organic chemistry alkanes Flashcards
what is the general formula of an alkane
CnH2n+2
Are alkanes saturated or unsaturated
saturated as only contain single bonds
what are the common characteristics of a homologous series
1) similar chemical properties 2) same general formula 3) gradual trend in physical properties 4) same functional group 5) each member differs by CH2
explain and describe the reactivity of alkanes
very unreactive, although they do burn and react with halogens eg. chlorine
what is crude oil
a finite, non renewable resource a mixture of 150 different hydrocarbons - no use in its raw form so we must separate components (and modified if needed)
where does crude oil come from
deposits and natural gas usually occur together, formed by the slow decay of marine animals and plants over millions of years under heat and pressure in the absence of air
what is a hydrocarbon
a compound ONLY containing hydrogen and carbon - crude oil is a hydrocarbon, majority are straight chain alkanes but also contains cycloalkanes and arenes (etc benzenes)
what is a crude oil fraction
a mixture of hydrocarbons of similar sized chain length and similar BP the difference of physical properties of the fractions allow crude oil to be seperated
what are the basic principles of fractional distilation
1) crude oil is vaporized 2) vapour passes through into column with a negative temp gradient (hot at bottom and cold at top) 3)vapour rises and cools 4) molecules will condense at different heights as they have different BP 5) the larger the molecule, the higher the BP, the lower down the column it will condense
why do the alkanes have different boiling points
- no polar bonds - weak van der waals forces holding molecules together (just breaking VDW) - longer chain molecules have larger electron clouds and more contact area SA for VDW
explain the polarity of alkanes
- almost non-polar as H+C have similar EN - strongest intermolecular is VDW
explain the BP of alkanes
- increase with chain length - C1-C4 are gas and C5 + are liquid - branched chains have lower `BP
explain the solubility of alkanes
- insoluble in water - mix with other non polar solvents
explain the viscosity of alkanes
increases with chain length
explain the volatility and colour of alkanes
(volatile = how easily it turns into a gas) - volatility decreases with chain length - molecules get darker in colour with chain length
how do alkanes react
relatively unreactive - do not react with acids, bases and oxidising agents or reducing agents -however they do combust and react with halogens when with UV light
what is the issue with supply and demand of different fractions of alkanes
for some fractions the supply and demand figures do not match the more useful fractions tend to be shorter chain length as they tend to make better fuels
what is cracking
chemical splitting of long chain alkanes into shorter chain length alkanes or alkenes , involves splitting of the ‘C-C’ bond eg. decane to octane and ethene C10H22 —> C8H18 + C2H4
what are the two methods of cracking
thermal and catalytic
explain the conditions for thermal cracking
900 degrees celsius 70 atm presssure makes alkenes
explain the conditions for catalytic cracking
450 degrees celsius 1-2 atm zeolite catalyst and makes aromatics (cyclic alkanes, branched alkanes)
what are the products and uses from cracking
thermal produces alkenes, eg. ethene can be used to make polymers catalytic makes aromatics eg. branched alkanes which can used for motor fuels
explain the combustion of alkanes
readily burn in the presence of oxygen, this combustion is highly exothermic explaining their use as fuels - products of complete combustion is carbon dioxide and water - if incomplete combustion (not enough oxygen) occurs CO (carbon monoxide) or C (soot solid) is produced
why is carbon monoxide toxic
binds to the same site in haemoglobin molecules in red blood cells as oxygen - prevents oxygen being transported around the body
give examples for complete and incomplete combustion of alkanes
combustions of propane (C3H8)
complete : C3H8 + 5O2 —> 3CO2 + 4H2O
incomplete: C3H8 + 31/2O2 —> 3CO + 4H2O
C3H8 + 2O2 —> 3C(s) + 4H2O
how is CO2 pollutant formed, problems caused and ways to reduce it
- formed from complete combusion of any carbon fuel
- green house gas and contributes to global warming
- burn less fuel
how is CO pollutant formed, problems caused and ways to reduce it
- incomplete combustion
- toxic
- ensure a good supply of oxygen and use a catalytic converter
how is C pollutant formed, problems caused and ways to reduce it
- incomplete combusion
- blackens buildings, global dimming , exacerbates asthma
- ensure a good supply of oxygen
how is water pollutant formed, problems caused and ways to reduce it
- combustion of fuels containing hydrogen
- green house gas
- however part of larger water cycle so not too much concern
how is sulfur dioxide pollutant formed, problems caused and ways to reduce it
combustion of sulfur containing compounds in fuels
- causes acid rain
- remove sulfur from fuel before burning and flue gas desulfurisation
how is oxides of nitrogen (NO, NO2) pollutant formed, problems caused and ways to reduce it
reaction of N2 and oxygen in the air
- needs a very high temp to react (spark of ignition in a car)
- causes acid rain
- use a catalytic converter
how is unburned hydrocarbon pollutant formed, problems caused and ways to reduce it
some of fuels may not burn due to lack of oxygen
waste issues
ensure the engine is well serviced and good supply of oxygen and use of a catalytic converter
how is sulfur dioxide formed from burning of fuels
- sulfur impurities are found in petroleum fraction leading to sulfur dixoide when they are burned which leads to acid rain
S + O2 —> SO2
can be removed from the waste gases from furnaces by flue gas desulfurisation
how does flue gas desulfurisation work
the gases pass through a scrubber containing calcium oxide or calcium carbonate which react with the sulfur dioxide
CaO and CaCO3 are bases as they are metal oxided and SO2 is acidic
CaO + SO2 —> CaSO3 (calcium sulfite not sulfate)
CaCO3 + SO2 —> CaSO3 + CO2
this is a acid-base reaction
product is calcium sulfite (+4) which is gypsum (makes plaster board)
explain the process of internal combusion within an engine
petrol (gasoline fraction which consists of liquid alkanes) are used in the interal combustion engine where the alkanes are vapourised and combusted with air
catalytic converters remove CO, NOx and unburned hydrocarbons
explain how catalytic converters work
remove CO, NOx and unburned hydrocarbons ag. octane from the exhaust pipe, turning them into CO2, N2 and H2O
The converters have a ceramic honeycomb coated with a thin layer of catalytic metals eg. Pt, Pd and Rh, this structure is to give a large SA with minimal metal as they are expensive
reactions:
explain the impact of burning hydrocarbons and global warming
the burning of these fossil fuels releases carbon dioxide into the atmosphere
carbon dioxide methane and water vapour are all green house gases,they absorb infrared radiation emitted by the earth due to bond vibrations (these bond vibrations lead to changes in dipoles that lead to the absorption of infrared)
water is the main greenhouse gas (but is natural) followed by CO2 then methane
explain the impacts of global warming on the earth
methane levels increased due to agriculture
- higher temps and more heat waves
- changing in rain and slow patterns
- damaged corals
- rising sea levels
- thawing permafrost
- changes in plant life cycles
define a mechanism
a picture of how things react, a series of steps
what is free radical substitution
H atoms are replaced by halogen atoms (eg. F, Cl, Br and I). For every one H atom that is replaced one X2 molecule is used up and one HX molecule is formed (X being a halogen)
give an example of the overall reaction and process of FRS
Depending on how many hydrogens you are replacing this must be highlighted with how many moles of the halogen you use
for example replace 3 hydrogens must use three moles
write the formulas in structural formula (eg. the correct order of atoms) or you will not be able to see correct structure
show an overall equation of FRS where structural formula is needed
what is a free radical
a species with an unpaired electron, they are very reactive and this is why they react with alkanes
- represent with a dot above the chemical symbol
explain and name the first stage of free radical substitution
Initiation
when exposed to a UV light a molecule of a halogen breaks apart into two halogen atom free radicals
why is a UV light needed in initiation
provides the energy to break the covalent bond between 2 halogen atoms
- homolytic bond fission
acts as a condition
show an example equation of initiation
explain and name step 2 for free radical sub.
Propagation
For every H that is replaced there is one pair of propagation reactions
part 1: the alkane/ haloalkane reacts with the free radical halogen, removing a hydrogen from the alkane- this produces a HF/HCl/HBr/HI and a C based free radical
part 2: the C based radical from step one reacts with F2/Br2/I2/Cl2 to put one of these atoms onto the C based radical, this produces another halogen free radical to continue the chain
show an equation for step 2 in FRS
make sure to look at the structural form and where the H was removed from and therefor where the free radical C is
explain and name stage 3 of free radical substition
termination
If two free radicals collide they will form a molecule and stop the chain. Any two free radicals involved in the mechanism could collide in this way
in general : radical + radical —> molecule
show examples of termination reactions
CF3CH3F in CH4 –> CF4
.CF3 + .CH2F —> CF3CH2F
the radicals must be present in the mech.
what are the equations for the catalytic converters
2CO + 2NO → 2CO2 + N2
C8H18 + 25NO → 8CO2 + 12.5N2 + 9H2O
