alkanes (3.3.2)
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what is the reaction of chlorine and methane an example of?
a free radical substitution mechanism
what are the stages in the reaction of chlorine and methane?
• initiation • propagation (2 steps) • termination
what happens during initiation?
a chlorine molecule absorbs the energy of a single quantum of UV light and splits into two chlorine free radicals
what is the equation for the initiation stage?
Cl2 -> (UV light) 2Cl.
what happens in the propagation stage?
• a chlorine free radical and methane molecule collide and the Cl free radical gains a H atom from methane producing a methyl free radical and HCl • the methyl free radical collides with a chlorine molecule and gains a chlorine atom producing CH3Cl and a chlorine free radical
what are the equations for the propagation stage - steps 1, 2 and overall?
1- Cl. + CH4 -> HCl + C.H3 2- Cl2 + C.H3 -> CH3Cl + Cl. overall - CH4 + Cl2 -> HCl + CH3Cl
what happens in the termination stage?
two free radicals collide?
what are the 3 possible equations for the termination stage?
Cl. + Cl. -> Cl2 C.H3 + C.H3 -> C2H6 Cl. + C.H3 -> CH3Cl
what are alkanes?
saturated hydrocarbons
what is petroleum?
a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation
what happens during fractional distillation?
• crude oil is heated to high temperatures turning it into a vapour • crude oil is put into the bottom of the fractioning column which has a heat gradient, so it is hottest at the bottom and gets cooler as you go up • long chain hydrocarbons with high boiling points condense at the bottom of the column forming liquid fractions • the other hydrocarbon molecules rise upwards and condense into fractions when they reach a level at their boiling point • short chain hydrocarbons with low boiling points condense at the top of the fractionating column
what is a fraction?
a mixture of hydrocarbons that have similar boiling points and similar numbers of carbon atoms eg. petrol, naphtha, kerosene
what is cracking?
when longer chain, less useful hydrocarbons are broken up into shorter chain, less useful ones
what does cracking involve?
breaking C-C bonds in alkanes
what are the two types of cracking?
thermal and catalytic
what are the conditions for thermal cracking?
high temperature - 450-900 degrees Celsius high pressure - 70 atm
what are the conditions for catalytic cracking?
high temperature - 450 degrees slight pressure - 1-2 atm presence of zeolite catalyst
what are the main products of thermal cracking?
high percentage of alkenes
what are the main products of catalytic cracking?
branched chain alkanes, cyclic alkanes, aromatic compounds eg. benzene
what are the economic reasons for cracking?
crude oil generally contains a higher proportion of long chain hydrocarbons, so they are in greater supply but there is less demand short chain hydrocarbons are in low supply but there is more demand, so cracking allows more short chain hydrocarbons to be produced to increase the supply and meet the demand for them
what are alkanes used as?
fuels
what are the types of combustion which alkanes and other organic compounds can undergo?
complete - excess oxygen, produces CO2 and water incomplete - limited oxygen, produces CO and water (possibly also solid C/soot)
what can internal combustion engines produce, and how can the products be removed?
pollutants including NOx, CO, carbon and unburned fuels remove gaseous pollutants using catalytic converters
pollution from carbon-based fuels - nitrogen oxides?
origin - N2 + O2 -> 2NO / N2 + O2 -> 2NO2 (high temperature cause nitrogen from air to react with oxygen in an engine) problems - acid rain (4NO2 + 2H2O + O2 -> HNO3) removal - 2CO + 2NO -> 2CO2 + N2