Alkanes pt2 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
Features of thermal cracking
-High pressure (7000 kPa)
-High temperature (400°C to 900°C)
Produces:
* Mostly small alkenes e.g. ethene used for making polymers and ethanol
* Slightly larger alkanes
What are the conditions needed for catalytic cracking
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
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 and what is the use of the products of catalytic cracking
branched and cyclic alkanes and aromatic hydrocarbons
Making motor fuel
Why is catalytic cracking cheaper than thermal cracking
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
and what are the products of complete combustion
An excess oxygen
CO2 and H2O
What are the conditions required for incomplete combustion
And What are the products of incomplete combustion
a limited amount of oxygen
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 to produce large amounts of energy
Used to power vehicles
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 consequence does the pollutant Carbon monoxide (CO) have
CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
Oxygen is transported to organs
Carbon dioxide is removed as waste material from organs
Why do alkanes not react with many reagents?
Because the C-C bond and the C-H bond are relatively strong
