Unit 6 chemistry

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Chemistry
Unit 6
There are several types of hydrocarbon. This unit deals with naming
hydrocarbons and their structure. Alkanes and Alkenes undergo a
variety of important reactions, many of which are important
processes in the chemical industry.
Hydrocarbons
A hydrocarbon is a substance which contains only hydrogen and
carbon. The atoms within a hydrocarbon are held together by
covalent bonds.
Methane
Methane has a formula CH4.
Remember: Carbon is in Group 4 so has a valency of 4 (it forms 4
bonds). Hydrogen is in group 1 so has a valency of 1 (it forms 1
bond).
Ethane
Ethane is another hydrocarbon gas. Molecules of ethane have two
carbon atoms. If we build a model of a hydrocarbon with two
carbon atoms we find we need six hydrogen's to meet the rule that
each carbon must form 4 bonds and each hydrogen 1 bond.
C2H6 is an easy way of writing the formula for the molecules of
ethane. It does not give information about the structure of the
molecules.
Alkanes
Methane and ethane are the first two numbers of a series of
hydrocarbons called the alkanes.
The alkanes are a subset of the set of hydrocarbons.
Each member of the alkane series has a name which ends in '-ane'

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Chemistry
and a prefix which indicates the number of carbon atoms in the
molecule.
Prefix: meth, No. of Carbon Atoms in the Molecule: 1, i.e. CH4
(Methane).
Prefix: eth, No. of Carbon Atoms in the Molecule: 2, i.e. C2H6
(Ethane).
Prefix: prop, No. of Carbon Atoms in the Molecule: 3, i.e. C3H8
(Propane).
Prefix: but, No. of Carbon Atoms in the Molecule: 4, i.e. C4H10
(Butane).
Prefix: pent, No. of Carbon Atoms in the Molecule: 5, i.e. C5H12
(Pentane).
Prefix: hex, No.…read more

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Chemistry
Ethane
Hydrocarbons exist where the carbon atoms are joined by a
covalent double bond c=c. The simplest one is called ethane.
CH2=CH2 --> C2H4.
In the shortened structural formula it is useful to show the double
bond.
Alkenes
Ethane is the first member of a series of hydrocarbons called the
alkenes. The alkenes are another subset of hydrocarbons.
Each member of the alkene series has a name which ends in '-ene'
and a prefix which indicates the number of carbon atoms in the
molecule.…read more

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Chemistry
However, it is possible to put atoms together in a different
arrangement.
What we have formed are two isomers of butane. Both have the
same molecular formula but different structural formula.
Pentane has three isomers. All have the same formula C5H12 but all
have different structures. Butane and Cyclobutane are isomers.
Both have formula C4H8 but the structures are different
It is actually possible to create another two isomers, both types of
butane:
Note: Alkanes de-colourise bromine water.…read more

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Chemistry
C3H8 + Br2 ---> C3H7Br +HBr.
When a bromine solution is added to an alkene it de-colourises the
bromine solution very quickly.
Saturated or Unsaturated
Saturated hydrocarbons have only simple bonds. Unsaturated
hydrocarbons contain at least one double (or triple) C to C bond.
We can use the bromine solution test to determine whether not a
hydrocarbon is saturated or unsaturated. Unsaturated
hydrocarbons will de-colourise bromine solution very quickly due to
the presence of a C to C bond.…read more

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Chemistry
Safety Note: Watch for suck-back. Do not stop heating the test-tube
before removing the delivery tube from the water.
Products of Cracking
For the following examples octane -C8H18- will be used but the
same principle applies to all alkanes. When octane is cracked the
molecule can break up in many different ways.
Vocabulary
Alkane - a hydrocarbon containing only single covalent carbon to
carbon bonds.
General Formula - a formula that can be applied to all the members
of a homologous series e.g.…read more

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Chemistry
Isomers 0 compounds having the same molecular formula but
different structural formula.
Addition reaction 0 a reaction in which an alkene breaks open its
double bond and adds atoms (Br2) to adjacent carbon atoms.
Cracking 0 the splitting of large saturated molecules to give a
mixture of small molecules, some saturated and some unsaturated.
Unit 6 - Hydrocarbons Summary
The simplest family of hydrocarbons is called the alkanes.
Families of this kind are called homologous series.…read more

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Chemistry
Each homologous series has a general formula. For alkanes it is
CnH2n+2, for alkenes it's CnH2n.
Butane and Cyclobutene are isomers since they have the same
molecular formula (C4H8) but different structural formula.
Alkenes are named after the equivalent alkanes with the ending
'-ene' to '-ane'.
All hydrocarbons burn producing carbon dioxide and water. The
smaller molecules are all dangerously flammable.
Hydrocarbons do not mix with water, but usually float on top.…read more

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Chemistry
The general formula for the alkane series is CnH2n+2.
The alkanes are members of a homologous series with the same
general formula, similar chemical reactions i.e. substitution and as
the molecular weight increases the boiling point increases.
Cycloalkanes are Carbon and Hydrogen atoms joined in rings. The
simplest is Cyclopropane.
Cycloalkanes have the general formula CnH2n. The three you
should know are Cyclopropane (C3H6), Cyclobutane (C4H8) and
Cyclopentane (C5H1O).
Isomers are compounds with the same molecular formula and
different structural formula.…read more

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Chemistry
A mixture of compounds is produced by cracking larger molecules
into smaller and more useful molecules. One of this mixture
however is unsaturated and can be used to make plastics.
A main use of the cracking of hydrocarbons is to increase the
volume of petrol required to meet demand.…read more

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