9: Alkanes

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59 Terms

1

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

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2

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

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3

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

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4

trend of boiling points in straight chain alkanes

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

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5

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

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6

branched chain alkenes trend

no. branches increases = bp decreases

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7

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

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8

crude oil is a mixture of

mainly alkane hydrocarbons

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9

fractional distillation

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

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10

step 1

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

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11

step two

The vapour rises up the column and creates a temperature gradient

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12

step three

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

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13

step 4

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

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14

step 5

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

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15

order of fractions

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

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16

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

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17

two types of cracking

thermal and catalytic

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18

thermal cracking temperature and pressure

very high temperatures and very high pressure

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19

products - cracking thermal

alkanes and high percentage of alkenes

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20

Why - thermal cracking products

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

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21

What are the products used for

to make polymers

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22

catalytic cracking temperature and pressure

high temperature and a slight pressure

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23

conditions

in the presence of a zeolite catalyst

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24

products

cycloalkanes, branched alkanes, aromatic hydrocarbons such as benzene

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25

What r products used for

used as motor fuels

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26

complete combustion products

co2, h2o

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27

equation

CnH2n+2 + O2 --> CO2 + H2O

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28

incomplete combustion products

co, h2o

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29

equation

CnH2n+2 + O2 --> CO + H2O

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30

when does incomplete combustion occur

when there is a limited supply of o2

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31

further incomplete combustion products

solid C (soot), h2o

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32

equation

CnH2n+2 + O2 --> C + h2o

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33

when does this occur

when there is a very limited supply of oxygen

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34

pollutants from combustion

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

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35

unburned hydrocarbons effect + production

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

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36

carbon dioxide effect + production

effect: global warming
production: complete combustion of fuels

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37

carbon monoxide effect + production

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

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38

carbon effect + production

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

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39

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

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40

sulphur dioxide effect + production

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

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41

What do catalytic converters remove

CO, NO and unburned hydrocarbons

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42

structure catalytic converters

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

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43

Why is a thin layer of metals used

to reduce the amount needed - to reduce the cost

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44

Why is a honeycomb structure used

a large surface area

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45

removal of NO + CO, + equation

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

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46

removal of unburnt hydrocarbons 1

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

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47

removal of unburnt hydrocarbons 2

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

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48

flue gas desulfurization

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

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49

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

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50

alkanes s are generally unreactive because

CC and CH bonds are strong
alkanes are nonpolar

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51

do halogens react with alkanes to form halogenoalkanes

yes they do

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52

reagent + conditions

halogen eg Cl2 or Br2 ok
UV light

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53

type of reaction

substitution because the hydrogen atom is replaced by a halogen atom

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54

ethane + chlorine formula equation

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

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55

how were position isomers of halogen alkenes formed

when three or more Cs react with a halogen

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56

further substitution

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

<p>if an alkane is reacted with an excess halogen each hydrogen atom can be replaced in turn by a halogen atom</p>
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57

radical

a species with an unpaired electron

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58

the 3 stages of free radical substitution

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

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59

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

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