Chapter 12 - Alkanes Flashcards
what are alkanes
a holologous series of saturated hydrocarbons with the general formula CnH2n+2
they are very unreactive, aside from burning and reacting with halogens
describe and explain the trend in boiling points as the carbon chain increases in length
comment on branched isomers
they generally increase as the chain increases because the strength of van der waals forces in between molecules increase as the total number of electrons increases.
for chained isomers, the more branches there are, the lower the boiling point because the molecules cannot pack as close togehter resulting in lower van der waals forces
what is crude oil and how is it formed
crude oils are fossil fuels containing a mixture of hydrocarbons
they are formed during the slow decay of marine organisms over millions of years in high heat, high pressure, and anaerobic conditions
on what basis are compound separated in fractional distilation
different length chains have different boiling points so we can separate these using fractional distillation
how is fractional distillation carries out
crude oil is vaporided
the vapour is passes into a tower which is cold at the top and hot at the bottom
as the vapour rises, it cools
different length molecules will condense at different heights (shorter at top, longer at bottom)
what is cracking
the breaking of c-c bonds in long hydrocarbons to make shorter hydrocarbons which are more in demand
it is the thermal decomposition of alkanes
what are the conditions of thermal cracking
900 Celsius
70atm
no catalyst
produces alkenes used for polymers for example
what are the conditions of catalytic cracking
450 Celsius
1-2 atm
zeolite catalysts
produces motor fuels (aromatics, cyclic alkanes, branched alkanes)
what is reforming and why is it used
a process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic
alkanes. Both these products burn more efficiently and are used in petrol for cars.
why are alkanes useful as fuels
because when burned in the presence of oxygen, the reaction is highly exothermic
what are the products of complete alkane combustion
carbon dioxide and water
what are the products of incomplete alkane combustion
either carbon monoxide and water
or carbon and water
when does incomplete combustion occur
when there is not enough oxygen
how is CO2 formed, what problem does it cause, and how can you reduce this
complete combustion of carbon based fuels
it is a greenhouse gas
burn less fossil fuels
how is CO formed, what problem does it cause, and how can you reduce this
incomplete combustion of carbon based fuels
it is highly toxic
ensure a good supply of oxygen when burning fuels
how is soot formed, what problem does it cause, and how can you reduce this
incomplete combustion of carbon based fuels
global dimming, respiratory problems, blackens buildings
ensure a good supply of oxygen when burning fuels
how is H2O formed, what problem does it cause, and how can you reduce this
combustion of fuels containing hydrogen
it isnt a major problem
how is SO2 formed, what problem does it cause, and how can you reduce this
combustion of sulfur compounds in fuels
acid rain
remove sulfur from fuel before burning, use flue gas desulfurisation
how are nitrate oxides (NOx) formed, what problem does it cause, and how can you reduce this
reaction of N2 in the air with O2 in the air at very high temperatures
acid rain
use catalytic converter (in cars)
how are unburned hydrocarbons formed, what problem does it cause, and how can you reduce this
some fuel just doesnt burn/react
this is a waste of fuel
ensure all engines are well tuned and have a good oxygen supply
how can you remove SO2 from waste gases produced in fuel combustion
using flue gas desulfurisation
this involves passing the gases through a scrubber containing calcium oxide or calcium carbonate which reacts with the SO2 to form CaSO3 (gypsum) which can be used to make plasterboard
what do catalytic converters do
they remove CO2, NOx and unburned hydrocarbons, turning them into CO2, NOx, and H2O
how do greenhouse gases work
They absorb infrared radiation emitted by the Earth – this is due to bond vibrations (these vibrations lead to changes in dipoles that lead to the absorption of infrared).
state a condition necessary for free radical mechanism reactions
the presence of UV rays
name the steps in free radical mechanisms
initiation
propagation
termination
what is a free radical
an non metal atom with a free electron in its outer shell
they are very reactive because of their instability
give the equations for the propagation steps when ozone decomposes into oxygen
O3 + Cl° —> ClO° + O2
ClO° + O3 —> 2O2 + Cl°
give the propagation step in the decomposition of ozone due to chloroflourocarbons CFCs
CF3Cl —> Cl° + °CF3
why should CFCs not be used for household products anymore
when released into the atmosphere, they do not react but rather decompose in homolytic fission due to the UV radiation present to form radicals which react with the ozone to give oxygen
what is the catalyst in the decomposition of ozone
chlorine free radicals
how are radicals formed
UV radiation causes homolytic fission to diatomic molecules with low enough bond energies such as Cl2
what is homolytic fission
when a molecule splits evenly and half the shared electrons go to one atom and the other half go to the other atom
why are free radical substitutions chain reactions until termication
because each propagation reaction releases more free radicals which react with substances in the reaction
this repeats until termination
