Alkanes Flashcards
what is petroleum?
Petroleum is a mixture consisting mainly
of alkane hydrocarbons
what is a petroleum fraction?
Petroleum fraction: mixture of hydrocarbons with
a similar chain length and boiling point range
what are key points of fractional distillation?
- 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
what is the use of a vacuum distillation unit?
• Heavy residues from the fractionating column are distilled
again under a vacuum.
• Lowering the pressure over a liquid will lower its boiling
point.
what is the benefit of a vacuum distillation unit?
Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down.
what is the process of fractional distillation in a laboratory?
- 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.
define cracking
conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
what is the formula for cracking?
High Mr alkanes -> smaller Mr alkanes+ alkenes + (hydrogen)
what are some 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,
what does the process of cracking involve?
This is a chemical process
involving the splitting of
strong covalent bonds so
requires high temperatures.
what are the conditions of thermal cracking?
High pressure (7000 kPa) High temperature (400°C to 900°C)
what are the conditions of catalytic cracking?
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
what are the products of thermal cracking?
produces mostly alkenes e.g. ethene used
for making polymers and ethanol
sometimes produces hydrogen used in the
Haber Process and in margarine manufacture
what are the products of catalytic cracking?
Produces branched and cyclic
alkanes and aromatic hydrocarbons
Used for making motor fuels
why is catalytic cracking better than thermal cracking?
-Branched and cyclic hydrocarbons burn more
cleanly and are used to give fuels a higher octane
number
-Cheaper than thermal cracking because it saves
energy as lower temperatures and pressures are used
what does combustion result in?
Fuel: releases heat energy when burnt
why are alkanes used as fuels?
Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels.
what are the products of complete combustion?
The products of complete combustion are CO2 and H2O.
what are the products of incomplete combustion?
+ why is complete combustion favoured?
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)
Incomplete combustion produces
less energy per mole than
complete combustion
Carbon (soot) can cause global
dimming- reflection of the sun’s
light
what is the product of the combustion of SO2?
-Sulfur-containing impurities are found in petroleum fractions which
produce SO2 when they are burned.
-Coal is high in sulfur content, and
large amounts of sulfur oxides are
emitted from power stations.
-SO2 will dissolve in atmospheric water and can produce acid rain.
how is SO2 removed?
give the formula
SO2 can be removed from the waste gases from furnaces (e.g. coal fired
power stations) 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
Sulfur containing impurities are found in petroleum fractions which
produce SO2 when they are burned. Coal is high in sulfur content, and
large amounts of sulfur oxides are
emitted from power stations. S+ O2 SO2
what can calcium sulfite be used for?
The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard. SO2 + CaO CaSO3
how is NO2 formed?
give the equation?
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
what is the environmental consequence of NO2?
NO is toxic and can form acidic gas NO2
NO2
is toxic and acidic and forms acid rain
what is the environmental consequence of carbon monoxide?
toxic
what is the environmental consequence of carbon dioxide?
Contributes towards global warming
what is the environmental consequence of unburnt hydrocarbons?
Contributes towards the formation of smog
what is the environmental consequence of soot (C)?
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
how do catalytic converters work?
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium – to give a large surface area.
what are the 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
explain the 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 do alkanes not generally react with many reagents?
In general, alkanes do not react with many reagents. This is because the C-C bond and the C-H bond is relatively strong
what is the mechanism for the synthesis of halogenoalkanes?
free radical substitution
what are the steps of free radical substitution?
- initiation
- propagation
- termination
what is a condition for initiation to occur?
UV light
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.
what are the equations for each step of the overall reaction of CH4 + Cl2 -> CH3Cl + HCl?
initiation Cl2 --> 2Cl. propagation CH4 + Cl. --> HCl + .CH3 .CH3 + Cl2 --> CH3Cl + Cl. termination .CH3 + Cl . --> CH3Cl .CH3 + .CH3 --> CH3CH3
Write the overall reaction equation for the formation of CCl4
from CH4 + Cl2
CH4 + 4 Cl2 –> CCl4 + 4 HCl
Write the overall reaction equation for the formation of CFCl3
from CH3F + Cl2
CH3F + 3 Cl2 –> CFCl3 + 3 HCl
