CRACKING AND COMBUSTION Flashcards
what is cracking
process in which long chain hydrocarbon molecules are broken into shorter chain molecules
shorter molecules are more useful
cracking involves the breaking of C-C bonds in alkanes
economical reasons for cracking
smaller chain molecules are in higher demand
products from cracking are more valuable
thermal cracking
long chain molecules heated to high temps
700-1200K
high pressures
up to 7000kPa
short time to avoid too much decomposition
1 second
products of thermal cracking
rich in alkenes
shorter chain molecules
catalytic cracking
long chain heated under pressure in presence of a zeolite catalyst
temp : 450 celcius
pressure : 1-2atm
time: 2-4 seconds
mechanism = carbocation / heterolytic fission
zeolite catalyst
acidic material made of aluminium oxide and silicon dioxide
honeycomb structure to allow for greater surface area which increases rate of reaction
products of catalytic cracking
contain mainly branched alkanes, cycloalkanes and aromatic compounds
usually produces motor fuels
fuels
substances that release heat energy when they undergo combustion
alkanes are used as fuels
more carbons present the greater the heat output
examples of alkane fuels
methane
propane
butane
petrol
paraffin
two types of combustion of alkanes
complete
incomplete
complete combustion
plentiful supply of o2
products formed : co2 and h2o
methane + oxygen = carbon dioxide + water
air can be interchanged for oxygen
incomplete combustion
limited supply of oxygen
products formed : carbon monoxide , water , carbon particulate
what is produced in incomplete combustion when there is an even more limited supply of oxygen
carbon particulates (soot)
examples of atmospheric pollutants
carbon monoxide
nitrogen oxides
sulfur dioxide
carbon particulate
unburnt hydrocarbons
carbon dioxide
water vapour
which gas is not a polllutant
nitrogen gas
describe an internal combustion machine
small amount of fuel and large amount of oxygen is drawn into the combustion chamber
mixture is compressed and ignited with an extreme temp spark.
mixture burns explosively , forcing movement of the engine parts
products of combustion exit via the exhaust
catalytic converters
can remove gaseous pollutants from internal combustion engines
honeycomb made of a ceramic material coated with platinum and rhodium metals - catalysts
why is the catalyst coated on ceramic honeycomb
bigger surface area
increased rate of reaction
removes more of the gases
ensures complete reaction
what can nitrogen oxides form
acid rain
why is a thin layer of metal used on the honeycomb ceramic support
reduce amount of metals needed
increase surface area
what happens as pollutants pass over the catalyst
react with each other , to form less harmful products
how does sulfur dioxide affect us
forms acid rain
acid rain destroys trees and vegetation . corrodes buildings , kills fish in lakes
how can sulfur dioxide be removed
flue gas desulfurisation
CaO and CaCO3 can be used to remove sulfur dioxide in an acid-base reaction to
produce gypsum.
Gypsum is a sealable product used to make builders plasters and plaster
boards.
Calcium carbonate and calcium oxide are used as they are basic so they
neutralise the acid.
greenhouse effect
infrared radiation from the sun passes through layers of gas in the atmosphere to hit the earth which warms the earth up
the ir from the earth travels back through the atmosphere where some is prevented from escaping into space by atmospheric gases such as co2 , h2o and ch4
this effect os trapping energy from the sun is called the greenhouse effect
global warming
term we give to the increasing average temperature of the atmosphere at the surface of the earth
caused by a change in concentration of co2 in the atmosphere
combustion of alkanes releases both water and carbon dioxide
