CRACKING AND COMBUSTION

Question 1

what is cracking

Answer 1

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

Question 2

economical reasons for cracking

Answer 2

smaller chain molecules are in higher demand
products from cracking are more valuable

Question 3

thermal cracking

Answer 3

long chain molecules heated to high temps
700-1200K
high pressures
up to 7000kPa
short time to avoid too much decomposition
1 second

Question 4

products of thermal cracking

Answer 4

rich in alkenes
shorter chain molecules

Question 5

catalytic cracking

Answer 5

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

Question 6

zeolite catalyst

Answer 6

acidic material made of aluminium oxide and silicon dioxide
honeycomb structure to allow for greater surface area which increases rate of reaction

Question 7

products of catalytic cracking

Answer 7

contain mainly branched alkanes, cycloalkanes and aromatic compounds
usually produces motor fuels

Question 8

fuels

Answer 8

substances that release heat energy when they undergo combustion
alkanes are used as fuels
more carbons present the greater the heat output

Question 9

examples of alkane fuels

Answer 9

methane
propane
butane
petrol
paraffin

Question 10

two types of combustion of alkanes

Answer 10

complete
incomplete

Question 11

complete combustion

Answer 11

plentiful supply of o2
products formed : co2 and h2o
methane + oxygen = carbon dioxide + water
air can be interchanged for oxygen

Question 12

incomplete combustion

Answer 12

limited supply of oxygen
products formed : carbon monoxide , water , carbon particulate

Question 13

what is produced in incomplete combustion when there is an even more limited supply of oxygen

Answer 13

carbon particulates (soot)

Question 14

examples of atmospheric pollutants

Answer 14

carbon monoxide
nitrogen oxides
sulfur dioxide
carbon particulate
unburnt hydrocarbons
carbon dioxide
water vapour

Question 15

which gas is not a polllutant

Answer 15

nitrogen gas

Question 16

describe an internal combustion machine

Answer 16

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

Question 17

catalytic converters

Answer 17

can remove gaseous pollutants from internal combustion engines
honeycomb made of a ceramic material coated with platinum and rhodium metals - catalysts

Question 17

why is the catalyst coated on ceramic honeycomb

Answer 17

bigger surface area
increased rate of reaction
removes more of the gases
ensures complete reaction

Question 18

what can nitrogen oxides form

Answer 18

acid rain

Question 19

why is a thin layer of metal used on the honeycomb ceramic support

Answer 19

reduce amount of metals needed
increase surface area

Question 20

what happens as pollutants pass over the catalyst

Answer 20

react with each other , to form less harmful products

Question 21

how does sulfur dioxide affect us

Answer 21

forms acid rain
acid rain destroys trees and vegetation . corrodes buildings , kills fish in lakes

Question 22

how can sulfur dioxide be removed

Answer 22

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.

Question 23

greenhouse effect

Answer 23

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

Question 24

global warming

Answer 24

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