3.2 Alkanes Flashcards
What are alkanes
A homologous series of saturated hydrocarbons
with the general formula CnH2n+2
Very unreactive though they burn and react with halogens
—> the longer the carbon chain, the higher the boiling point
- due to stronger vanderwalls forces
- for alkane isomers, the more branched a chain
- the lower the boiling point due to less vdw forces
- molecules cant pack properly (are distanced)
What is crude oil
Also called petroleum
It’s a mixture containing many alkane hydrocarbons
that can be separated through fractional distillation
What is fractional distillation and a fraction
It’s the continual evaporation and condensation
Of a mixture, causing substances to separate due to different bps.
A fraction is a group of compounds with similar bps
And are removed at the same level of fractionating column
—> in fractional distillation, these are the less complex mixtures formed
.
Crude oil is heated in a furnace until a portion turns into vapour
The liquid vapour mixture goes up column (cooler at top)
And condenses at a layer that’s cool enough
—> shorter chains condense higher up due to low bps
What are the fractions in a column?
Refinery gas (eg. Methane)
Petrol (gasoline)
Naphtha
Kerosine
Diesel oil
Fuel oil and lubricating oil
Tar/bitumen
Lowest bps are most useful but are in most demand
What is cracking
Long chain hydrocarbons are broken
Into shorter chain molecules wch are in higher demand
As more useful
— thermal cracking
Long chain alkanes are heated to high temperatures and pressures (900c , 70atm)
This produces cracking products with shorter alkane chains and lots of alkenes
These temps and pressures can decompose the molecule completely
Producing just C and H
—> this is prevented by making the time the conditions are applied short (1s)
..
— catalytic cracking (450c, 1-2atm)
Long chain alkane is heated under pressure with (aluminosilicate) zeolite catalyst
The zeolite is an acidic mineral with a honeycomb structure
Giving large SA to increase rate of reaction
Produces mainly branches alkanes, cycloalkanes, aromatic compounds
—> used to produce fuels for road vehicles
Where is methane produced
In some forms of anaerobic respiration
Like in the gut of sheep and cows, rice fields
Compost heaps and landfill tips
What are physical properties of alkanes
Are nonpolar so only have vdw forces between
Strength of vdw between molecules increase as Mr of molecule increases
So bp and mps increase too
Branched chain isomers have lower bps
Due to smaller SA in contact with each other
And don’t fit as closely
Reducing strength of vdw forces
Alkenes are also insoluble in water but
Will dissolve in other non polar liquids like hexane or cyclopentane
What’s complete combustion
Alkanes are fuels as they combust readily releasing lots of heat energy
They burn in a plentiful oxygen supply to produce CO2 and water
Methanes a major component of natural gas (found in coal and oil deposits)
Methane + 2oxygen —> carbon dioxide + 2water
Butane is a major component of liquid petroleum gas
Produced by fractional distillation of crude oil
Butane + 6.5Oxygen —> 4Carbon Dioxide + 5Water
Incomplete combustion?
Alkanes burn to form water and CO in limited supply of oxygen
Methane + 1.5Oxygen —> carbon monoxide + 2Water
Butane + 4.5Oxygen —> 4Carbon monoxide + 5Water
What does CO do
A toxic, colourless and odourless gas
Reacts with haemoglobin ogle in red blood cells preventing them from carrying O2 round body
—> absorbed 200x faster than O2
—> so difficult to eliminate so classed as cumulative poison
Mild exposure (slight headache, nausea, tiredness)
Medium exposure (severe throbbing headache, drowsiness, fast heart)
Extreme exposure (unconsciousness, cardio respiratory failure, death)
All gas fuelled appliances are regularly checked
To prevent CO forming due to lack of O2 for combustion
CO detectors can warn dangerous levels
When is soot formed
If oxygen supply is further limited soot is formed (cause asthma)
Methane + Oxygen —> C + 2H2O
Butane + 2.5Oxygen —> 4C + 5H2O
What’s combustion in an internal engine
Most cars in UK are fuelled by an alkane fuelled internal combustion engine
A small amount of fuel mixed with excess air
Is drawn into a combustion chamber.
The mixture is compressed and ignited
with extreme temperature spark (spark plug)
The mixture burns explosively forcing movement of engine parts
Products of combustion exit by exhaust
Process of intake , compression, combustion, and exhaust
takes place 100s of times per minute.
What are the unwanted side reactions in alkane fuelled internal combustion engines
Primary reaction facilitating from chemical to kinetic energy
Is combustion of alkane fuel in oxygen
Pressure and temperatures of combustion chamber
Can cause Nitrogen oxides (NO or NO2)
Sulfur dioxide can be produced too
Sulfur is from impurities from crude oil which end up in fuel
Excess hydrocarbons in fuel wch remain unburned
Form ground level ozone - respiratory problems
What is global warming
Earth is surrounded by an atmosphere of several gas layers
- Infrared radiation from sun passes through them to reach earth
- warming it up; it then from earth travels back through atmosphere
- where some of it is prevented reaching space by atmospheric gases (CO2, CH4, H3O)
-
- the effect trapping from sun is greenhouse effect.
- these gases causing this are greenhouse gases
An important natural process as without it
The earth’s surface temperature will be 60°c lower(life unable)
..
Global warming is the increasing average temperature of atmosphere
Caused by changing concentration of greenhouse gases on earth’s surface
Recent human activity has rapidly increased CO2 in atmosphere (fossil fuels burning)
As a result more heat from the sun is trapped so earth’s temp increases
What are the greenhouse gases and how are they produced
Water vapour (occurs naturally in atmosphere)
Carbon dioxide (produced in respiration as a product of combustion)
Methane (produced as product of digestion by cows and sheep and other natural processes)
