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1

All about Alkanes

Saturated hydrocarbons containing C-H bonds only. The general formula for all chain alkanes is CnH2n+2

Unbranched chains are often called straight cha ins but the C-C-C angle is 109.5°. This means that the chains are not actually straight.
Physical properties-

Polarity
Alkanes are almost non-polar because the electronegativities of carbon (2.5) and hydrogen (2.1) arc so similar. As a result, the only intermolecular forces between their molecules are weak van der Waals forces, and the larger the molecule, the stronger the van der Waals forces.

Boiling points-
This increasing intermolecular force is why the boiling points of alkanes increase as the chain length increases, chain length of about 18 carbons, the alkanes become solids at room temperature. The solids have a waxy feel. Alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms. This is because they cannot pack together as closely as unbranched chains and so the van der Waals forces are not so effective.
Longer chain= higher BP Bcuz VDW IMF increase w/longer chains as molecule gets bigger (so more electrons)
Branching= lower BP bcuz VDW decrease bcuz main chain gets shorter AND the shape changes so moleculuse unable to pack close together so VDW not so effective

Solubility-
Alkanes are insoluble in water. This is because water molecules are held together by hydrogen bonds which are much stronger than the van der Waal's forces that act between alkane molecules. However, alkanes do mix with other relatively non-polar liquids.

How alkanes react
Alkanes are relatively unreactive. They have strong carbon-carbon and carbon- hydrogen bonds. However, they do burn and they will react with halogens under suitable conditions.

2

What causes acid rain?

Crude oil contains small amounts of other compounds dissolved in it. These come from other elements in the original plants and animals the oil was formed from, for example, some contain sulphur. These produce sulphur dioxide, SO2 , when they are burnt. This is one of the causes of acid rain - sulphur dioxide reacts with oxygen high in the atmosphere to form sulphur trioxide. This reacts with water in the atmosphere to form sulfuric acid

3

Fractional distillation process

• The crude oil is first heated in a furnace.

• A mixture of liquid and vapour passes into a tower that 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 arrive at a tray that is sufficiently cool (at a lower temperature than their boiling point). Then they condense to liquid.

• The mixture of liquids that condenses on each tray is piped off.

• The shorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler, because they have lower boiling points.

• The thick residue that collects at the base of the tower is called tar or bitumen. It can be used for road surfacing but, as supply often exceeds demand, this fraction is often further processed to give more valuable products

4

What is crude oil?

-World's main source of organic chemicals.

-It is called a fossil fuel because it was formed millions of years ago by the breakdown of plant and animal remains at the high pressures and temperatures deep below the Earth's surface. -

-Because it forms very slowly, it is effectively non-renewable.

-Crude oil is a mixture mostly of alkanes, both unbranched and branched.

5

As C chain gets longer, hydrocarbons...

Become more viscous

Harder to ignite

Less volatile

Have higher boiling points

6

Why is cracking used? Give the conditions for Thermal and Catalytic cracking

To make use of excess larger hydrocarbons and supply demands for shorter ones.

Catalytic Cracking
Temp 450
Pressure 1-2 atm(slight pressure)
Catalyst zeolites
Products motor fuels (aromatics, cyclic alkanes, branched alkanes

Thermal
Temp 90
Pressure 70 atm
Catalyst none
Products alkenes

7

What is reforming?

Process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic alkanes. Both these products burn more efficiently and are used in petrol for cars.

8

What are the products of complete combustion? Products of incomplete combustion and cause?

Complete: CO2 and H2O
Incomplete, not enough oxygen: CO and/or C and H2O

9

Write the equation for the complete combustion of propane and two for the incomplete combustion

C3H8 + 5O2 --> 3CO2 + 4H2O

C3H8 + 3.5O2 --> 3CO + 4H2O

C3H8 + 2O2 --> 3C + 4H2O

10

Pollutants from burning fossil fuels

Carbon dioxide, carbon monoxide, carbon, water, sulfur dioxide, nitrous oxide, unburned hydrocarbons

11

How are the pollutants from burning fossil fuels formed?

Sulfur dioxide- combustion of S containing compounds (sulfur impurities)

Nitrous oxide- reaction of N2 in air with O2 in air at very high temps

12

What problems are caused by pollutants, and how can they be reduced

Carbon dioxide- greenhouse gases- burning less fossil fuels

Carbon monoxide- toxic- ensure there's a good supply of oxygen when burning fuels

Carbon (soot)- blackens buildings, can cause respiratory systems, global dimming- see above

Sulfur dioxide- acid rain- remove S from burning fuel (desulfurisation)

Nitrous oxide- acid rain- use catalytic converter in cars
Unburnt hydrocarbons- wastes fuel- well-tuned engines and good supply of oxygen

13

Formation of Sulfur dioxide and how it is solved

Sulfur containing impurities are found in petroleum fractions which produce SO2 when they are burnt, which leads to acid rain

-Can be removed from waste gases from furnaces (e.g coal fired power stations) by FLUE GAS DESULFURISATION

--Gases pass through a scrubber/mesh containing carbon dioxide or calcium carbonate which reacts with the sulfur dioxide
CaO + SO2 --> CaSO3 or
CaCO3 + SO2 --> CaSO3 + CO2

-This is an acid-base reaction as CaCO3 and CaO3 are bases and SO2 is an acidic oxide (non-metal oxide)

-The reaction forms gypsum (calcium sulfate (IV) also known as calcium sulphite (everyday name)) which is used to make plasterboard

14

Internal combustion engine (e.g cars)- what is in it, about catalytic converters, the equation for it, what are converters coated in, etc

-Petrol (gasoline fraction- liquid alkanes) used in the the internal combustion engine where the alkanes are vaporised and combusted with air

-Catalytic converters remove CO, NOx unburnt hydrocarbons (e.g octane C8H18) from the exhaust gases, turning them into CO2, N2 and H2O

- 2CO + 2NO --> 2CO2 + N2
C8H18 + 25NO --> 8CO2 + 12.5N2 + 9H2O

Converters have a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pd, Rh)- to give a large SA

15

Global warming- greenhouse gases, how free radicals formed, molecs w/polar bonds, causes of global warming, methan and carbon dioxide

Gases that absorb IR radiation given off by the Earth but do NOT absorb the higher freq UV/vis light radiation given off by the sun- breaks Halogen molec into two free radicals

-Molecs that contain polar bonds absorb IR radiation to make bonds vibrate

Donc molecs that vibrate: CO2( C=O polar) H2O (O-H polar) and CH4 ( C-H slightly polar)
Molecs that dont: O2, N2 don't absorb IR radiation and aren't greenhouse gases- no polar bonds

-Earth getting warmer- due to increasing greenhouse gases in atmoshphere

-Burning of fossil fuels= CO2 in atmosphere- risen sig. recently

-CO2 particularly effect greenhouse gas- largely responsible for global warming

-Methane lvls increased sig. due to agriculture (rice production) and farming (cows)

16

What are free radicals?

Species with single unpaired e-, very reactive

Represented by X (halogen) • (unpaired e-) e.g F• Cl•

Formed by homolytic fission- bond splits evenly, 1e- to each atom

Photochemical reaction - catalysed by light. Requires UV to break halogen bond

CH4+ Cl2 ----> CH3Cl + HCl
alkane + halogen -----> alkane halogen + hydrogen halide

Free radical substitution

17

What happens in free radical substitution reactions?

H atoms are replaced by halogen atoms (e.g Br, Cl, F)

For every one H atom that is replaced, one X2 molecule is used and one HX molecule formed

18

Initiation

When exposed to UV light, halogen molec breaks into two halogen atoms. UV provides energy to break the covalent bonds between the two halogen atoms.

Forms free radicals: F2---> 2F•
Cl2---> 2Cl•

19

Propagation

For every H that is replaced, there is one pair of propogation reactions

Step 1) The alkane/halogenalkane reacts w/ F•/Cl•/Br•/I•- this removes a H atom from the alkane/halogenalkane- this produces HF/HCl/HBr/HI and a C based free radical

C molecule + halogen radical → radical C molecule + halide

Step 2) The C based radical from step 1 reacts w/F2/Cl2/Br2/I2 to put an atom of the halogen molecule onto the C based radical- this also produces another halogen free radical (F•/Cl•/Br•/I•) to continue the chain reaction in another step 1


radical C molecule + halogen molecule → C molecule + halogen radical

20

Termination equation

2 radicals → molecule

No. of atoms in radicals should be same no. of atoms in molecule

If two free radicals collide, they will form a molecule and stop the chain reaction. Any two free radicals involved in the mechanism could collide in this way

e.g CH3-CH3 in CH4 → CH3Cl ....
2•CH3→CH3-CH3