2. Alkanes
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What is petroleum another name for?
Crude oil
What is crude oil a mixture of?
Hydrocarbons, the majority of which are alkanes
What needs to be done to crude oil in order for it to be useful?
It needs to be separated into mixtures of hydrocarbons with similar chain lengths
What are fractions?
Mixtures with similar chain lengths
What is a hydrocarbon?
A molecule that contains only hydrogen and carbon atoms
What is an alkane?
A hydrocarbon with the general formula C₂H₂ₙ₊₂
Why are alkanes relatively unreactive?
They only contain strong C-C and C-H bonds
Why are alkanes useful as fuels?
They burn
What are alkanes held together by?
Weak van der waal’s forces
What happens to van der waal’s forces as size of the alkane increases?
The strength increases
What happens to boiling point of alkanes as size increases?
It increases
Why does boiling point of alkanes increase with size?
Because the strength of the van der waal’s forces in between increases
Why do branched alkanes tend to have lower boiling points than straight chain alkanes?
Because there is reduced surface area contacts with branched isomers, and so fewer van der waal’s forces - and so less energy required to break them
How is petroleum separated into useful fractions?
In an oil refinery by fractional distillation
What equipment is used for fractional distillation?
A fractionating column
Which fractions condense at the top of the fractionating column?
• those with short chains * those with low melting points
Which fractions condense at the bottom of the fractionating column?
• those with long chains * those with high melting points
What happens during fractional distillation?
• vaporised crude oil enters a fractionating column - which is hot at bottom and cool at top • vapour from the oil rise • vapour condense when they become cool enough
What is cracking?
The chemical splitting of long-chain alkanes into hydrocarbons of shorter chain length by breaking a C-C bond
What is produced when long chain alkanes are cracked?
Short chain alkanes and alkenes
What are the two main types of cracking?
• thermal cracking * catalytic cracking
What are the conditions required for thermal cracking?
• 700-1200K (900*C) * 7000 kPa
Why are intense conditions required to break alkanes down?
They are unreactive
What provides the energy to break C-C bonds in thermal cracking?
High temperatures