2. Alkanes Flashcards
What is petroleum another name for?
Crude oil
What is crude oil a mixture of?
Hydrocarbons, the majority of which are alkanes
What needs to be done to crude oil in order for it to be useful?
It needs to be separated into mixtures of hydrocarbons with similar chain lengths
What are fractions?
Mixtures with similar chain lengths
What is a hydrocarbon?
A molecule that contains only hydrogen and carbon atoms
What is an alkane?
A hydrocarbon with the general formula C₂H₂ₙ₊₂
Why are alkanes relatively unreactive?
They only contain strong C-C and C-H bonds
Why are alkanes useful as fuels?
They burn
What are alkanes held together by?
Weak van der waal’s forces
What happens to van der waal’s forces as size of the alkane increases?
The strength increases
What happens to boiling point of alkanes as size increases?
It increases
Why does boiling point of alkanes increase with size?
Because the strength of the van der waal’s forces in between increases
Why do branched alkanes tend to have lower boiling points than straight chain alkanes?
Because there is reduced surface area contacts with branched isomers, and so fewer van der waal’s forces - and so less energy required to break them
How is petroleum separated into useful fractions?
In an oil refinery by fractional distillation
What equipment is used for fractional distillation?
A fractionating column
Which fractions condense at the top of the fractionating column?
- those with short chains
* those with low melting points
Which fractions condense at the bottom of the fractionating column?
- those with long chains
* those with high melting points
What happens during fractional distillation?
- vaporised crude oil enters a fractionating column - which is hot at bottom and cool at top
- vapour from the oil rise
- vapour condense when they become cool enough
What is cracking?
The chemical splitting of long-chain alkanes into hydrocarbons of shorter chain length by breaking a C-C bond
What is produced when long chain alkanes are cracked?
Short chain alkanes and alkenes
What are the two main types of cracking?
- thermal cracking
* catalytic cracking
What are the conditions required for thermal cracking?
- 700-1200K (900*C)
* 7000 kPa
Why are intense conditions required to break alkanes down?
They are unreactive
What provides the energy to break C-C bonds in thermal cracking?
High temperatures
What is there an absence of in the conditions for thermal cracking?
Air
What is a high percentage of the products made from thermal cracking?
Alkenes
How many free radicals does cracking form?
Two
In thermal cracking, where do long-chain hydrocarbons tend to break at the lower end of the temperature scale?
In the middle of the molecule
In thermal cracking, where do long-chain hydrocarbons break as temperature is increased?
The cracking shifts from the middle of the molecule towards one end of the molecule
What does using even higher temperatures in thermal cracking lead to?
A higher percentage of short chain alkenes as molecules are cracked closer to the end
How would you get shorter-chained molecules in thermal cracking?
Use higher temperatures
What is thermal cracking also known as?
Free radical reaction
When happens when the C-C bond breaks in thermal cracking?
Each of the carbon atoms gets one of the shared electrons - producing two free radicals
What happens after two free radicals are formed during thermal cracking?
One free radical can take a hydrogen atom to form an alkane and the other will lose a hydrogen atom to form an alkene
Why are free radicals highly reactive?
Due to an unpaired electron
How long are conditions in thermal cracking maintained for? Why is this?
Approximately 1 second, as they are so intense
What type catalyst of is used in catalytic cracking?
A zeolite catalyst
What are zeolite catalysts?
Crystalline aluminosilicates
What are the conditions required for catalytic cracking?
- high temperature - 720K (450*C)
- slight pressure - 1-2 atm
- zeolite catalyst
What structure does the catalyst used in catalytic cracking have?
Honeycomb
Why does the zeolite catalyst in catalytic cracking have a honeycomb structure?
To ensure a large surface area
How does one carbon atom end up with a positive charge in catalytic cracking?
The C-C bond breaks in a way that both electrons from the shared pair remain with one carbon (leaving the other with a positive charge)
What is mainly produced during catalytic cracking?
Branched alkanes, cycloalkanes and aromatic compounds
Why are mainly branched alkanes, cycloalkanes and aromatic compounds produced in catalytic cracking?
The C-C bond breaks in a way that leaves one carbon with a positive charge
Why are short-chain branched chain hydrocarbons used for motor fuels?
They burn more easily
What are short-chain branched chain hydrocarbons used for?
Motor fuels
Which burns easiest; straight or branched chain hydrocarbons?
Branched chain
What are the aromatic compounds produced in catalytic cracking used for?
Starting materials for many substances such as dyes and polymers
What do the products of cracking need to be separated?
Fractional distillation
What type of hydrocarbons are more in demand?
Short-chained
What happens to long-chained hydrocarbons that are produced from fractional distillation? Why?
They are cracked again, as short-chained hydrocarbons are more in demand
What are the most useful fractions produced from fractional distillation of crude oil? Why is this?
Those with shorter chains as they are more in demand
Does the supply of useful fractions from fractional distillation only meet demand?
No
How does demand and supply compare for long and short chained hydrocarbons?
- short chained - supply exceeds demand
* long chained - demand exceeds supply
How is the demand for short-chained hydrocarbons met?
Long chain hydrocarbons are cracked into more useful fractions to help meet demand
Why are alkenes industrially valuable?
They have a reactive nature due to the presence of a double bond
What are alkenes used as a starting material for?
Different products such as polymers and pharmaceuticals
What are alkanes used for?
Fuels
What are the types of combustion of organic compounds?
- complete
* incomplete
When does complete combustion occur?
When there is a plentiful supply of oxygen
What are the products of complete combustion?
Carbon dioxide and water
Why are carbon dioxide and water produced in the complete combustion of alkanes?
- carbon dioxide due to oxidation of carbon in the hydrocarbon
- water due to oxidation of hydrogen in the hydrocarbon
When does incomplete combustion occur?
When there is an insufficient supply of oxygen
What can be produced instead of carbon dioxide in incomplete combustion?
- carbon monoxide
* soot
What is soot?
A black solid
Where might incomplete combustion take place?
An internal combustion engine (car engine)
What is formed in incomplete combustion?
- carbon monoxide and water
* soot and water
What additional products are formed during combustion in internal combustion (car) engines?
Oxides of nitrogen (NOx) and unburned hydrocarbons
What happens in combustion engines at high temperatures and pressures?
Oxygen and nitrogen in the air can react together to form the nitrogen oxides:
- NO
- NO2
Under what conditions is NO produced in a combustion engine?
High pressure/temperature
When is NO2 produced in a combustion engine?
Under high temperatures/pressures, when NO reacts further
What is produced in combustion engines when NO reacts further?
NO2
Which compound is responsible for the haze on hot days (ie. smog)?
NO2
What happens when NO2 dissolves in rainwater?
Produces acid rain (weak solution of nitric acid)
What are catalytic converters made from?
Platinum or palladium
What are cars fitted with to remove harmful gases?
Catalytic converters
What are catalytic converters supposed to do?
Remove most of the harmful gases produced in engines
Example of the equations used to show how catalytic converters work?
- C3H8 + 5O2 -> 3CO2 + 4H20
* 2NO + 2CO -> N2 + 2CO2
What do most crude oil and coal deposits contain as an impurity?
Sulphur
Why is sulphur present in many fuels?
Most crude oil contains sulphur as an impurity
What happens to sulphur when hydrocarbon fuel undergoes combustion?
The sulphur reacts with the oxygen to produce sulphur dioxide
What do sulphur-containing hydrocarbons burn to produce?
SO2
What is sulphur dioxide?
An acidic gas that causes air pollution
What happens when sulphur dioxide dissolves in rainwater?
Produces weak solution of sulphuric acid - acid rain
What are flue gases?
Gases coming out of chimneys/flues
What do flues (chimneys) in factories contain to remove flue gases?
Calcium carbonate or calcium oxide
What does the calcium carbonate/oxide in flues react with?
SO2 to form calcium sulphate
How is SO2 removed from factory flue gases?
Reacts with calcium carbonate/oxide in flues to produce calcium sulphate
What is the process known as to remove SO2 from flue gases?
Flue gas desulphurisation
What is flue gas desulphurisation?
The removal of SO2 from flue gases
Equation for reaction of SO2 with calcium oxide in flue gas desulphurisation?
CaO + SO2 -> CaSO3
Equation for reaction of SO2 with calcium carbonate in flue gas desulphurisation?
CaCO3 + SO2 -> CaSO3 + CO2
Which is the better calcium compound to use in flue gas desulphurisation?
Calcium oxide as calcium carbonate produces carbon dioxide
How can sulphur dioxide emissions be reduced?
- flue gas desulphurisation
* sulphur impurities removed from petrol before it is burnt
Why is water vapour harmful to the environment?
It is a greenhouse gas
What are halogenoalkanes?
Alkanes in which one or more hydrogen atoms are replaced by a halogen atom
What does the halogen replace in a halogenoalkane?
A hydrogen atom
What happens in the substitution reaction for the formation of halogenoalkanes?
When a hydrocarbon and a halogen react, at least one H atom will be replaced by a halogen
What must happen when methane reacts with chlorine to form a halogenoalkane?
At least one H atom will be replaced with chlorine
Under what conditions will methane undergo a substitution reaction with chlorine?
In UV light
What will the substitution reaction of methane with chlorine produce?
Chloromethane and hydrogen chloride
Symbol equation for the substitution reaction of methane with chlorine?
CH4 + Cl2 → CH3Cl + HCl
What can happen after the substitution reaction of methane with chlorine?
The reaction can continue until all hydrogen atoms are replaced (one at a time)
What is formed after all hydrogen atoms in methane are replaced with chlorine?
Tetrachloromethane
In which type of reaction can all of the hydrogen atoms in an alkane be replaced with halogens?
Substitution reaction
What does the equation for a substitution reaction not show?
Which bonds are broken and formed, when and how
How is the substitution of halogens in alkanes shown in more detail?
Through the reaction mechanism - in this case free radical substitution
What is the reaction mechanism for when halogenoalkanes are formed?
Free radical substitution
How many stages does free radical substitution occur in?
3
What are the three stages in free radical substitution?
- Initiation
- Propagation
- Termination
What does the reaction stage in free radical substitution involve?
The homolytic fission of a covalent bond
What is the first stage of free radical substitution?
Initiation
What is the second stage of free radical substitution?
Propagation
What is the third stage of free radical substitution?
Termination
What is homolytic fission?
When a covalent bond breaks between two identical atoms, so that one electron from the shared pair goes to each atom
What conditions are required for homolytic fission?
UV light
Why is UV light required for homolytic fission?
Provides energy to split Cl-Cl bond
Why are the C-H bonds not broken in homolytic fission, but the Cl-Cl are?
It takes more energy to break C-H bonds than Cl-Cl, so there is not enough energy
What is the equation for the initiation step in free radical substitution (chlorine and methane)?
Cl - Cl → Cl• + Cl•
Where do the electrons go in homolytic fission?
One electron from the shared pair foes to each atom
What type of bond is broken in homolytic fission?
Covalent
What is a radical?
A species with an unpaired electron
What is propagation?
The reaction of a free radical with a molecule to produce a different free radical as well as a stable molecule
What is produced in propagation?
A stable molecule and a free radical
How many steps are involved in propagation?
2
What is the first step of propagation (chlorine and methane)?
The chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, leaving a methyl free radical, •CH3
What is the equation for the first step of propagation (chlorine and methane)?
Cl• + CH4 → HCl + •CH3
What is the second step of propagation (chlorine and methane)?
The methyl free radical reacts with a chlorine molecule - this produces another chlorine free radical and a stable compound (e.g. chloromethane)
What is the equation for the second step of propagation (chlorine and methane)?
•CH3 + Cl2 → CH3Cl + Cl•
What are the propagation equations for chlorine and methane?
- Cl• + CH4 → HCl + •CH3
2. •CH3 + Cl2 → CH3Cl + Cl•
What is the effect of the propagation steps in free radical substitution? (e.g. chlorine and methane)
Produces HCl, chloromethane, and a new Cl• free radical
What is the chain part of the chain reaction that is free radical substitution?
- in propagation
- HCl, chloromethane and Cl• produced
- free radical ready to react with more methane
What is termination?
The combination of any two free radicals to form a single stable molecule with no unpaired electrons
Equation examples of how termination can occur?
- Cl + •Cl → Cl2
- Cl + •CH3 → CH3Cl
- CH3 + •CH3 → C2H6
How many free radicals are involved in termination?
Two
What does termination show?
That other products are formed as well the main ones (e.g. chloromethane and hydrogen chloride)
How is it ensured that chloromethane is the major product in a reaction of methane and chlorine?
Using excess methane
What is excess methane used for in creating chloromethane?
In order to ensure that chloromethane is the major product
Why are a range of products formed in the termination step?
There are a range of free radicals which can combine
How can the three steps of free radical substitution be easily identified?
- initiation produces free radicals from stable molecules
- propagation has free radicals as both reactants and products
- termination has radicals as reactants but only stable molecules as products
What are chloroalkanes used for?
Coolants
What are chlorofluroalkanes used for?
Coolants
What are chlorofluroalkanes?
An alkane where one or more hydrogen atoms have been replaced by chlorine and fluorine atoms
What happens if long-chained alkanes are used in free radical substitution?
Many products and isomers will be formed
Why are chain reactions of long-chained alkanes not very useful?
Due to the mixture of products formed
Is UV required in the termination step?
No
