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Writing Equations for Cracking
We can use the general formulae for alkanes and alkenes to check that we have correctly written out equations for cracking.
Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules.
Ethene as the starting material for the production of alcohol and butane as a fuel.
The equation for this reaction is:
C6H14 ⟶ C4H10 + C2H4
Note that the starting compound for this reaction is an alkane and thus the general formula CnH2n+2 applies.
Butane is also an alkane and so the same rule applies.
Ethene is an alkene and so its formula will follow the C2H2n rule.
Definition:
Crude oil -
Oil we find underground.
It is formed from partially decayed plants and animals, under mud, millions of years ago.
Hydrocarbon:
The definition of a hydrocarbon is a compound that contains only hydrogen and carbon atoms.
Hydrocarbons are the simplest organic compounds and they have very useful properties.
Crude oil is also called petroleum and is a complex mixture of hydrocarbons which also contains natural gas.
Hydrocarbons are compounds that contain large molecules made of carbon and hydrogen only.
The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms.
The mixture contains molecules with many different ring sizes and chain lengths.
It is a thick, sticky, black liquid that is found under porous rock (under the ground and under the sea).
Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals.
Since it is being used up much faster than it is being formed crude oil is a finite resource.
Separation of crude oil
The hydrocarbons in crude oil are separated using fractional distillation.
This happens at an oil refinery in a tower called a fractionating column.
In the fractioning column the hydrocarbons in crude oil are separated as they have different boiling points
Crude oil as a mixture isn’t a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are useful, with each fraction having different applications.
Each fraction consists of groups of hydrocarbons of similar chain lengths.
The fractions in petroleum are separated from each other in a process called fractions distillation.
The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain.
The size and length of each hydrocarbon molecule determines in which fraction it will be separated into.
The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains.
Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them.
Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top.
Crude oil enters the fractionating column and is heated so vapours rise.
Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column.
Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off.
The different fractions condense at different heights according to their boiling points and are tapped off as liquids.
The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases.
The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column.
Properties of the main fractions of crude oil
Viscosity: This refers to the ease of flow of a liquid. High viscosity liquids are thick and flow less easily. If the number of carbon atoms increases, the attraction between the hydrocarbon molecules also increases which results in the liquid becoming more viscous with the increasing length of the hydrocarbon chain. The liquid flows less easily with increasing molecular mass
Colour: As carbon chain length increases the colour of the liquid gets darker as it gets thicker and more viscous
Melting point/boiling point: As the molecules get larger, the intermolecular attraction becomes greater. So more heat is needed to separate the molecules. With increasing molecular size there is an increase in boiling point
Volatility: Volatility refers to the tendency of a substance to vaporise. With increasing molecular size hydrocarbon liquids become less volatile. This is because the attraction between the molecules increases with increasing molecular size
Chemical Properties :
Fuel: Substance which when burned, releases heat energy.
Complete combustion: Occurs when there is an unlimited supply of air so that elements in the fuel react fully with oxygen.
Complete combustion of hydrocarbons:
During the complete combustion of hydrocarbons, carbon dioxide and water will be produced:
Carbon will oxidise to form carbon dioxide
Hydrogen will oxidise to water
Equation:
Hydrocarbon + Oxygen → Carbon dioxide + Water
Incomplete combustion: Occurs when there is a limited supply of air so that elements in the fuel do not fully react with Oxygen
During the incomplete combustion of hydrocarbons, water will still be produced but instead of carbon dioxide, carbon monoxide (soot) will form
Equation:
Hydrocarbon + Oxygen → Carbon Monoxide + Water
In a substitution reaction, one atom is swapped with another atom. Alkanes undergo a substitution reaction with halogens under the presence of ultraviolet radiation.
Example: Under ultraviolet (UV) radiation, methane reacts with bromine
This reaction is a substitution reaction because one of the hydrogen atoms from the methane is replaced by a bromine atom
CH4 + Br2 =CH3Br + HBr
CH3Br + Br2 = CH2Br2 + HBr
CH2Br2 +Br2 =CHBr3 +HBr
CHBr3 +bR2 = CBr4 +HBr
Overall reaction: CH4 +4Br2 = CBr4 + 4HBr
Why Cracking is Necessary
Although there is use for each fraction after the fractional distillation of crude oil, the amount of longer chain hydrocarbons produced is far greater than needed.
However, the amount of shorter chain hydrocarbons produced is far less than needed (e.g. gasoline fraction) and there is a higher demand for shorter chain hydrocarbons. This is why cracking is necessary to increase the supply of shorter chain hydrocarbons.
Short chain hydrocarbons burn well and flow well.
Cracking is used to produce petrol for cars.
Cracking also produces alkenes which are raw material in the plastic industry.
Cracking is used to produce hydrogen gas which is a raw material in manufacture of ammonia in Haber process.
Example:
The Cracking of hexane (C6H14) to produce butane (C4H10) and ethene (C2H4):
C6H14 (g) → C4H10 (g) + C2H4 (g)
Catalytic Cracking
Long-chain hydrocarbons: large number of hydrocarbon molecules. More viscous and less flammable so less useful.
Short-chain hydrocarbons: small numbers of hydrocarbon molecules.
Cracking is simply splitting of larger molecules to simpler ones.
Explanation:
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules.
Fractions containing large hydrocarbon molecules are heated at 600 – 700°c to vaporise them.
Vapours will then pass over a hot catalyst of silica or alumina
This process breaks covalent bonds in the molecules, causing thermal decomposition reactions.
As a result, cracking produces smaller alkanes and alkenes. The molecules are broken up in a random way which produce a mixture of alkanes and alkenes.
Distinguishing between alkanes and alkenes
Alkanes and alkenes have different molecular structures
All alkanes are saturated and alkenes are unsaturated
The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot
This allows us to tell alkenes apart from alkanes using a simple chemical test:
Explanation:
Bromine water is an orange coloured solution of bromine
When bromine water is shaken with an Alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution
But when bromine water is shaken with an alkene, the alkene will decolourise the bromine water and turn colourless as alkenes do have double carbon bonds (C=C)
The bromine atoms add across the C=C double bond hence the solution no longer contains the orange coloured bromine
This reaction between alkenes and bromine is called an addition reaction
Writing Equations for Cracking
We can use the general formulae for alkanes and alkenes to check that we have correctly written out equations for cracking.
Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules.
Ethene as the starting material for the production of alcohol and butane as a fuel.
The equation for this reaction is:
C6H14 = C4H10 + C2H4
Note that the starting compound for this reaction is an alkane and thus the general formula CnH2n+2 applies.
Butane is also an alkane and so the same rule applies.
Ethene is an alkene and so its formula will follow the C2H2n rule.
Alkanes can be cracked into
one alkane and one one alkene
one alkane and 2 alkenes
one alkene and hydrogrn gas.
Homologous Series:
A series of compounds in which successive members differ from one another by a CH2 unit is called a homologous series. Thus, the series CH4, C2H6, C3H8 . . .
General formula of Alkanes:
CnH2n+2, is an example of a homologous series.
Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. Alkanes are often called saturated hydrocarbons because they have the maximum possible number of hydrogens per carbon. In Section 1.7, thealkane molecule, ethane, was shown to contain a C-C sigma bond. By adding more C-C sigma bond larger and more complexed alkanes can be formed. Methane (CH4), ethane (C2H6), and propane (C3H8) are the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH2 unit. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a homologous series. The members of such a series, called homologs, have properties that vary in a regular and predictable manner.
Molecular Formulas
Alkanes are the simplest family of hydrocarbons - compounds containing carbon and hydrogen only. Alkanes only contain carbon-hydrogen bonds and carbon-carbon single bonds. The first six alkanes are as follows:
Isomerism
All of the alkanes containing 4 or more carbon atoms show structural isomerism, meaning that there are two or more different structural formulas that you can draw for each molecular formula. Isomers (from the Greek isos + meros, meaning "made of the same parts") are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Alkanes with 1-3 carbons, methane (CH4), ethane (C2H6), and propane (C3H8,) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds. However, C4H10, has more than possible structure. The four carbons can be drawn in a row to form butane or the can branch to form isobutane. The two compounds have different properties—for example, butane boils at −0.5°C, while isobutane boils at −11.7°C.
methane CH4 ethane C2H6
propane C3H8 butane C4H10
pentane C5H12 hexane C6H14
Isomerism
All of the alkanes containing 4 or more carbon atoms show structural isomerism, meaning that there are two or more different structural formulas that you can draw for each molecular formula. Isomers (from the Greek isos + meros, meaning "made of the same parts") are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Alkanes with 1-3 carbons, methane (CH4), ethane (C2H6), and propane (C3H8,) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds. However, C4H10, has more than possible structure. The four carbons can be drawn in a row to form butane or the can branch to form isobutane. The two compounds have different properties—for example, butane boils at −0.5°C, while isobutane boils at −11.7°C.
CH4 Meth CH4 methane
C2H6 Eth CH3CH3 ethane
C3H8 Prop CH3CH2CH3 propane
C4H10 But CH3CH2CH2CH3 butane
C5H12 Pent CH3CH2CH2CH2CH3 pentane
C6H14 Hex CH3(CH2)4CH3 hexane
C7H16 Hept CH3(CH2)5CH3 heptane
C8H18 Oct CH3(CH2)6CH3 octane
C9H20 Non CH3(CH2)7CH3 nonane
C10H22 Dec CH3(CH2)8CH3 decane
Exercises
1) Give all the isomers for C6H14O that contain a straight-chain hexane and an OH group.
2) Draw all possible isomers for the following compounds.
a) C6H14 (There are five total)
b) C3H6
Of the structures show above, butane and pentane are called normal alkanes or straight-chain alkanes, indicating that all contain a single continuous chain of carbon atoms and can be represented by a projection formula whose carbon atoms are in a straight line. The other structures, isobutane, isopentane, and neopentane are called called branched-chain alkanes. As the number of carbons in an akane increases the number of possible isomers also increases.
Unsaturated Hydrocarbons
All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C.
All alkenes contain a double carbon bond, which is the alkene functional group and is what allows alkenes to react in ways that alkanes cannot.
The names and structure of the first four alkenes are shown below.
The general formula for alkenes is:
CnH2n
E.g. a straight chain alkene with 8 carbons has 8 x 2 = 16 hydrogen atoms, so it has twice as many hydrogens as carbons.
Compounds that have a C=C double bond are also called unsaturated compounds.
That means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group.
Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds.
This makes them much more reactive than alkanes.
The numbers in butene and pentene refer to the carbon atom in which the C=C begins, counting from the left. E.g. pent-2-ene, C5H10 has the C=C between the 2nd and 3rd carbon atoms. In pent-3-ene the C=C bond is between the 3rd and 4th carbon atoms from the left.
Addition Reactions of Alkenes
The chemistry of the alkenes is determined by the C=C functional group.
Since all members of the alkene homologous series contain the same functional group then they all react similarly.
Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
The carbon carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbon.
Hydrogen, water and the halogens can take part in these reactions.
Hydrogenation
Alkenes undergo addition reactions with hydrogen in which an alkane is formed.
These are hydrogenation reactions and occur at 150ºC using a nickel catalyst.
Hydrogenation reactions are used to change vegetable oils into margarine to be sold in supermarkets.
Hydration
Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction.
The reaction is very important industrially for the production of alcohols and it occurs using the following conditions:
Temperature of around 330ºC.
Pressure of 60 – 70 atm.
Concentrated phosphoric acid catalyst.
When the reaction is complete, the reaction chamber holds unreacted ethene, ethanol and water.
The contents are transferred to a condenser where ethene is separated easily as it has a much lower boiling point than ethanol and water:
Ethanol: 78oC
Ethene: -103oC
Water: 100oC
The ethanol and water are separated afterwards by fractional distillation.
Halogenation
The halogens also participate in addition reactions with alkenes.
The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond.
Alkenes are more reactive than alkanes due to the presence of the carbon carbon double bond which contains an area of high electron density.
Combustion of Alkenes
These compounds undergo complete and incomplete combustion but because of the carbon carbon double they tend to undergo incomplete combustion, producing a smoky flame in air.
Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:
C4H8+ 6O2→ 4CO2+ 4H2O
Incomplete combustion occurs when there is insufficient oxygen to burn so a mixture of products can form, e.g:
C4H8+ 5O2→ 2CO + 4H2O + 2CO2
CH4+ 3O2→ 2C + 2H2O + 2CO
Bromination of Ethene
Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
The reaction between bromine and ethene is an example of an addition reaction .
The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond.
Cycloalkanes: Cycloalkanes with one ring have the general formula CnH2n compared with the general formula CnH(2n + 2) for acyclic alkanes. Cycloalkanes have two fewer hydrogen atoms than alkanes because another carbon–carbon bond is needed to form the ring.
Isomerism in Alkenes
Alkenes show both structural isomerism and geometrical isomerism.
Structural Isomerism
Ethene and propene have only one structure. Alkenes higher than propene have different structures. Let us see how many structural isomers an alkene with formula C4H8 has.
Branched And Unbranched Alkenes ( Isomerism in Alkenes):
Isomers can be created in Alkenes by changing the direction of Carbon atoms or by changing the position of Carbon Carbon double bonds (C=C).
They have the same molecular formula C4H8, but have different structural formula.
Butene is a straight-chain unsaturated hydrocarbon while methylpropene is a branched-chain, unsaturated hydrocarbon.
Butene and methylpropene have different melting and boiling point.
As the # of Carbon atoms increases, the number of isomers also increases.
As they have same molecular formula, therefore their percentage composition by mass remains the same.
Ring-chain Isomerism
Alkenes forms ring chain isomers like cycloalkanes
Alkynes are ring-chain isomers with cycloalkenes.
Many cycloalkanes are used in motor fuel, natural gas, kerosene, diesel and other heavy oils.