water.
Uses of ethene
Ethene is used for manufacturing organic compounds such as ethyl alcohol and ethylene glycol. Ethylene glycol is used for making artificial fibbers like polyesters.
Ethene is used for manufacture of plastics. These plastics are made from polymerization of ethene into polythene. Polythenes are used for making bags, electrical insulation, etc.
Ethene is used artificial ripening of fruits such as mangoes, bananas, etc.
Hydrocarbons that have two carbon atoms in a triple bond are called alkynes. They are unsaturated bonds.
Their general formula is CnH2n-2 and their names are derived from the alkanes by changing the ending “ane’ of the alkane by “yne”, for example, ethyne, propyne, butyne, etc.
The simplest of alkynes has two carbon atoms in triple bond and is called ethyne. The table below gives names of the first three alkynes.
Known commonly as
Number of C-atoms
Number of H-atoms
The structural formula that is the actual arrangement of different atoms in space of the simplest alkyne namely ethyne (CH ≡ CH ) is shown below. H - C ≡ C - H Acetylene (contains a triple bond)
Chemical Properties of Ethyne
Ethyne burns in air with a sooty flame. It forms carbon dioxide and water and gives out heat. HC ≡ CH + 5O2 → 4CO2 + 2H2O + heat
The sooty flame is due to higher amount of carbon in ethyne than in methane.
All the carbon atoms cannot get oxidized while burning this makes the flame sooty.
But if ethyne is burnt with a proper control, for example, if the gas is made to pass through a small nozzle, then it gets ample air mixture to burn completely.
This type of complete combustion is used for acetylene lamps in industries. Acetylene lamps produce very luminous non-sooty flame.
Ethyne combined well with oxygen can burn to give a flame whose temperature is 30000C.
This oxy-acetylene flame is used for welding metals, where very high temperatures are required.
Alkynes are more reactive than the alkanes or alkenes due to the presence of unsaturated bonds. Such a reaction is called addition reaction.
In an addition reaction, the alkynes will become an alkane. For example if ethyne is reacted with chlorine, it becomes 1,1,2,2 tetra-chloro-ethane.
Similarly, addition reaction with bromine will give rise to 1,1,2,2, tetra-bromo-ethane. Bromine water decolorizes on reaction with ethyne. This is a prominent test for testing unsaturated nature of hydrocarbons.
When hydrogen is added to ethyne, and heated in the presence of nickel, it becomes ethene and then proceeds to become ethane. The bonds become saturated.
This is known as the process of hydrogenation. The addition of hydrogen to a double or triple bonded hydrocarbon leads to saturation of the bonds.
When hydrochloric acid is added to ethyne, it becomes first chloro-ethene and then 1,1- dichloro-ethane. The reaction is shown below.
Uses of ethyne
Ethyne burns in oxygen to give a very luminous light. Hawkers use this as lamps.
Ethyne is used for oxy-acetylene flame used for industrial welding.
Ethyne is used for manufacture of synthetic plastics, synthetic rubbers, and synthetic fibers.
Ethyne is also used making many industrially useful organic compounds like acetaldehyde, acetic acid, etc.
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