4.1.2 - Alkanes
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What is an alkane?
A saturated hydrocarbon containing C-H bonds only
What is the general formula of an alkane?
Cn H2n+2
Are alkane bonds polar? Why / why not?
Non polar because carbon and hydrogen have similar electronegativities
What is the shape and angle of an alkane?
Tetrahedral 109.5 degrees
Describe the sigma bond in alkane
The sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms
What type of intermolecular forces do alkanes have> why?
London force -> Induced dipole - dipole interaction Because the bonds are nonpolar
What happens to the boiling point as alkane chain length increases? Why?
The boiling point increases because there is more surface area and so more number of induced dipole - dipole interaction. Therefore more energy required to overcome the attraction
Does a branched molecule have lower or higher boiling point compared to equivalent straight chain? Why?
The branched molecule has a lower boiling point because they have fewer surface area and hence less induced dipole-dipole interactions
Are alkanes soluble in water? Why?
Insoluble Because hydrogen bonds in water are stronger than alkane’s london forces of attraction
How reactive are alkanes?
Very unreactive
What reactions will alkanes undergo?
Combustion and reaction with halogens
What type of reaction is combustion?
Oxidation reaction
What is complete combustion?
Combustion that occurs with plentiful supply of air
What are the products of complete combustion when alkanes are used?
Carbon dioxide and water
What is the colour of the bunsen burner flame during complete combustion?
Blue flame
What is incomplete combustion and what products are formed in the case of alkanes?
Combustion in a limited supply of oxygen Products: water, carbon dioxide and carbon monoxide
Write an equation for the complete combustion of propane
C3H8 + 5O2 -> 3CO2 + 4H2O
What type of hydrocarbon are most likely to undergo incomplete combustion?
Longer chains
What is the environmental impact of carbon monoxide?
It is toxic / poisonous
What is the environmental impact of soot (carbon)?
Asthma, cancer, global warming
How are halogenoalkanes formed from alkanes?
Radical substitution
In the presence off what does alkane react with halogens?
UV light
What are the three stages of free radical substitution?
Initiation Breaking halogen bond to form free radicals Propagation Chain part of the reaction where products are formed but free radical remains Termination Free radicals removed, stable products formed
Write equations for the reaction of CH4 with Cl2 to form CH3Cl
Initiation: Cl2 -> 2CL radicals (in presence of UV light) Propagation: CL radical + CH4 -> HCL and CH3 radical CH3 radical + cl2 -> CH3Cl + Cl radical Termination: CH3 radical + CH3 radical = CH3CH3 CH3 radical + cl radical = CH3Cl 2Cl radicals = Cl2