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Reactions of Alkanes: Combustion hydrocarbons (only contain C & H) complete combustion alkanes burn in an excess supply of oxygen to form carbon dioxide and water: example: C 8 H 18 (g) + 12 ½ O 2 (g) → 8 CO 2 (g) + 9 H 2 O (l) exothermic (-∆H) incomplete combustion if oxygen supply is limited, the gas carbon monoxide and carbon is formed C 8 H 18 (l) + O 2 (g) → C (s) + CO (g) + CO 2 (g) + H 2 O (l) (notice left over carbon (black soot) and dangerous CO)
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Alkanes and AlkenesTopic 10.2 and 10.3
Alkanes have low reactivity
bond enthalpies are relatively strong348 kJ mol-1 to break a C-C bond412 kJ mol-1 to break a C-H bond
low polarity only readily undergo combustion reactions
with oxygen (very flammable) and substitution reactions with halogens in UV light
Reactions of Alkanes: Combustion hydrocarbons (only contain C & H) complete combustion
alkanes burn in an excess supply of oxygen to form carbon dioxide and water: example:
C8H18 (g) + 12 ½ O2 (g) → 8 CO2 (g) + 9 H2O (l)
exothermic (-∆H) incomplete combustion
if oxygen supply is limited, the gas carbon monoxide and carbon is formed C8H18 (l) + O2 (g) → C (s) + CO (g) + CO2 (g) + H2O (l)(notice left over carbon (black soot) and dangerous CO)
Reactions of Alkanes (methane and ethane) with Halogens (Cl and Br) alkanes do not react with halogens in the
dark at room temperature, but will react in the presence of sunlight (UV) a substitution reaction will occur where
some or all of the hydrogens will be replaced with a halogen C2H6 (g) + Br2 (g) → C2H5Br (l) + HBr (g) Cl2 + CH4 CH3Cl + HCl
this happens by a process know as free radical substitution that happens in 3 steps1. Initiation2. Propagation3. Termination
initiation initiated by UV light breaking a chlorine molecule into two free radicals by a process called homolytical fission (* = unpaired electron)
Cl2 Cl* + Cl* propagation
keeps the chain going (radical in reactants and products)
CH4 + Cl* CH3* + HCl
CH3* + Cl2 CH3Cl + Cl*
termination this removes free radicals (*) from the system without
replacing them by new ones
Cl* + Cl* Cl2 CH3
* + Cl * CH3Cl CH3
* + CH3* CH3CH3
each resulting atom receives one unpaired electron, known as free radicals that have lots of energy
each resulting atom receives one unpaired
electron, known as free radicals
Reactions of Alkenes (Topic 10.3.1):the general mechanism alkenes react with many substances to
form a new substance catalysts, acids or other substances may
be required to complete the reaction:C2H4 + XY → CH2XCH2Y
process occurs by breaking the double bond.
Reactions of Alkenes (10.3.2) : with hydrogen
alkenes react with hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC:
C2H4 + H2 → CH3CH3
Reactions of Alkenes: with halogens
alkenes react readily with chlorine or bromine to create a dihalogenalkane (general name)
C2H4 + 2 Cl2 → CH2Cl CH2Cl
H
H H
H
+HH
ClHH
ClCl Cl
1,2-dichloroethane
Reactions of Alkenes (10.3.2)with hydrogen halides
alkenes react readily with hydrogen halides to create a halogenalkanesC2H4 + HBr → CH3CH2Br
H
H H
H
+HH
HHH
ClH Cl
Reactions of Alkenes with water alkenes do not react readily with water. if sulfuric acid is used as a catalyst, an
alcohol to be created remember that H2O can be dissociated into H+ and
OH-
C2H4 + H2O → CH3CH2OH
H
H H
H
+ OHH
a c idO
H
H
H H
H
H
Distinguish between alkanes and alkenes using bromine water
bromine water (a red liquid) tests for unsaturated hydrocarbons (alkenes) alkanes → stay yellow/orange
no reaction alkenes → turn clear / colourless
because of reaction with unsaturated hydrocarbon
http://www.youtube.com/watch?v=NjIuBvod2eM
Reactions of Alkenes: Polymerization naming polymers
put “poly-” in front of the name of the monomer
there are 3 polymerization mechanisms that you need to be familiar with:
1. polyethene2. polychloroethene3. polypropene
Polyethene monomer: ethene CH2=CH2
undergoes additions reactions with itself to make a chain
n CH2=CH2 [-CH2-CH2-]n
Polychloroethene each chloroethene contains 1 chlorine therefore when the chloroethene
molecules polymerize, every other carbon will bond to 1 chlorine
this is PVC
Polypropene
+ =
Teflon non-stick pans
Outline the economic importance of the reactions of alkenes (10.3.5) making margarine
hydrogenation (addition of H) of vegetable oils (alkenes)
making ethanol ethene + water
drink it or use as fuel
H
H H
H
+ OHH
a c idO
H
H
H H
H
H
making plastics Teflon
tetrafluoroethene
PVCpolychloroethen
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