3.3.2- Alkanes (PAPER 2) Flashcards
What are alkanes?
Saturated (only single bonds between the carbon atoms) hydrocarbons (compounds containing only hydrogen and carbon)
Alkane homologous series increases by CH2 each time.
What is the general formula for alkanes?
CnH2n+2
What are the 3 physical properties of alkanes?
1) Non-polar: electronegativities of carbon and hydrogen are very similar so the electrons are shared equally, C-C bonds require large amount of energy to overcome so also unreactive.
2) Insoluble in water as water is a polar solvent so will dissolve polar substances, they dissolve in other non-polar liquids. Attractions between alkane and water are VDW’s which do not provide enough energy to break H bonding in water.
3) Relatively low boiling points: 1st 4 alkanes are gas at room temp and pressure, larger alkanes are volatile liquids.
How does boiling point change with with increasing size alkanes?
The larger the molecule- with increasing Mr, the stronger the vdw forces between the molecules and the higher the boiling point.
What are the most volatile components of crude oil?
Shorter hydrocarbon molecules which have a lower Mr so lower boiling point and higher volatility.
What are alkanes used for and why?
They are unreactive as their carbon-carbon bonds are strong BUT
Used mainly as fuels: the combustion of alkanes releases large amounts of energy which can be used to drive the cylinders in an engine.
Lubricants: reduce friction
What is crude oil?
A fossil fuel made from the breakdown of plant and animal remains at high temp+ pressure under surface of earth.
Non-renewable: forms at such a slow rate
Mixture of different alkanes of different lengths and branching.
Describe fractional distillation
NO COVALENT BONDS BROKEN!!!!!
Crude oil heated above its boiling point so it is vaporised, passed into fractionating column passing through trays until it is cool enough for them to condense back into a liquid, collect into the tray and leave as liquid alkane.
There is a negative temperature gradient.
Shorter alkanes rise further up column as they have lower boiling points, some do not condense at all. Largest alkanes leave as liquids that did not vaporise as it was not warm enough for them.
What occurs with the residue from fractional distillation?
Further distillation.
The residue still contains useful materials, the residue is further distilled under reduced pressure (vacuum distillation) which reduces temperature and prevents decomposition, solutions boil at a lower temperature.
Why is vacuum distillation helpful?
Less heat energy required so less energy to break C-C bonds and C-H strong covalent bonds preventing decomposition.
Give the name, length of carbon chain and use of the fractions in crude oil
LPG= 1-4 C chain, calor gas
Petrol= 4-12 C chain, petrol for cars
Naptha= 7-14 C chain, petrochemicals
Kerosene= 11-15 C chain, jet fuel
Gas oil= 15-19 C chain, central heating fuel
Mineral oil= 20-30 C chain, lubricating oil
Fuel oil= 30-40 C chain, fuel for ships
Wax/grease= 40-50 C chain, candles
Bitumen= 70+ Carbon chain, roofing
Properties of shorter alkanes
Lower boiling points: turn into gases more easily
More volatile: can turn from a liquid to a gas at normal temperatures
More flammable
Less viscous (thick) and more runny as the intermolecular forces are weaker
Less coloured than longer alkanes
What is cracking + 2 types?
Cracking is a chemical process where C-C bonds are broken in large alkane molecules to make smaller more useful alkanes.
2 types: thermal and catalytic
Describe thermal cracking
High temperatures (around 1000K) and high pressure (around 7000KPa).
C-C bonds break and one electron from each pair in covalent bond goes to each carbon atom- free radicals are formed through homolytic fission.
There are not enough H atoms to produce 2 alkanes: one of the new chains must have at least one C=C and is an alkene. The chain could break at any point- can limit the time the alkanes spend in these conditions to prevent decomposition.
Write thermal cracking equation of C15H32 to form octane and ethene in 1:2 ratio with one other product.
C15H32—-> C8H18 + 2C2H4 + C3H6
Describe catalytic cracking
Still high temperatures of around 723K, pressure only slightly above atmospheric.
Zeolite catalyst (silicon dioxide + aluminium oxide) with a honeycomb structure which has a large SA.
Produces branched alkanes, cycloalkanes and aromatic compounds- mainly produces motor fuels.
Ions are formed through heterolytic fission- the covalent bond breaks and the shared pair of electrons are taken by one of the atoms= uneven distribution.
What are the economic reasons for cracking?
-Shorter chain products found in crude oil are more useful as fuels
-Shorter chain products are more economically valuable than long chain products.
-Alkenes are converted to a wide range of products like polymers- eg ethene used for poly(ethene)
What is the difference between complete and incomplete combustion?
Complete combustion occurs when the alkane is burnt in excess oxygen to form CO2 (g) and H20 (l) whereas incomplete combustion occurs when the alkane is burnt in a limited supply of oxygen producing CO (g) or C (s) but still producing H20 (l)
What are the pollutants that can be made when burning alkanes? (7)
carbon monoxide- poisonous gas made through incomplete combustion,
carbon particulates (soot) can exacerbate asthma,
carbon dioxide, water are greenhouse gases,
sulfur dioxide if alkanes have sulfur dissolved in them S+O2–> SO2= can react with water to cause acid rain,
nitrogen oxides are most commonly produced in petrol engines due to high temperatures N2+ O2—-> 2NO can react with water to cause acid rain,
unburnt hydrocarbons can cause photochemical smog if they react with nitrogen oxides to create airborne particles
which can interfere with sunlight.
How to remove sulfur dioxide from the atmosphere?
Removed from flue gases by desulfurisation
Using CaO
CaO + SO2 -> CaSO3
CaSO3 is safely disposed of
Using CaCO3
CaCO3 + SO2 -> CaSO3 + CO2
How can catalytic converters reduce the escape of toxic gases?
The gases (eg CO, NOx, unburnt hydrocarbons) enter the catalytic converter and it is a honeycomb structure which has a large SA, maximising the opportunity for collision between exhaust gases and catalyst.
Expensive catalysts used: eg platinum, palladium, rhodium.
The exhaust gases react with eachother to form less harmful products.
Eg- 2CO(g) + 2NO (g) —–> N2 (g) + 2CO2 (g)
What is carbon footprint?
the total amount of carbon dioxide (and other greenhouse gases such as carbon dioxide, methane, nitrous oxide and CFCs (chlorofluorocarbons) which are emitted over the full life cycle of a product, service or event.
What is carbon neutral?
fuel is described as carbon-neutral if the production and use of the fuel has no net increase on the amount of carbon dioxide in the atmosphere.
What are chloroalkanes?
Compounds in which some H atoms of alkanes have been replaced by one or more chlorine atoms.
What are the conditions and reagents for the reaction between chlorine and methane?
Reagents: chlorine and methane
Conditions: UV light/ sunlight
What is the overall equation for the reaction between chlorine and methane?
CH4 + Cl2 —-> CH3Cl + HCl
What occurs during the initation?
Chlorine radicals formed in the presence of UV light.
Cl2 —-> 2Cl*
What occurs during propagation? 2 steps
Chlorine radicals react with methane molecules to form new radicals and molecules.
CH4 + Cl* —-> HCl + C*H3
CH3 + Cl2 —-> CH3Cl + Cl
What occurs during termination?
Radicals react with eachother to form molecules.
CH3 + Cl —–> CH3Cl
OR
CH3 + CH3 —-> C2H6
How to stop further reactions from occurring?
Use excess alkane
