Alkanes Flashcards
What is cracking
conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
High Mr alkanes —> smaller Mr alkanes+ alkenes + (hydrogen)
What are the Economic reasons for cracking(3)
- The alkanes with shorter C chains 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.)
Why does cracking require large amounts of energy
It is a chemical process which involves the splitting of strong covalent bonds so it requires high temperatures
What are the two types of cracking
Thermal and catalytic. They need different conditions and are
used to produce different products
What are the conditions required for Thermal cracking
High pressure (7000 kPa)
High temperature (400°C to 900°C)
What are the conditions needed for catalytic cracking
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
What does thermal cracking produce
- Mostly small alkenes e.g. ethene used for making polymers and ethanol
- Slightly larger alkanes
- Sometimes produces hydrogen which is used in the Haber Process and in margarine manufacture.
An example equation 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.
What does catalytic cracking produce
branched and cyclic alkanes and aromatic hydrocarbons
What is the use of the products of catalytic cracking
Making motor fuel
Why is catalytic cracking cheaper than thermal cracking
g because it saves
energy as lower temperatures and pressures are used
Define what a Fuel is
something that releases heat energy when burnt
What are the conditions required for complete combustion
An excess oxygen
What are the products of complete combustion
CO2 and H2O
What are the conditions required for incomplete combustion
a limited amount of oxygen
What are the products of incomplete combustion
CO (which is very toxic) and/or C (producing a sooty
flame)
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 is the differnevce between CC and IC (comple and incomplete combustion)
Incomplete combustion produces
less energy per mole than
complete combustion
What is a negative for the release of Carbon
Carbon (soot) can cause global
dimming- reflection of the sun’s
light
What is Petroleum
A mixture consisting mainly of alkane hydrocarbons
What are the steps for fractional distillation? (6)
- Crude oil is heated until it evaporates.
- Crude oil vapour is put into a fractionating column at the bottom and rises upwards.
- The temperature is highest at the bottom of the column.
- Long-chain hydrocarbons condense at the bottom
- Short-chain hydrocarbons have lower boiling points.
- They condense at lower temperatures nearer the top.
What causes acid rain
SO2 will dissolve in atmospheric water which produces acid rain
How are nitrogen oxides formed
- 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
What environmental consequence does the pollutant NO and NO2 have
NO is toxic and can form acidic gas
NO2 is toxic and acidic and forms acid rain
