9: Alkanes

hydrocarbon structure

each carbon atom in an alkane is surrounded by 4 bps of electrons so the shape around each carbon atom is a tetrahedral and the bond angles are 109.5 degrees

polarity, structure, bonding

• carbon and hydrogen have similar electronegativity so the bonds are nonpolar

• this means that all alkane molecules will also be nonpolar

• alkanes have a simple molecular structure with van der Waals. these are weak intermolecular forces

solubility

The forces of attraction between water molecules are hydrogen bonds which are much stronger than the van der Waals in alkanes, therefore alkanes are not soluble in water

trend of boiling points in straight chain alkanes

As the length of the carbon chain increases, the boiling point of the alkane increases

explanation

this is because there are more electrons in the molecule so the van der Waals forces between the molecules become stronger and require more energy to break

branched chain alkenes trend

no. branches increases = bp decreases

explanation - brancehd bp

this is because there are fewer points of contact between the molecules so the van der Waals forces between molecules become weaker and require less energy to break

crude oil is a mixture of

mainly alkane hydrocarbons

fractional distillation

The separation of the components of a liquid into fractions which differ in boiling point

step 1

1. crude oil is vaporized and vapour is introduced near the bottom of the column

step two

The vapour rises up the column and creates a temperature gradient

step three

because the alkanes have different boiling points they condense at different levels and the frictions are collected

step 4

the hydrocarbons with the lowest boiling points do not condense and I drawn off as gases at the top of the tower

step 5

The largest hydrocarbons do not vaporize at all and are collected at the base of the tower as a thick residue

order of fractions

20°: petroleum gas
150°: gasoline (petrol)
200°: kerosene
300°: diesel
370°: industrial fuel oil
400°: lubricating oil, paraffin wax and bitumen

Why are hydrocarbons cracked

longer, less useful alkanes are converted to more useful shorter molecules in which CC bonds are broken. The demand for petrol, diesel and jet fuel does not match the natural abundancies in a barrel of crude oil

two types of cracking

thermal and catalytic

thermal cracking temperature and pressure

very high temperatures and very high pressure

products - cracking thermal

alkanes and high percentage of alkenes

Why - thermal cracking products

The CC bonds can break at different positions in the chain to give a mixture of products.

What are the products used for

to make polymers

catalytic cracking temperature and pressure

high temperature and a slight pressure

conditions

in the presence of a zeolite catalyst

products

cycloalkanes, branched alkanes, aromatic hydrocarbons such as benzene

What r products used for

used as motor fuels

complete combustion products

co2, h2o

equation

CnH2n+2 + O2 --> CO2 + H2O

incomplete combustion products

co, h2o

equation

CnH2n+2 + O2 --> CO + H2O

when does incomplete combustion occur

when there is a limited supply of o2

further incomplete combustion products

solid C (soot), h2o

equation

CnH2n+2 + O2 --> C + h2o

when does this occur

when there is a very limited supply of oxygen

pollutants from combustion

unburned hydrocarbons, carbon dioxide, carbon monoxide, carbon, nitrogen oxides, sulphur dioxide

unburned hydrocarbons effect + production

effect: low level ozone (causes respiratory problems)
production: reacts with NOx gas to form low level ozone

carbon dioxide effect + production

effect: global warming
production: complete combustion of fuels

carbon monoxide effect + production

effect: toxic gas
production: incomplete combustion of fuels in limited supply of oxygen

carbon effect + production

effect: particles exacerbate asthma
production: further incomplete combustion in very limited supply of oxygen

nitrogen oxides effect + production

effect: acid rain and photochemical smog
production: N2 + O2 from the air react at high temperatures in engine, e.g N2 + O2 --> 2NO

sulphur dioxide effect + production

effect: acid rain
production: s from fuel impurities reacts with O2 in air, S + O2 --> SO2

What do catalytic converters remove

CO, NO and unburned hydrocarbons

structure catalytic converters

contain honeycombed structure with a thin layer of Pt/Pd/Rh metals

Why is a thin layer of metals used

to reduce the amount needed - to reduce the cost

Why is a honeycomb structure used

a large surface area

removal of NO + CO, + equation

NO + CO react to produce less polluting products
2NO + 2CO --> 2CO2 + N2

removal of unburnt hydrocarbons 1

by reacting with o2
C8H18 + 12.5O2 --> 8CO2 + 9H2O

removal of unburnt hydrocarbons 2

by reacting with NO
C8H18 + 25NO --> 8CO2 + 9H2O + 12.5N2

flue gas desulfurization

when power stations burn coal or natural gas to produce electricity sulphur dioxide is also produced

What are chimneys coated with which absorb and react with sulphur dioxide produced. include equations.

calcium oxide or calcium carbonate.
SO2 + CaO --> CaSO3
SO2 + CaCO3 --> CaSO3 + CO2

alkanes s are generally unreactive because

CC and CH bonds are strong
alkanes are nonpolar

do halogens react with alkanes to form halogenoalkanes

yes they do

reagent + conditions

halogen eg Cl2 or Br2 ok
UV light

type of reaction

substitution because the hydrogen atom is replaced by a halogen atom

ethane + chlorine formula equation

H H H H
| | | |
H-C-C-H + Cl2 --> H-C-C-Cl + HCl
| | | |
H H H H

how were position isomers of halogen alkenes formed

when three or more Cs react with a halogen

further substitution

if an alkane is reacted with an excess halogen each hydrogen atom can be replaced in turn by a halogen atom

radical

a species with an unpaired electron

the 3 stages of free radical substitution

initiation: formation of radicals
propagation: formation of products
termination: removal of radicals

initiation, propagation and termination for:

CH4 + Cl2 --> Ch3Cl + HCl

1. initiation: Cl2 --> 2Cl•

2. propogation:

CH4 + Cl• --> •CH3 + HCl (•CH3 is an intermediate)

•CH3 + Cl2 --> CH3Cl + Cl• (Cl• is a catalyst)

3. termination:

•CH3 + Cl• --> CH3Cl

Cl• + Cl• --> 2Cl

•CH3 + •CH3 --> C2H6