Hydrocarbons Flashcards
Are alkanes saturated or unsaturated hydrocarbons?
Saturated.
What is the general formula of alkanes?
CnH2n+2
What bond shape is around each carbon in alkanes?
Tetrahedral.
Why is there a bond angle of 109.5 degrees around each carbon in an alkane?
Each carbon atom has four pairs of bonding electrons around it and they repel each other equally, forming a tetrahedral shape around each carbon.
What is a sigma bond?
An overlapping of orbitals between the bonding atoms.
Where can sigma bonds form in an alkane?
C-C and C-H bonds.
Do sigma bonds rotate?
Yes.
Why does an alkane with a longer chain have a higher boiling point?
The longer the chain, the stronger the London forces because there’s more surface contact and electrons to interact. This means it requires more energy to break the London forces and so the boiling point is higher.
Why do branched alkanes tend to have lower boiling points?
Branched alkanes cannot pack as closely together and so there’s less surface area contact. This means that London forces are reduced and so it requires less energy to break these forces. This results in a lower boiling point.
How reactive are alkanes?
Relatively unreactive.
Show the equation for the complete combustion of propane. Use state symbols.
C3H8 (g) + 5O2 (g) -> 3CO2 (g) + 4H2O (g)
What has to happen to liquid alkanes before they are combusted?
They have to be vaporised into a gas.
Why do alkanes make good fuels?
They release a lot of energy when burned.
What is formed in free radical substitution reactions, from halogens reacting with alkanes?
Haloalkanes.
What are the 3 main steps of free radical substitution?
Initiation, propagation and termination.
What is required in the initiation stage of free radical substitution?
UV light.
What type of bond fission is the initiation stage an example of and why?
Homolytic because the bond splits equally and forms free radicals.
What happens in the propagation stage?
Free radicals are used up and created in a chain reaction.
Show the propagation stages using methane and chlorine.
