6.2 Hydrocarbons: Alkanes and alkenes

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  • Created by: Dane
  • Created on: 02-02-20 15:42
What is a hydrocarbon
They are compounds containing only carbon and hydrogen
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Explain the two types of hydrocarbons 1.) Aliphatic hydrocarbons
They are those with chains of carbon atoms which may be branched or unbranched and with rings that are not aromatic and are alkanes or alkenes example is cyclohexene
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2.) Aromatic hydrocarbons
They are ring compounds in which there are delocalised electrons. They are called aromatic because of their smell. These hydrocarbons are sometimes called arenes
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What is the general formula for alkanes
CnH[2n+2]
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Why are they called unsaturated compounds
Because they have only single bonds between their atoms in their molecules
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What happens as the type of alkane observed gets larger
Their intermolecular forces increase, therfore, the melting and boiling temperatures rise as the number of carbon atoms per molecule increase
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Alkanes in the range from C1 to C4 are in the state of...
a gas
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Alkanes in the range from C5 to C17 are in the state of...
a liquid
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Alkanes from C18 upwards are in the state of...
a solid
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Describe the bonds of C-C and C-H
Their bond enthalpies are relatively high so they are difficult to break and these bonds are non-polar so they are unreactive
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Nmae 3 type of reactions of alkanes involving homolytic bond breaking and free radicals
Combustion, halogenation and cracking
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Describe the oxidation of alkanes
Lots of air is required and carbon dioxide and water are the products, the reaction is exothermic
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What happens if air is in short supply
The products include soot, carbon monoxide and carbon dioxide
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What type of mechanism is involved in the combustion of alkanes
A free radical mechanism
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Why is the burnig of alkanes important for society
It is used to generate energy for power stations, furnances, domestic heaters, cookers, candles and vehicles
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What is the down side of burning alkanes
It is a major cause of global warming and increases the greenhouse effect
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How do alkanes react with chlorine and bromine
This is done by heating the substance or on exposure to ultraviolet light
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Describe the reaction of methane and chlorine in sunlight
It is an explosive reaction forming the products chloromethane and hydrogen chloride
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What is meant by a substitution reaction
This is a type of reaction in which an atom or a group of atoms are replaced by another atom or a group of atoms
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What is free radical substitution
It is the replacement of hydrogen atoms in a molecule by halogen atoms in a reaction which involves free radicals
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What is meant by initiation
These are the steps which produces free radicals from molecules
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What is probagation
This is a step which forms more products and more free radicals
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What is termination
This is steps which remove free rdicals by turning them into molecules
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What is a chain reaction
This occurswhen a product in a reaction can react with a starting material so the reaction continues
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What are the three main process crude oil undergoes
Fractional distillation, cracking and reforming
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Explain the process of fractional distillation
A furnace heats the crude oil to about 400 degrees celcius, oil then flows into a fractionating tower containing 40 or so horizontal trays
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What is meant by the term octane number
It is a measure of the performance of fuel by comparison with 2,2,4-trimethylpentane which is given the number 100
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What are the three main methods used to increase the octane number of fuels
1.) Cracking - which makes smaller molecules and converts straight-chain hydrocarbons to branched and cyclic hydrocarbons
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secondly...
2.) Reforming - Which turns straight chain alkanes into branched-chain or cyclic alkanes or arenes such as benzene and methylbenzene and turns cyclic alkanes into arenes
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thirdly...
3.) Adding ethanol and esters - such as ETBE or gasohol (90% petrol and 10% alcohol)
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Why is cracking more useful than fractional distillation
It converts heavier fractions such as diesel and fuel oil into more uesful hydrocarbons by breaking up larger molecules into smaller ones
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Explain what cracking is
cracking converts long-chain alkanes with 12 or more carbon atoms into smaller more useful molecules in a mixture of branched alkanes, cycloalkanes, alkenes and branched alkenes
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What condition causes more alkenes to be produced
A high temperature
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What condition causes more alkanes to be produced
The presence of a catalyst
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What type of catalyst is used
Zeolites, a synthetic sodium aluminium silicate
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Why are zeolites excellent catalysts
They can be developed with active sites to favour the shapes and sizes of those molecules which react to give a desired product
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Describe reforming
Reforming converts straight chain alkanes into branched-chain or cyclic alkanes or arenes such as benzene and methylbenzene and turns cyclic alkanes into arenes
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What catalyst is used for this process
Platinum and rhodium supported by an inert material such as aluminium oxide
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What temperature does the process operate at
500 degress celcius
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What are the main reasons burning petrol or diesel can pollute the air we breath
1.) They do not burn the fuel completely 2.) The fuel contains impurities 3.) They run at such high temperatures that oxygen and nitrogen in th air can react
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If the air supplied is insufficient what do alkanes burn into
They burn to form water with carbon monoxide or carbon
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What are the main impurities in crude oil
Sulfur compounds
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What temperature is sufficient enough to satisfy the activation energy for nitrogen to react with oxgygen to form nitrogen monoxide
2800 degrees celcius
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Complete the equation 2NO + O2 -> ???
2NO2 Nitrogen dioxide
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Nitrogen dioxide then reacts with water and oxygen to form what product
Nitric acid (The cause of acid rain)
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Insunlight what molecules do nitrogen dioxide breakdown into
Nitrogen monoxide and oxygen free radicals
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Why is ozone bad for the enviornment
Becuase it is a pollutant, the reaction of ozone with hydrocarbons forms a complex mixture of irritant chemicals which mixed with the wind can for a smog
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What do catalytic converters do
They improve the air quality by removing the pollutants that would otherwise be released by car exhausts
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In the presence of a catalyst what does carbon monoxide and unburned hydrocarbonsreact with nitrogen oxides to form
They form carbon dioxide and water
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Describe the converter
The converter contains a honeycomb of crematic material coated with a thin layer of metals such as rhodium, platinum or palladium, the large surface area increases the rate of reaction so 90% of the pollutant gases are removed in a fraction of a seco
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How were fossil fuels formed
By anaerobic decomposition of the remains of organisms that settled at the bottom of the sea
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Why are they known as non renewable energy resources
This becuase supplies of them are being used up faster than they can be replaced
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What are some alternatives to fossil fuels
Nuclear, solar, wind and wave power. Renewable energy such as biofuels (bioethanol and biodiesel)
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Why does biofuels have no effect on the air
Because the carbon dioxide that is being uptaken by photosynthesis is replaced when biodiesels are burned to release carbon dioxide
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What is the process called carbon neutral
This is if the carbondioxide released by combustion is balanced by actionswhich remove an equivalent amount of carbon dioxide from the atmosphere
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What does fermentation do
It converts starch to gluecose and then gluecose to ethanol and carbon dioxide
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How is biodiesel produced
It is produced by extracting and processing the oils from crops such as rapeseed
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What is a better solution to making biofuels
To produce ethanol from non food sources such as woody plants and agricultural wastes including straw
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What is meant by the tern unsaturated
Unsaturated compounds contain one or moredouble or triple bonds btween atoms in their molecules
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What causes molecular orbitals to form
Molecular orbitals result when atomic orbitals overlap forming bonds between atoms
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How can sigma bonds form
They are formed by the overlap of two s orbitals, an s and a p orbital or two p orbitals
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Where are Pi bonds found (go to page 190 to see what it looks like)
They are found in molecules with double and triple bonds. The bonding electrons are in a PI orbital formed by the sideways overlapping of two atomic p orbitals
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Where is the electron density concentrated in a Pi bond
One above and the other below the plane of the other molecule on either side of the line between the nuclei of the two atoms joined by the bond
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Why is rotation restricted around a Pi bond
Because these regions have a high electron density
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In alkane why is it possible for the whole molecule to rotate around the sigma bond
Because the bond does not affect the the overlap of the orbitals. In a double bond rotation would involve breaking the Pi bond and this requires more energy than the molecule posses at normal temperatures
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What is meant by the term E/Z Isomerism
This occurs where there is restricted rotation about a bond and also different groups are attatched to the carbon atoms at each end of the bond
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What is stereoisomerism (see page 191)
This occurs when molecules with the same molecular formula and the same structural formula have different spatial arrangements of bonds
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What does E/Z Isomerism involve molecules with
The involve molecules with restricted rotation about the bond, and different groups attatched to the carbon atoms at each end of the bond
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What is cis-trans isomerism
This occurs when at least one group on each carbon is the same
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What is a cis isomer
Similar groups are on the same side of the double bond
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What is a trans isomer
This is when similar groups are on the opposite side of the double bond
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Other cards in this set

Card 2

Front

Explain the two types of hydrocarbons 1.) Aliphatic hydrocarbons

Back

They are those with chains of carbon atoms which may be branched or unbranched and with rings that are not aromatic and are alkanes or alkenes example is cyclohexene

Card 3

Front

2.) Aromatic hydrocarbons

Back

Preview of the front of card 3

Card 4

Front

What is the general formula for alkanes

Back

Preview of the front of card 4

Card 5

Front

Why are they called unsaturated compounds

Back

Preview of the front of card 5
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