Alkanes Flashcards
What type of hydrocarbons are alkanes
Saturated contains no C=C bonds
Alkanes general formula and what type of alkanes don’t follow this rule
CnH2n+2
Cycloalkanes don’t follow this general rule
How does boiling points change and why
Chain length increases - SA increases - more points of contact between neighbouring marks - Van der walls forces increases.
Petroleum
A mixture of crude oil & natural gas formed from decayed dead life form
Crude oil consists of…
A complex mixture of hydrocarbons - mainly alkanes and some impurities (e.g. sulfur compounds)
How is crude oil formed
Marine life dies and falls to the bottom of the sea and is covered by sediment. It decomposes anaerobically (without oxygen) and undergoes lots of heat, pressure and after millions of years crude oil forms.
What is Fractional distillation of crude oil
Fractional distillation is used to seperate a mixture of different liquids and uses the fact that they have different boiling points.
Short (low molecular mass) alkanes have low boiling points and so will condense higher in the column.
Long (high molecular mass) alkanes have higher boiling points and will condense lower in the column
Fractional distillation of crude oil - process
There is a temperature gradient in the fractionating column - being hot at the bottom and cooler at the top.
Crude oil is heated and vaporised before it is fed into the bottom.
Some compounds with very high molecular masses are still liquid and collect at the bottom in a fraction called the residue. This fraction contains carbon chains with over 70 carbon atoms and are used for roads / roofing.
Other molecules are gases at the high temperature at the base of the column. These hot vapours rise up the column and pass through holes called bubble caps that occur in metal trays at regular intervals in the column.
As the vapour rises it becomes cooler - when the vapour reaches a temperature which is equal to its boling point it will condense as a liquid which will collect on the nearest tray and be siphoned off.
What is naphtha and its uses?
Contains hydrocarbon molecules of 5-9 carbon atoms and is used to make chemicals
What are LPG’s and how many carbon atoms do they contain
Liquid petroleum gases (C1 - C4)
Supply and demand of crude oil fractions - which are in higher / lower demand compared to supply
When heated in the absence of air bonds between alkane atoms break, Which breaks more easily C-C or C-H
C-H has a greater electronegativity difference than C-C so C-C bonds are weaker and need less energy to break - This allows cracking to happen.
What is Cracking
Cracking is the breakdown of larger less useful alkane molecules into shorter more useful alkanes and alkene molecule. It is also possible to produce branched, cyclic and aromatic hydrocarbons. This is known as Homolytic fission
General equation for cracking reactions:
Give 2 equations for how C12H26 may be cracked giving different alkanes in the gasoline fraction
2 types of cracking
Thermal cracking
Catalytic cracking
Thermal cracking
In thermal cracking, the bonds are broken using a high TEMPERATURE (400 - 900°C) and a high PRESSURE (70 atmospheres).
The high temperatures mean that the molecule breaks near the end of the chain, giving a high percentage of small alkenes such as ETHENE.
Most thermal cracking reactions involve the formation of one of smaller alkane molecules and one alkene molecule. Naphtha (C7 - C14) is usually used as the starting material.
This process is known as homolytic fission
This type of cracking precedes a free radical mechanism.
Catalytic Cracking
In catalytic cracking, the bonds are broken using a high TEMPERATURE (450 °C, which is generally lower than in thermal cracking),
a slight PRESSURE (slightly greater than 1 ATMOSPHERE),
and a zeolite catalyst.
Catalytic cracking is cheaper and more efficient than thermal cracking as it uses a lower temperature and pressure.
Carbocation mechanism
Catalytic cracking products
The zeolite catalyst favours the formation of branched ALKANES and cycloalkanes, which are widely used in motor fuels.
The most important product of catalytic cracking is 2-methylheptane, which is the major component of petrol. It also produces aromatic HYDROCARBONS such as benzene, which have a variety of uses.
This type of cracking proceeds a CARBOCATION mechanism.
Advantages of producing more branched alkanes using catalytic cracking
Branched Alkanes burn more efficiently in a car engine (results in a higher octane rating)
What is a fuel?
A fuel is a store of energy that can be released on demand
Complete combustion of Alkanes
Occurs when O2 is plentiful
Alkanes + O2 -> CO2 +H2O
Incomplete combustion of alkanes
Occurs when O2 is limited
Alkanes + O2 -> CO +H2O
E.g. C4H10 -> 9/2 O2 -> 4CO + 5H2O
If oxygen is limited further then reaction will be:
Alkanes + O2 -> C + H2O
E.g. C4H10 -> 5/2 O2 -> 4C + 5H2O
Environmental problems from burning petroleum and other hydrocarbons
CO2 increases greenhouse effect
CO is toxic
NOx gases causes respiratory problems and acid rain
SO2 causes acid rain
Unburnt hydrocarbons forms photochemical smog and some cause cancer
Lead causes neuralogical disorders and lowers iq in children
How is SO2 formed in car exhaust fumes using the fact that oil contains a small proportion of sulphurous compounds such as CH3SH
CH3SH + 3O2 -> CO2 + 2H2O + SO2
How is SO2 removed from flue gases
Metal oxides are basic and and non metal oxides such as SO2 are acidic so they neutralise one another
How do NOx gases form in a car engine
From N2 and O2 reacting in the engine at high temperature and high pressure
N2 + 2O2 -> 2NO2
N2 + O2 -> 2NO
Environmental problems from burning petroleum and other hydrocarbons - Global warming
CO2 produced by complete combustion causes an increase in greenhouse gases causing the greenhouse effect to increase causing an increase in global warming
Environmental problems from burning petroleum and other hydrocarbons - Carbon monoxide
CO is a colourless and oudourless toxic gas - it binds irreversibly to haemoglobin preventing the body from getting oxygen
Environmental problems from burning petroleum and other hydrocarbons - lead compounds
Leaded petrol can burn to release lead compounds which are toxic to humans and cause neurological diseases and lowers IQ in children
Environmental problems from burning petroleum and other hydrocarbons - Photochemical smog
Mixture of unburnt hydrocarbons and NOx gases react in UV light to form photochemical smog
Environmental problems from burning petroleum and other hydrocarbons - Acid rain
SO2 & NO2 produced can dissolve in water to produce acidic solutions:
2NO2 + H2O -> HNO3 + HNO2
SO2 +H2O -> H2SO3
Catalytic converters
These are used to treat the exhaust fumes of cars to reduce the emission of gases that can harm the environment. They help produce less CO, NOx and unburnt hydrocarbons.
What does a catalytic converter consist of
The converter consists of a ceramic material made into a honeycomb arrangement - the metals platinum, rhodium and palladium are the active catalysts and coat the ceramic honeycomb.
This structure increases the surface area of the catalyst resulting in increased efficiency
Explain how greenhouse gases are thought to cause a warming of the earths atmosphere
Greenhouse gases absorb some of the IR radiation emitted by the earth and re-emit some of it back towards the earth. This causes the earth’s average temperature to increase.
Explain how molecules like CO2 can trap heat energy
The absorbed IR radiation causes the bonds within a CO2 molecule to vibrate.
Halogenation of alkanes
Alkanes can react with halogens in the presence of UV radiation to form a haloalkane compound with a substitution reaction.
Mechanism for halogenisation of alkanes
It is an example of free radical substitution. It is also a chain reaction and occurs in different stages:
Initiation - 1 step
Propagation - 2 steps
Termination - 3 steps
halogenisation of alkanes - Initiation
UV light is needed to break the bond between the two
Chlorine atoms in Cl2.
This type of bond breaking is called homolytic fission.
Particles are produced which have an unpaired electron
these are called free radicals and are a very reactive species
Cl2 -> Cl* + CL* Where * = free radicals
halogenisation of alkanes - propagation
halogenisation of alkanes - Termination
