Alkanes

-Alkanes are saturated hydrocarbons that contain only C-C and C-H single bonds. 

-among the least reactive organic compounds and are used for fuels, lubricants and starting materials for other compounds 

CnH2n+2 for straight and unbranched chains and CnH2n for ringed molecules as 2 end H's arent required

-methane, ethane and propane have no isomers but after that the number of possible isomers increases with the number of C atoms 

- almost non-polar as electronegativity of C and H is very similar. so only van der waal forces, which increase in strength as size increases

- boiling points increase as size increases as stronger van der waals 

- insoluble in water as the H bonding in water is much stronger than the van der waals 

-alcohols only burn or react with halogens 

crude oil contains a mixture of alkanes

also contains a small amount of other compounds- some of which contain S which forms SO2 when burnt. this reacts with oxygen in atmosphere to form SO3 which then reacts with water to form sulphuric acid

Process of fractional distillation in a fractioning tower:

• crude oil heated in a furnace to become a mixture of vapours
• passes into the bottom of a tower which is cooler at the top than at the bottom
• the vapours pass up the tower via a series of trays containing bubble caps until they reach a tray at a low enough temperature for it to condense to form a liquid 
• the mixture of liquids that condenses at each tray is piped off 
• shorter chain HC's condense nearer the top where its cooler 
• tar (thick residue) collects at bottom of tower which can be used for roads but usually is processed further to give shorter more valuable products. 

longer chain molecules arent in as high demand as shorter chain ones so longer chain fractions are usually cracked which has 2 results:

• produces shorter, more useful alkanes
• produces alkenes which are more reactive and important starting materials 

Thermal cracking:

• high temp- 700-1200K
• high pressure- 7000kPa 
• the C-C bonds break in a way that one electron from the pair goes to each C atom so each new chain has a free radical. these then react in a variery of ways to form shorter chain molecules
• forming an alkane and an alkene as not enough hydrogen to produce 2 alkanes
• the alkanes are only kept in these conditions for a short period of time to prevent thermal decomposition 

Catalytic cracking:

• lower temp (720K) and lower temp 
• zeolite catalyst- silicon dioxide and aluminium oxide 
• zeolites have a honeycomb structure with an enourmous SA
• tends to produce branched alkanes, cycloalkanes and aromatic compounds 
• products obtained are seperated by fractional distillation 
• used to produce motor fuels 
• products are mostly gases

shorter chain alkanes burn completely in plentiful supply of oxygen to give CO2 and H2O 

-combustion reactions give out heat and large -ve enthalpies of combustion. the more C's present, the greater the heat output- good for use as fuels

Fuels are substances that release heat energy when they undergo combustion. They also store large amounts of energy for a small weight

Incomplete combustion:

carbon monoxide (and water) formed but with even less oxygen, soot can be formed- usually occurs with longer chain hydrocarbons as they require more oxygen

-CO- poisonous gas from incomplete combustion

- NOx (nitrogen oxides) form when there is enough energy for oxygen and nitrogen in the air to react (in engines when sparks ignite the fuel). these my react with water to form nitric acid- acid rain

- SO2- acid rain

-carbon particulates- cause cancer and worsen asthma 

- unburnt hydrocarbons- greenhouse gases 

-CO2- greenhouse gas

-water vapour- greenhouse gas 

Flue gas desulfurisation:

-when SO2 reacts with oxygen and water in the air it forms sulfuric acid 

-one way to remove SO2 before it is released:

• calcium oxide and water is sprayed into the flue gases
• calcium sulfite is formed
• can be further oxidised to calcum sulfate used to make plaster 
• CaO + 2H2O+ SO2+ 0.5O2-----> CaSO4.2H2O

or you can use calcium carbonate instead of calcium oxide to still form calcium sulfate

Catalytic converters: 

-cars are fitted with catalytic converters which reduce the output pollutants in the exhaust gas mixture. 

- they are honeycombs made of ceramic material coated with platinum and rhodium- large SA

when you put a mixture of an alkane and a halogen into bright light, they will react to form a halogenoalkane. The UV component of light starts the reaction 

a substitution reaction takes place- one or more H atom is replaced by a halogen and fumes of a hydrogen-halide gas eg hydrogen bromide is given off 

Free radical substitution- chain reactions in three stages. 

1. Initiation:

• breaking the X-X (halogen) bond to form 2 halogen atoms. does this by absorbing UV light  and because the energy of the quantum of light is greater than the bond energy, it breaks 
• the bond breaking results in 2 free halogen radicals forming 
• the C-H bond does not break at this stage as it needs more than 1 quantum of UV light 

2. propagation:

• halogen free radical takes a H from the alkane to form a stable hydrogen halide. But this leaves a alkane free radical eg methyl eg Cl. + CH4 ---> HCl + CH3. 
• this methyl free radical is very reactive so will react with a halogen molecule- producing another halogen free radical and halogen compound  
• eg CH3. +Cl2---> Cl. + CH3Cl 
• the free radical is ready to react with more alkane and repeate the 2 steps 
•  chain part of the reaction and may take place 1000's of time before termination

3. Termination:

• the step in which free radicals are removed by:
• 2 halogen free radicals reacting together to give a halogen molecule
• 2 of the alkane free radicals (methyl) reacting together to give a compound 
• or a chlorine free radical and an alkane free radical reacting together to give a compound 

some other products can be formed as well as the main ones 

• eg at the termination stage when the 2 alkane radicals react to form a longer chain alkane molecule 
• with longer chain alkanes there will be many isomers formed because the Cl can replace any of the H atoms
• chain reactions arent useful as they produce a mixture of products and dont occur without bright light

• CFC's (chloroflourocarbons) in the atmosphere are destroying the ozone layer 
• ozone is important as it protects the earth from UV rays 
• Cl free radicals are formed from CFC's as the C-Cl bond breaks homolytically in the presence of UV radiation. these then attack ozone molecules Cl. + O3 ---> ClO. +O2 
• the resulting radicals (ClO.) also attack ozone and regenerate Cl free radicals: ClO. + O3 ---> 2O2 +Cl.