Alkanes Flashcards
What is petroleum and the petroleum fraction?
Petroleum is a mixture consisting mainly of alkane hydrocarbons
Petroleum fraction: mixture of hydrocarbons with a similar chain length and boiling point range
Key points to learn about fractional distilation
- Oil is pre-heated
• then passed into column.
• The fractions condense at different heights
• The temperature of column decreases upwards
• The separation depends on boiling point.
• Boiling point depends on size of molecules.
• The larger the molecule the larger the van der waals forces
• Similar molecules (size, bp, mass) condense together
• Small molecules condense at the top at lower temperatures
• and big molecules condense at the bottom at higher temperatures.
Why is fractional distillation a physical process?
This is a physical process because it involves the splitting of
weak van der waals forces
between molecules.
Why is a vacuum distillation unit used in fractional distillation?
Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down.
• Heavy residues from the fractionating column are distilled again under a vacuum.
• Lowering the pressure over a liquid will lower its boiling
point.
Steps of fractional distillation in the labatory
Heat the flask, with a Bunsen burner or electric mantle
• This causes vapours of all the components in the mixture to be produced.
• Vapours pass up the fractionating column.
• The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
• The thermometer should be at or below the boiling point of the most volatile substance.
• The vapours with higher boiling points condense back into the flask.
• Only the most volatile vapour passes into the condenser.
• The condenser cools the vapours and condenses to
a liquid and is collected.
What is cracking and the general equation of it?
Cracking is the conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
This is is a chemical process
involving the splitting of strong covalent bonds so requires high temperatures.
High Mr alkanes —> smaller Mr alkanes+ alkenes + (hydrogen)
Economic reasons for cracking
• The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
• To make use of excess larger hydrocarbons and to supply demand
for shorter ones, longer hydrocarbons are cracked.
• The products of cracking are more valuable than the starting
materials (e.g. ethene used to make poly(ethene), branched alkanes
for motor fuels, etc.)
What are the conditions of thermal cracking
High pressure (7000 kPa)
High temperature (400°C to 900°C)
What does thermal cracking produce
It produces mostly alkenes e.g. ethene used for making polymers and ethanol
sometimes produces hydrogen used in the Haber Process and in margarine manufacture.
Example equations of thermal cracking
C8H18 —> C6H14 + C2H4
C12H26 —> C10H22 + C2H4
Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission.
Conditions for catalytic cracking
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
Therefore cheaper than thermal cracking because it saves
energy as lower temperatures and pressures are used
What does catalytic cracking produce
Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number
Produces branched and cyclic
alkanes and aromatic hydrocarbons
Combustion of alkanes and what does it produce
Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use
as fuels.
Fuel : releases heat energy when burnt
Complete combustion
In excess oxygen alkanes will burn with complete combustion
The products of complete combustion are CO2 and H2O.
C8H18(g) + 12.5 O2(g) 8CO2(g) + 9 H2O(l)
Incomplete combustion
Incomplete combustion produces
less energy per mole than
complete combustion
If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame)
CH4(g) + 3/2 O2(g) —> CO(g) + 2 H2O(l)
CH4(g) + O2(g) —> C(s) + 2 H2O(l)
Sulfur dioxide as a product of combustion
Sufur containing impurities are found in petroleum fractions which
produce SO2 when they are burned.
S+ O2 —> SO2
CH3SH+ 3O2 —> SO2 + CO2 + 2H2O
Coal is high in sulfur content, and
large amounts of sulfur dioxide are
emitted from power stations.
SO2 will dissolve in atmospheric water and can produce acid rain
How can sulfur dioxide be removed from waste gases?
By flue gas desulfurisation.
The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction
The calcium sulfite which is
formed can be used to make
calcium sulfate for plasterboard.
SO2 + CaO CaSO3
calcium sulfite
How do oxides of nitrogen arise?
Nitrogen oxides form from the reaction between N2 and O2 inside the car engine.
The high temperature and spark in the engine provides sufficient energy to break strong N2 bond
N2 + O2 —> 2NO
N2 + 2O2 —> 2NO2
Environmental consequence of nitrogen oxide
NO is toxic and can form acidic gas NO2
NO2 is toxic and acidic and forms acid rain
Carbon monoxide environmental consequence
Toxic
Carbon dioxide environmental consequence
Contributes to global warming
Unburnt hydrocarbons environmental consequences
Contributes to formation of smog
Soot (carbon) environmental consequence
Global dimming and respiratory problems
What do catalytic converters do
These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18)
from the exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O.
2 CO + 2 NO —> 2 CO2 + N2
C8H18 + 25 NO —> 8 CO2 + 12½ N2 + 9 H2O
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium
– to give a large surface are
Greenhouse gases
Carbon dioxide (CO2), methane (CH4) and water vapour (H2O) are all greenhouse gases.
Water is the main greenhouse gas (but is natural), followed by carbon dioxide and methane.
Mechanism of the greenhouse effect
UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up Earth’s surface.
The Earth radiates out infrared long wavelength radiation.
The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere.
This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
Why is carbon dioxide largely responsible for global warming
Carbon dioxide levels have risen significantly in recent years due to
increasing burning of fossil fuels.
Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming.
Reaction of alkanes with bromine / chlorine in UV light
In the presence of UV light alkanes react with chlorine to form a mixture of products with the halogens substituting hydrogen atoms.
Overall reaction
CH4 + Cl2 —> CH3Cl + HCl
methane to chloromethane
This is the overall reaction, but a more complex mixture of products is actually formed
What is the mechanism called and the steps involving reactions of chlorine/bromine with UV light
this reaction is called a free radical substitution
Step one: initiation
Step two: propagation
Step three: termination
Step one of free radical substitution
Initiation
Essential condition: UV light
Cl2 —> 2Cl.
The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others because it is the weakest.
The bond has broken in a process called homolytic fission.
When a bond breaks by homolytic fission it forms free radicals.
Free radicals do not have a charge and are represented by a each atom gets one electron from the covalent bond
What is a free radical
A free radical is a reactive species which
possess an unpaired electron.
Step two of free radical substitution
Propagation
The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical
CH4 + Cl. —> HCl + .CH3
The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical
.CH3 + Cl2 —> CH3Cl + Cl.
What do all propagation steps have?
All propagation steps have a free radical in the reactants and in the products.
As the Cl free radical is regenerated, it can react with several more alkane molecules in a chain reaction.
Step three of free radical substitution
Termination
.CH3 + Cl . —> CH3Cl
.CH3 + .CH3 —> CH3CH3
Collision of two free radicals does not generate further free radicals:
the chain is terminated.
Minor step leading to impurities of ethane in product.
Write this step using structural formulae and do not
use molecular formulae.
Applying mechanism to bromine
Initiation -
Essential condition: UV light
Br2 —> 2Br.
Propagation
CH3CH2CH3 + Br.—> HBr + CH3CH2CH2.
CH3CH2CH2. + Br2 —> CH3CH2CH2Br + Br.
Termination
CH3CH2CH2. + Br. —> CH3CH2CH2Br
CH3CH2CH2. + CH3CH2CH2 . —> CH3CH2CH2CH2CH2CH3
Proagation steps for substituting a halogen on a ‘middle’ carbon
CH3CH2CH3 + Br. —> HBr + CH3CH. CH3
CH3CH. CH3 + Br2 —> CH3CHBrCH3 + Br.
If the question asks for the halogen to be substituted onto a middle carbon in the chain, it is important to
put the free radical ‘dot’ on the correct carbon in the propagation stages.
Further substitution reactions
Excess Cl2 present will promote further substitution and could produce CH2Cl2, CHCl3 and CCl4
These reactions could occur
CH3Cl + Cl2 —> CH2Cl2 + HCl
CH2Cl2 + Cl2 —> CHCl3 + HCl
CHCl3 + Cl2 —> CCl4 + HCl
Example propagation steps that would lead to further substitution
CH3Cl + Cl. HCl + .CH2Cl
. CH2Cl + Cl2 CH2Cl2 + Cl.
Overall reaction equations
Example 1. Write the overall reaction equation for the formation of CCl4 from CH4 + Cl2
CH4 + 4 Cl2 —> CCl4 + 4 HCl
Example 2. Write the overall reaction equation for the formation of CFCl3 from CH3F + Cl2
CH3F + 3 Cl2 —> CFCl3 + 3 HCl
Note HCl is always the side product – never H2
