3.2 Alkanes Flashcards
(30 cards)
Describe the process of fractional distillation
- The mixture is vaporised and fed into the fractionating column
- Vapours rise, cool and condense at their boiling points
- Products are siphoned off for different uses
Where are products with short chains collected and why
- Short chains have lower boiling points
- Therefore, they rise higher up the column before reaching their boiling points
- They are collected at the top of the column, where it is cooler
What happens to compounds collected from the fractionating column
They are broken down further by cracking
Why is cracking done
To break longer, less useful chains into smaller, more useful ones.
What are the 2 main types of cracking
Thermal cracking and catalytic cracking
What are the conditions used for thermal cracking
- High temperatures (around 1200K)
- High pressures (around 7000kPa/ 70atm)
What are the typical products of thermal cracking
High proportions of alkanes and alkenes
What are the conditions used for catalytic cracking
- Lower temperatures (around 720K)
- Just above 1atm (100kPa)
- Zeolite catalyst (to compensate for less harsh conditions)
What are the typical products of catalytic cracking
Aromatic compounds with carbon rings.
Why do alkanes make good fuels
They release a lot of energy when burned
What is complete combustion
When alkanes are burned with sufficient oxygen
What are the products of complete combustion
Carbon dioxide and water
What are the products of incomplete combustion
Water, Carbon monoxide, and/or Carbon particulates (soot).
What is a catalytic converter
It converts harmful products (like CO) into more stable products like Carbon dioxide or water.
What type of catalyst is used in a catalytic converter
A Rhodium catalyst.
What is free radical substitution
- Alkanes react with halogens in the presence of UV light to produce halogenoalkanes
- The UV light breaks down the halogen bonds, producing reactive intermediates called free radicals
- These attack the alkanes, resulting in a series of reactions
What are the 3 stages in free radical substitution
- Initiation
- Propagation
- Termination
What happens during initiation
A halogen molecule is broken down by UV light to produce 2 halogen free radicals
What happens during propagation
- A halogen free radical reacts with a hydrogen atom in the alkane, producing a H-halogen (e.g. HCl)
- This also produces an alkane free radical
- The alkane free radical reacts with a halogen molecule, producing a halogenoalkane and the halogen free radical is reformed (acts as a catalyst)
What happens during termination
Two free radicals join together to end the chain reaction, forming a stable product.
What is flue gas desulfurisation
A process that removes sulfur impurities
What is used during the process of flue gas desulfurisation
Calcium oxide and gypsum
Why are sulfur impurities harmful
They can lead to the acidification of water in the earth’s atmosphere as they react to form a weak form of sulfuric acid.
Describe the polarity of alkanes and therefore the types of bonds between molecules
- They are generally non-polar as the C-H bond is non-polar
- Because Carbon and Hydrogen have very similar electronegativities
- Therefore the bonds are weak and weak van der waals forces form between molecules
