Properties of the alkanes Flashcards
What are alkanes the main component of?
Natural gas and crude oil
Why are alkanes used in crude oil?
They are some of the most stable organic compounds and their lack of reactivity has allowed crude oil deposits to remain on earth for many years
What are alkanes mainly used as?
Fuels
Why are alkanes used as fuels?
They react with oxygen to generate heat
What is the general formula of alkanes?
CnH2n+2
What type of hydrocarbons are alkanes?
Saturated
How are the hydrogen and carbon atoms joined in alkanes?
By single covalent bonds
How many atoms is each carbon atom in an alkane bonded to?
Four
What are the covalent bonds in alkanes called?
Sigma bonds
What is a covalent bond?
A shared pair of electrons
How is a sigma bond formed?
It is the result of the overlap of two orbitals, one from each bonding atom
How many electrons does each overlapping orbital contain?
One
How many electrons are in a sigma bond?
Two
Where is a sigma bond positioned?
On a line directly between bonding atoms
How many sigma bonds does each carbon atom in an alkane have?
Four
What are the two possible sigma bonds in alkanes?
C-C and C-H
What is each carbon atom in an alkane surrounded by?
Four electron pairs in four sigma bonds
What shape do alkanes have?
Tetrahedral arrangement around each carbon atom
Why do alkanes have a tetrahedral shape?
Repulsion between the electron pairs in the sigma bonds causes a tetrahedral arrangement
What size are the bond angles in alkanes?
109.5 degrees
Why are alkane shapes not rigid (can move)?
The sigma bonds act axes around which the atoms can rotate freely, e.g. into a U shape
How do oil refineries separate crude oil into fractions?
By fractional distillation in a distillation tower
What does each fraction of crude oil contain?
A range of alkanes
Why is fractional distillation of alkanes possible?
The boiling points of the alkanes are different
What determines an alkane’s boiling point?
Chain length - boiling point increases as chain length increases
Why do boiling points increase as the chain length increases?
London forces hold molecules together in solids and liquids but, once broken, the molecules move apart from each other and the alkane becomes a gas
How are intermolecular forces related to boiling point?
The greater the intermolecular forces, the higher the boiling point
What molecules do London forces act between?
Molecules that are in close surface contact
What happens to molecules as chain length increases?
They have a larger surface area, so more surface contact is possible between them
Why does chain length affect boiling point?
London forces between molecules will be greater so more energy will be required to overcome the forces
What do isomers of alkanes all have in common?
Same molecular mass
How does branching affect boiling point?
Branched isomers have lower boiling points compared to straight-chain isomers
Why do branched structures have lower boiling points? (2 reasons)
There is fewer surface points of contact between molecules of branched alkanes, giving fewer London forces
Branches also get in the way and prevent molecules getting as close together as straight-chain molecules, further decreasing intermolecular forces
