Alkanes Flashcards
How does fractional distillation separate alkanes?
Crude oil heated until it has evaporated
Gas then rises through fractioning tower
Fractioning tower cools as you go up
The longer the fraction (C chain), the higher the boiling/condensation point - therefore condense fairly near the bottom
The shorter chains have lower condemnation points, and so condense nearer the top as the temp. cools
What are the conditions of thermal cracking?
High temps (500-1000*C) and high pressure
What kind of length chain does thermal cracking produce?
Shorter chain alkanes and alkenes
What are the conditions of catalytic cracking?
Takes place at lower temp (450*C) and pressure than thermal
Uses a zeolite catalyst
What kind of products does catalytic cracking produce?
Products mainly branched and cyclic alkanes and aromatic compounds (benzene based)
What are the products of catalytic cracking mostly used for?
Motor fuels
What are the products of complete combustion?
CO2 and H2O ONLY
What is the difference between complete and incomplete combustion?
Complete only produces CO2 and H2O
Incomplete may also produce CO or particulates
Why is CO harmful?
Colourless and poisonous - reduces carrying O2 capacity of blood
What is an effect of particulates?
Smog and global dimming
CO2, H2O and possibly CO or particulates are formed in the combustion of petrol. What other pollutants are formed?
SO2 gas from sulphur impurities, various oxides of nitrogen
What is an effect of sulphur of the environment?
Acid rain
What is an effect of nitrogen oxides of the environment?
Acid rain
What does a catalytic converter do?
Remove pollutant gases formed from the combustion in the engine
What kind of structure does a catalytic converter have and why?
Honeycomb
Increases SA so increases efficiency and reduces cost
A catalytic converter consists of a mixture of metals. What are they?
Platinum, palladium and rhodium
The chlorination of alkanes involves what kind of mechanism reaction?
Free radical substitution
What are the stages of free radical substitution?
Initiation, propagation 1 and 2, termination
What happens in the initiation stage of free radical substitution (chlorination of an alkane)?
UV light breaks the Cl-Cl bond (homolytic fission)
2 free radicals of Cl produced
What is a free radical?
An atom or species with an unpaired electron
What happens in the propagation stages of free radical substitution (chlorination of an alkane)?
1 - Cl + CH4 –> HCl + CH3 (C = free radical)
2 - CH3 + Cl2 –> Ch3Cl + Cl
Why is propagation hard to control?
It’s a chain reaction - a free radical Cl is produced at the end each time, and so starts a new reaction
What happens in the termination stage of free radical substitution (chlorination of an alkane)?
2 free radicals combine
CH3 + Cl –> CH3Cl
CH3 + CH3 –> C2H6
How are further substitutions able to take place in free radical substitution?
It is possible for more than one H to be replaced if the product meets more Cl*
In free radical substitution, how can you increase the likelihood of CCl4 being produced?
Excess Cl2
Name the raw material from which hexane is obtained
Crude oil
Explain the economic reasoning for the cracking of alkanes
There is a higher demand for shorter chain alkanes
Give the general formula for alkanes
CnH2n+2
Sulphur dioxide is sometimes produced from reactions in the engine. How has it come to be present there?
Present as impurities in petroleum fractions
What is an alkane?
A saturated hydrocarbon
What does cracking involve?
Breaking C-C bonds in alkanes
What is an essential condition for the initiation stage of free radical substitution to occur?
UV light must be present
