Alkanes Flashcards
give a description of alkanes
they are a homologous series of saturated hydrocarbons with the general formula CnH2n+2
are alkanes reactive
however?
alkanes are very unreactive
however they burn in oxygen (combustion) and react with halogens to form halogenoalkanes
what happens to BP of alkanes as chain length increases and why?
boiling point increase as chain length increases because greater surface area contact and therefore more van der waal forces between molecules
what happens to BP as alkanes get more branched
BP decreases because they have less surface area contact between molecules and fewer van der waals forces between molecules
what is crude oil
crude oil is a complex mixture consisting mainly of alkanes (including cycloalkanes) aromatic compounds and others containing S,O,N.
is crude oil useful in its raw form ?
how does it become useful
no
by getting separated in fractional distillation
what is distillation
distillation is a method to separate mixtures of miscible liquids where separation is based on boiling point
list the essential features of fractional distillation of crude oil
1) crude oil is vaporised
2) the vapour is passed into a tower which is hotter at the bottom and cooler at the stop
3) as the vapour rises it cools
4) molecules will condense at different points as they have different boiling points
5) the larger the molecule the further down it condenses
6) this produces fractions ( a fraction is a mixture of hydrocarbons with similar boiling points
the boiling point of the alkanes increases as the strength of the van der waal forces increases . this is increased bevause:
increased mr
increased surface area contact
as the carbon chain gets longer, the hydrocarbons:
get more viscous
less flammable
less volatile
have higher BP
stronger van der waal forces between molecules
how do we supply demand for shorter hydrocarbons
longer hydrocarbons are cracked
what is cracking
cracking is the:
breaking of long chain alkane molecules into smaller ones
it involves breaking a strong C-C bond
what is naptha
short chain hydrocarbons (C7-C14)
why do we carry out cracking reactions
1) short chains are higher in demand economically
2) alkenes are used to make plastics and polymers
what are the 2 types of cracking
thermal and catalytic
describe thermal cracking
high temperature
high pressure
produces a high percentage of alkenes and straight chain alkanes
describe catalytic cracking
high temperature
moderate presssure
zeolite catalyst
produces branched alkanes (motor fuels) , cycloalkanes and aromatic hydrocarbons
why are alkanes used as fuels
they will burn
what is complete combustion
alkanes reacting with excess of oxygen
what is produced in complete combustion
alkane + oxygen —> carbon dioxide and water
which is more exothermic complete or incomplete combustion
complete
what’s incomplete combustion
reacting with limited oxygen
what’s produced in incomplete combustion
carbon monoxide CO (toxic gas)
carbon C (black solid
what’s produced in incomplete combustion
carbon monoxide CO (toxic gas)
carbon C (black solid)
and water
what other pollutants are made in internal combustion engines
NO or NO2
unburnt hydrocarbons
what other pollutants are made in coal burning power stations
SO2
how do we reduce the problem of carbon monoxide
use catalytic converters in cars
how do we reduce the problem of unburnt hydrocarbons
use catalytic converters in cars
how do we reduce the problem of NO/NO2
use catalytic converters in cars
what’s the problem with C carbon particulates
can cause respiratory problems
what’s the problem with unburnt hydrocarbons
photochemical smog
what’s the problem with NO/NO2
reacts with water to form acid rain
how is NO NO2 formed
combustion of N2 in the air and O2 in the air at very high temperatures in engines and furnaces
what’s the problem with sulfur dioxide
reacts with water to form acid rain
how to reduce problem of SO2
flue gas desulfurisation
what neutralises SO2
CaO and CaCO3 (bases)
where are sulfur compounds found
in coal and crude oil
how is SO2 formed
by burning of sulfur compounds
why are high temperatures needed to produce NO2
to break the strong triple bond between nitrogen atoms
name all the components of the catalytic converter
1) honey comb structure
2) made ceramic material
3) thin coating of metal catalyst ( rhodium palladium platinum)
why honey comb structure
increase surface area and cost effective
why ceramic material
withstands high temperatures of combustion engine
why thin coating of metal catalyst ( rhodium platinum palladium)
to reduce cost and increase surface area
what happens when dangerous gases pass over the catalyts
give example
the dangerous gases react together to produce less harmful gasses
2CO + 2NO —> 2CO2 and N2
give equation for formation of acid rain
4NO2 + 2H2O + O2 —> 4HNO3
