alkanes Flashcards
what are alkanes
a homologous series of saturated hydrocarbons
what is the general formula of an alkane
CnH2n+2
why are alkanes insoluble
they are not polar and cannot overcome / disrupt the hydrogen bonding between water molecules.
what is the trend in boiling points
As the alkane gets longer, the boiling point increases. This is because a longer alkane chain has increased intermolecular forces which require more energy to overcome and cause the state change from liquid to gas.
why are the boiling points of longer alkanes higher
stronger van der waals forces bwt the molecules which require greater amounts of energy to overcome- as there are more electrons
why are the boiling points of shorter alkanes lower
weaker VDW
what is the process in which alkanes are fractioned into different sections
fractional distillation
what is crude oil
Crude oil is a fossil fuel, formed from the decay of sea creatures over millions of years under high pressure and lack of oxygen
how does fractional distillation work
Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top
Crude oil enters the fractionating column and is heated so vapours rise
Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column
Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off
The different fractions condense at different heights according to their boiling points and are tapped off as liquids
The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases
The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column
The crude oil contains small amounts of other compounds
Some of these compounds may contain sulfur, and when burned sulfur dioxide can be produced
This is one of the causes of acid rain
what are the fractions of hydrocarbons then used for after FD
they are not useful as heavy alkanes so they are cracked into shorter alkanes and alkenes which are higher in demand
what is cracking
the thermal decomposition of alkanes
what is thermal cracking
temp = 900
pressure = 70atm
catalyst= none
products= alkanes
what is catalytic cracking
temp=450
pressure = 1-2 atm
cataylst= zeolites
products= alkenes + motor fuels
why do alkanes undergo combustion
Alkanes are combusted (burnt) on a large scale for their use as fuels
what is complete combustion
When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
what are the products of complete combustion
CO2 and H2O
what is incomplete combustion
When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
Some carbon is only partially oxidised to form carbon monoxide
For example, the incomplete combustion of octane to form carbon monoxide
what are the products of incomplete combsution
H2O
CO and/or C
when will carbon be produced
Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present
why is CO harmful
CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
Oxygen is transported to organs
Carbon dioxide is removed as waste material from organs
why are oxides of nitrogen and sulfur harmful
Normally, nitrogen is too unreactive to react with oxygen in air
However, in a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:
N2(g) + O2(g) → 2NO(g)
N2(g) + 2O2(g) → 2NO2(g)
The oxides of nitrogen are then released in the exhaust fumes into the atmosphere
Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)
Nitrogen oxides and sulfur dioxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain
Acid rain can cause corrosion of buildings, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health
how is carbon harmful
balckens buildings
can cause respiratory problems
global dimming
how is CO2 harmful
greenhouse gas
how does sulphur dioxide form
the sulphur in impurties in petroleum can react with oxygen in atmosphere or air producing SO2
how is SO2 removed from waste gases in furnaces
flue gas desulfuristation
how does flue gas desulfurisation work
sulpher reacts with either calcium oxide or calcium carbonate in a scrubber and undergoes an acid-base reaction
what does the reaction of SO2 + CaO/ CaCO3 form
gypsum which is used to make plasterboard
what are catalytic convertors
To reduce the amount of pollutants released in car exhaust fumes, many cars are now fitted with catalytic converters
Precious metals (such as platinum) are coated on a honeycomb to provide a large surface area and used as catalysts
The reactions that take place in the catalytic converter include:
Oxidation of CO to CO2:
2CO + O2 → 2CO2
or
2CO + 2NO → 2CO2 + N2
Reduction of NO to N2:
2CO + 2NO → 2CO2 + N2
Oxidation of unburnt hydrocarbons:
CnH2n+2 + (3n+1)[O] → nCO2 + (n+1)H2O
what are the catalyst involved in catalytic conervtors
platinum
palladium
rubidium
what is global warming
Climate change refers to long-term shifts in temperatures and weather patterns. -The Earth is getting warmer and many scientists believe it is due to increasing amounts of greenhouse gases in the
atmosphere.
how do greenhouses gases cause global warming
Greenhouse gases are gases that absorbs the IR radiation given off by the Earth, but do not absorb the higher
frequency UV/visible radiation given off by the sun.
* Molecules that contain polar bonds absorb IR radiation to make bonds vibrate.
* therefore, molecules such as CO2 (the C=O bonds are polar), H2O (the O-H bonds are polar) and CH4 (the C-H bonds
are slightly polar).
* The burning of fossil fuels (including alkanes) releases carbon dioxide into the atmosphere. Carbon dioxide levels have
risen significantly in recent years due to increasing burning of fossil fuels.
* Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global
warming.
* Methane levels have also increased significantly largely due to agriculture (e.g. rice production) and farming (e.g. cows).
why do some molecules not absorb IR
- molecules such as O2 and N2 do not absorb IR radiation and are not greenhouse gases as they do not have polar
bonds.
