Alkanes Flashcards
1
Q
what is a petroleum fraction
A
- a mixture of hydrocarbons with a similar chain length and boiling point range
2
Q
how does fractional distillation work?
A
- oil is preheated then passed into column
- the fractions condense at different heights
- temp of column decreases upwards
- seperation depends on boiling point
- boiling points depend on size of molecules
- similar molecules condense together
- small molecules condense at the top at lower temps
- big molecules comsense at the bottom at higher temps
3
Q
what is cracking?
A
- conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
4
Q
what are the economic reasons for cracking
A
- petroleum fractions with shorter C chains are more in demand than larger fractions
- to make use of excess larger hydrocarbons and to supply demand for shorter ones
- products of cracking are more valuable than the starting materials
5
Q
what are the conditions for thermal cracking
A
- high pressure ( 7000kPa )
- high temperature ( 400 to 900 )
6
Q
what are the conditions for catalytic cracking
A
- slight or moderate pressure
- high temperature ( 450 )
- Zeolite catalyst
7
Q
what are the products of thermal cracking
A
- produces mostly alkenes
- sometimes produces hydrogen
C8H18 > C6H14 + C2H4
8
Q
what are the products of catalytic cracking
A
- produces branched and cyclic allanes and aromatic hydrocarbons
9
Q
why is catalytic cracking cheaper than thermal cracking?
A
- saves energy as lower temperatures and pressures used
10
Q
what are the products for complete combustion of alkanes
A
- carbon dioxide and water
11
Q
what are the products for incomplete combustion of alkanes
A
CO and/or C and water
12
Q
whats the impact of carbon soot
A
- causes global dimming
13
Q
whats the impact of SO2 produced
A
- dissolves in atmospheric water and can produce acid rain
14
Q
how can SO2 be removed from waste gases from furnaces
A
- by flue gas desulfurisation
- gases pass through scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide
- SO2 + CaO > CaSO3
15
Q
how do nitrogen oxides form
A
- from the reaction between N2 and O2 inside the car engine
- the high temp and spark in engine provides enough energy to break strong N2 bond
