3.2 - Alkanes and Halogenoalkanes Flashcards
(35 cards)
How is crude oil separated?
Crude oil is separated by fractional distillation
What are the steps in separated crude oil?
1) The crude oil is vaporised at around 620K
2) The vaporised crude oil travels up the fractionating column
3) Due to different sized alkanes having different boiling points, they condense at different temperatures in the column
4) The smallest hydrocarbons do not condense, and the largest do not vaporise (they are a thick liquid)
What is cracking?
Cracking is the process of breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes)
What is thermal cracking?
Thermal cracking takes place at a high temperature (1250K) and a high pressure (up to 70 atm). Thermal cracking produces a lot of alkenes
What catalyst is used in catalytic cracking?
Catalytic cracking uses zeolite (hydrated aluminosilicate) as a catalyst
What is catalytic cracking?
A catalyst, slight pressure and a medium temperature (800K) are used to mainly produce aromatic hydrocarbons and the alkanes needed to produce motor fuels
What are the benefits of catalytic cracking?
Using a catalyst cuts costs and speeds up the rate of reaction
What are nitrous oxides?
Nitrogen oxides are a series of toxic and poisonous molecules that are produced in car engines
How is ozone formed in a car engine?
Unburnt hydrocarbons and nitrogen oxides react to form ozone, a major component in smog and can cause lung damage and aggravate asthma
What are catalytic converters?
Catalytic converters (made of precious metals like platinum, palladium and rhodium) remove pollutants like nitrogen oxides, unburnt hydrocarbons and carbon monoxide
How is sulphur dioxide removed from power stations?
It is reacted with calcium oxide or calcium carbonate to form calcium sulphate, a harmless salt
How does burning fossil fuels contribute to global warming?
The carbon dioxide released is a greenhouse gas, and therefore contributes to the enhanced greenhouse effect
What is formed when halogens react with alkanes in photochemical reactions?
They form a halogenoalkane. A hydrogen atom is substituted by a halogen; this is called free radical substitution
How are photochemical reactions started?
They are started by UV (ultraviolet) light
What is a free radical?
A free radical is a particle with an unpaired electron
How are free radicals formed?
Free radicals are formed when a covalent bond splits equally, giving one electron to each species.
How is a free radical shown?
A dot is put next to the atom
What are the three stages of free radical substitution?
Initiation, propagation and termination
What happens in the ‘Initiation stage?
Free radicals are produced under UV light e.g. Cl2 → 2Cl•
What happens during the ‘Propagation’ stage?
Free radicals are used up and created in a chain reaction. The first stage is when the free radical attack a molecule, forming another free radical and a separate compound. The second stage is when the free radical previously created attacks the halogen diatomic atom, creating the first free radical and a halogenoalkane
What are the two propagation stages in the formation of chloromethane?
First Stage: Cl• + CH4 → •CH3 + HCl
Second Stage: •CH3 + Cl2 → CH3Cl + Cl•
What happens in the reaction with chlorine and methane when the chlorine is in excess?
The products will go from chloromethane, dichloromethane, trichloromethane and tetrachloromethane
What happens during the ‘Termination’ stage?
Free radicals are used up; two free radicals join together. This ends the chain reaction
What are the three possible termination stages for the synthesis of chloromethane?
1) •CH3 + Cl• → CH3Cl
2) •CH3 + •CH3 → C2H6
3) Cl• + Cl• → Cl2
