Alekenes

HideShow resource information
  • Created by: Ambreen
  • Created on: 01-02-13 19:11
Preview of Alekenes

First 299 words of the document:

Alkanes 2812 1
ALKANES
General · members of a homologous series with general formula CnH2n+2 - non-cyclic only
· saturated hydrocarbons - all carbon-carbon bonding is single
· bonds are spaced tetrahedrally about carbon atoms.
Isomerism The first example of structural isomerism occurs with C4H10. Two structural isomers exist
CH3
CH3 CH CH CH3 CH3 CH CH3
2 2
butane 2-methylpropane
Structural isomers have different physical properties e.g. boiling and melting pt
Q.1 Draw out and name the structural isomers of C5H12 and C6H14.
Physical properties of alkanes
Boiling point · boiling point of alkanes increases as they get more carbon atoms in their formula
· the more atoms there are the greater the Van der Waals' forces holding them together
· the greater the intermolecular force, the more energy needed to separate the molecules
· the more energy required, the higher the boiling point
CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C)
difference gets less because mass is increasing by a smaller percentage each time
· straight chains have a larger surface area giving greater interaction between molecules
· branching makes molecules more compact and lowers the attraction between them
· the lower the intermolecular forces, the lower the boiling point
"The greater the branching, the lower the boiling point "
CH3
CH3 CH 2 CH2 CH3 CH3 CH CH3
b.p. - 0.5°C b.p. ______ °C
Q.2 Arrange the isomers of C5H12 in ascending boiling point order .
Melting point A general increase with molecular mass - not as regular as that for boiling point.
Solubility Are non-polar so are immiscible with water but soluble in most organic solvents.

Other pages in this set

Page 2

Preview of page 2

Here's a taster:

Alkanes
CHEMICAL PROPERTIES OF ALKANES
Introduction · fairly unreactive - their old family name, paraffin, means little reactivity
· consist of relatively strong, almost non-polar covalent bonds
· have no real sites that will encourage substances to attack them
Combustion · alkanes make useful fuels - especially the lower members of the series
· combine with oxygen in an exothermic reaction
complete combustion CH4(g) + 2O2(g) ------> CO2(g) + 2H2O(l)
incomplete combustion CH4(g) + 1½O2(g) ------> CO(g) + 2H2O(l)
· the greater the number of…read more

Page 3

Preview of page 3

Here's a taster:

Alkanes 2812 3
Pollution Processes involving combustion give rise to a variety of pollutants ...
power stations SO2 emissions produce acid rain
internal combustion engines CO, NOx and unburnt hydrocarbons
Q.4 What does the formula NOx stand for ?
Q.5 Why are the following classed as pollutants ?
· CO
PLATINUM, PALLADIUM, RHODIUM
· NOx
· unburnt hydrocarbons
Removal SO2 react effluent gases with a suitable basic compound (e.g.…read more

Page 4

Preview of page 4

Here's a taster:

Alkanes
Breaking covalent bonds
There are three different ways to split the shared pair of electrons in an unsymmetrical covalent bond.
UNEQUAL SPLITTING
+ + - produce IONS
}
a) X Y X Y
known as HETEROLYSIS or
- + HETEROLYTIC FISSION
b) X Y X + Y
EQUAL SPLITTING
produces RADICALS
c) X Y X + Y
known as HOMOLYSIS or
HOMOLYTIC FISSION
If several bonds are present the weakest bond is usually broken first.…read more

Page 5

Preview of page 5

Here's a taster:

Alkanes 2812 5
Chlorination
of methane Reagents chlorine and methane
Conditions UV light or sunlight - heat could be used as an alternative energy source
Equation(s) CH4(g) + Cl2(g) ----> HCl(g) + CH3Cl(g) chloromethane
CH3Cl(g) + Cl2(g) ----> HCl(g) + CH2Cl2(l) dichloromethane
CH2Cl2(l) + Cl2(g) ----> HCl(g) + CHCl3(l) trichloromethane
CHCl3(l) + Cl2(g) ----> HCl(g) + CCl4(l) tetrachloromethane
Mixture · free radicals are very reactive as they are trying to pair up their unpaired electron
· if there is sufficient chlorine, every hydrogen will eventually…read more

Page 6

Preview of page 6

Here's a taster:

Alkanes
CRACKING
Process · involves the breaking of C-C (and C-H) bonds in alkanes
· converts heavy fractions into smaller, higher value products such as alkenes
· two types THERMAL Free radical mechanism
CATALYTIC Carbocation (carbonium ion) mechanism
THERMAL · HIGH PRESSURE ... 7000 kPa
· HIGH TEMPERATURE ... 400°C to 900°C
· FREE RADICAL MECHANISM
· HOMOLYTIC FISSION
· PRODUCES MOSTLY ALKENES ... e.g. ETHENE for making polymers and ethanol
· PRODUCES HYDROGEN ...…read more

Page 7

Preview of page 7

Here's a taster:

Alkanes 2812 7
The Petrochemical Industry
Crude Oil In the past, most important organic chemicals were derived from coal. Nowadays, natural
gas and crude petroleum provide an alternative source.…read more

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all resources »