3.3.2 alkanes Flashcards
what type of molecule is an alkane?
a saturated hydrocarbon
how is crude oil separated into fractions?
fractional distillation
explain how crude oil is separated in the fractionating column
crude oil is heated to vaporise it
the vapour is passed into column
the top of tower is cooler than bottom
fractions then separated by the fact they have different boiling points
when the temp of the column is lower than the boiling point temp of the molecule, it condenses and is collected
low boiling point fractions are collected at the top and high boiling point fractions at the bottom
what is the relationship between size of the molecule and boiling point?
the larger the molecule, the higher the boiling point, there are more electrons in the molecule so Van der waals are stronger and require more energy to overcome
what is cracking?
the process of breaking longer chain hydrocarbons into smaller chain hydrocarbons
outline features of thermal cracking
requires high temperatures
requires high pressure
produces a large proportion of alkenes and lower proportion of branched alkanes
outline features of catalytic cracking
uses a zeolite catalyst
requires lower temperatures
requires slight pressure
produces a large proportion of branched alkanes and arenes
why are alkanes used as fuels?
combustion of alkanes is a highly exothermic process
this makes them good fuels as they release a large amount of of energy per gram of fuel
what are the products of complete combustion?
carbon dioxide and water
what are the products of incomplete combustion of alkanes?
one of CO or C and H20
what pollutants are produced by an internal combustion engine?
nitrogen monoxide/ dioxide, carbon monoxide, carbon and unburned hydrocarbons
how can these pollutants be removed from combustion engines?
catalytic converter
what catalysts does a catalytic converter have?
platinum and rhodium
outline the reaction that removes CO by reacting with oxygen
2CO + O2 — 2CO2
outline the reaction that removes CO and NO by reacting them together
2CO + 2NO — 2CO2 + N2
outline the reaction of the breakdown of NO
2NO — N2 + O2
what happens when sulphur in crude oil is burned?
it forms sulphur dioxide gas
what does sulphur dioxide and trioxide contribute to?
acid rain
how is sulphur dioxide removed from the air?
a neutralisation reaction with a base
flue gas desulphurisation
write an equation for the reaction of sulphur dioxide with a base
SO2 + CaO — CaSO3
what is the name of the mechanism for the reaction of alkanes to produce halogenoalkanes
free radical substitution
what are the 3 stages of free radical substitution?
initiation
propagation
termination
outline an equation for initiation
Cl2 — 2Cl .
explain the steps of initiation
in the presence of UV light, the bond in a chlorine molecule breaks forming a chlorine radical
a radical is an atom or group of atoms with an unpaired electron
homolytic bond fission occurs = bond breaks equally
outline equations for the propagation step
CH4 + Cl. — .CH3 + HCl
.CH3 + Cl2 — CH3Cl + Cl.
explain the steps of propagation
chlorine radical collides with the alkane breaking a C-H bond and taking the H atom to form an alkyl radical
the alkyl radical collides with more chlorine molecules to produce the halogenoalkanes and more chlorine radicals
outline equations for the termination step
.CH3 + Cl. — CH3Cl
Cl. + Cl. — Cl2
.CH3 + .CH3 — CH3CH3
explain the steps of termination
when radicals collide they form a stable molecule
there are multiple ways radicals for the reaction could combine
what is a radical?
an atom or group of atoms with an unpaired electron
