Alkanes Flashcards
What is an alkane?
A saturated hydrocarbon containing C-H bonds only.
What is the general formula of the alkane?
CnH2n+2
Are alkane bonds polar? Why/why not?
Nonpolar because carbon and hydrogen have similar electronegativities.
What is the shape and angle of an alkane?
Tetrahedral, 109.5 degrees
Describe the σ-bond in alkanes.
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 increased number of induced dipole-dipole interactions. Therefore more energy is 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 alkanes’ 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 is 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 dimming
How are halogenoalkanes formed from alkanes?
Radical substitution
In the presence of what does alkane react with halogens?
UV light
What are the three stages of free radical substitution?
Initiation - breaking halogen bond to form free radical
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* (uv light)
Propagation: Cl* + CH4 —> HCl + CH3*
CH3* + Cl2 —> CH3Cl + Cl*
Termination: CH3* + Cl* —> CH3Cl
2Cl* —> Cl2
CH3* + CH3* —> CH3CH3
