Organic 1 - Chapter 12: Alkanes Flashcards
What is an alkane?
Saturated hydrocarbon with only C-C and C-H bonds
What are alkanes used for and why?
Fuels, lubricants and feedstock due to being unreactive
What is the general formula for a chain alkane?
CnH2n+2
What is the general formula for a ring alkane?
CnH2n
Describe the physical properties of alkanes
Mention polarity, boiling point, solubility and reactivity
Polarity: Almost non-polar as electronegativity of carbon and hydrogen are similar
MP + BP: Bigger chain -> more Van der waals forces -> Higher MP+ BP
Solubility: Insoluble in water as hydrogen bonds in water are stronger than vdw forces between alkane molecules, can mix in other non-polar liquids
Reactivity: Relatively unreactive due to strong C-C and C-H bonds. They do burn and react with halogens under suitable conditions
Describe the steps for the fractional distillation of crude oil
Crude oil heated in furnace
Mixture of liquid + vapour pass into fractionating tower which is cooler at top than bottom
Vapours pass up tower via a series of trays containing bubble caps until they arrive at a tray where they can condense to liquid
The mixture of liquids that condense on each tray is piped off
Shorter chain hydrocarbons condense near top of tower as it is cooler
Thick residue collects at the base (bitumen) often processed to give more valuable products as supply > demand
What is broken during fractional distillation?
Vdw forces between the molecules break during vaporisation and reform on condensing
NOT COVALENT BONDS
What is fracking?
Natural gas is trapped within shale rock:
Drill into the shale
Force pressurised water with chemicals mixed with sand into the shale. Chemicals such as HCl and methanol added to break shale and prevent corrosion
Shale fractures -> trapped gas flows to surface
Why are people against fracking?
People do not like the infrastructure of wells and associated ‘traffic’ around them
Concern about the amount of water used
Concern about chemical additives polluting water supply
Fracking occasionally causes small earthquakes
Burning natural gas creates CO₂ which causes global warming
What are the benefits of cracking?
Shorter more useful chains produced
Some products are alkenes which are more reactive than alkanes
What are the conditions required for thermal cracking?
700-1200K
7000kPa
What are the main products of thermal cracking?
High proportion of alkenes
Alkanes and hydrogen can also be produced
Why are hydrocarbons not subjected to thermal cracking for long durations?
To avoid too much decomposition, ultimately to just carbon and hydrogen
How does thermal cracking work?
C-C bonds break
1 electron from the pair in the covalent bond goes to each carbon
This produces 2 shorter chains with a free radical at the end
They react to form shorter chain molecules
There is not enough hydrogen atoms to produce 2 alkanes, therefore one of the new chains must be an alkene
What are the conditions required for catalytic cracking?
720K
Just above atmospheric pressure
Zeolite catalyst