Akanes 6B Flashcards
what is the general formula of alkanes?
Cn H2n +2
what are the two types of fission?
homolytic and heterolytic
what is homolytic fission?
in homolytic fission, the bond breaks evenly and each binding atom receives one electron from the bonded pair. this means that two electrically uncharged radicals are formed. these are particles that have an unpaired electron (big dot next to the molecular formula)
what is heterolytic fission?
in heterolytic fission the bind breaks unevenly with one of the bonded atoms receiving both of the electrons from the bonded pair. two different substances can form - a positively charged cation or a negatively charged anion .
what is the reaction called in which halogens and alkanes react?
photochemical reaction. they are started by light - most reactions require ultraviolet light ignorer for them to start
what are the stages of a reaction mechanism?
initiation, propagation and termination
how are radicals reduced in halogenoalkane reactions
they are produced by initiation reactions.
sunlight produce enough energy to break the Cl-Cl bond - photodissociation
the bond splits equally and each electron gets to keep one electron - homolytic fission the aim becomes a highly reactive radical because of its unpaired electron
what happens in a propagation reaction?
radicals are used up and created in a chain reaction
Cl radical attacks a methane molecule
the new methane radical can attack another Cl2 molecule
the new Cl radical can can attack another methane molecule and so on and so on until the Cl2 or the methane are wiped out
what happens in a termination reaction?
the radicals are destroyed
if two free radicals join together, they make a stable molecule
there are lots of possible
what is the problem with reaction mechanisms?
you end up with a mixture if products
if your trying to make particular products you don’t only get the product that you want but a mixture of products
what is petroleum?
a different name for crude oil. mixture of hydrocarbons, it is usually made up of alkanes - they range from small alkanes to massive alkanes
how does fractional distillation work?
first the crude oil is vaporised at a out 350 oC
the vaporised crude oil then goes into the fractionating column and rises up through the trays. the largest hydrocarbons don’t vaporise at all as their boiling points are too high.
as the crude oil vapour goes up the fractionating column it gets cooler. as the alkane molecules have different chain lengths they have different boiling points so each fraction condenses at a different temperature
what are two types of cracking?
thermal cracking and catalytic cracking
what is thermal cracking
it takes place at a high temperature and a high pressure. it produces a lot of alkanes
what is catalytic cracking
catalytic cracking uses something called a zeolite catalyst (hydrated aluminosilicate), a slight pressure and high temperature
it mostly produces aromatic hydrocarbons and motor fuels
using a catalyst cuts costs, because the reaction can be done at a low temperature also it speeds up the reaction.
what can alkanes be reformed into?
Cycloalkanes and Aromatic Hydrocarbons
what is knocking?
it is where atoms explode out of their own accord when the fuel/air is mixed in a compressed engine
straight chain alkanes are the most likely to cause knocking. adding branches to the chain and cyclic hydrocarbons to the petrol mixture makes knocking less likely to happen so combustion is more important
why are alkanes useful as fuels?
if you burn alkanes with oxygen you get carbon dioxide and water - combustion reaction - if there is not much oxygen around the alkane will s till burn but it will produce a mixture of mainly carbon monoxide, carbon and water - incomplete combustion.
combustion reactions happen between gases, so liquid alkane have to be vaporised first
combustion reactions are exothermic
why do alkanes produce harmful Emissions?
Carbon monoxide is harmful - it is better at binding to the haemoglobin in the bloodstream than oxygen, so it binds before - less oxygen can move around the body leading to oxygen deprivation
sulfer dioxide and oxides of nitrogen lead to acid rain - dissolves in the moisture of the atmosphere and is converted to sulphuric acid
oxides of nitrogen dissolve in the moisture in the atmosphere and are converted to nitric acid
what do catalytic converters do?
they remove some pollutants from car emissions
they get rid of them by using a platinum catalyst to change them to harmless gases, like water vapour and nitrogen
why are fossil fuels non renewable?
the main fossil fuels - coal, oil and natural gas are relatively easily extracted however there is a finite amount of it
what is bioethanol?
the fermentation of sugar from crops to create ethanol.
what is biodiesel?
it is made from the refining of renewable fats and oils such as vegetable oil
