Alkanes Flashcards
What is petroleum mainly made of?
Alkanes
What are alkanes?
Alkanes are saturated hydrocarbons
What is the general formula of alkanes?
CnH2n+2
What are hydrocarbons?
Compounds that only contain carbon and hydrogen atoms
What is the definition of saturated?
There are only single bonds
What is the general formula of cycloalkanes?
CnH2n
What are the bonds like between the C and H bonds?
They are non-polar
What is petroleum?
It is a mixture of hydrocarbons ranging from small to long alkanes
What process allows the hydrocarbons in petroleum to be separated?
Fractional distillation
What happens to the van der Waals forces as the length of the chain increases?
As the chain length increases, the melting/boiling points increases which result in stronger van der Waals forces
Describe the fractional distillation process of petroleum
- crude oil is vapourised at about 350 degrees C
- the vapourised crude oil is passed into a fractionating column
- it is hot at the bottom and cold at the top
- the vapourised crude oil cools as it rises up the column
- the molecules condense at their boiling points
- larger molecules are collected lower down the column
- molecules with the lowest boiling points are collected as gases at the top of the column
What does primary distillation produce?
Primary distillation produces useful substances that boil above 350 degrees C at atmospheric pressure
What happens to the products of primary distillation at high temperatures?
The substances decompose at high temperatures and have to be distilled at lower pressure
What is cracking?
Cracking is the breaking of long alkane chains into more useful shorter alkane and alkene chains
What is the general formula for alkenes?
CnH2n
What are the two types of cracking?
Thermal cracking
Catalytic cracking
Describe the conditions for thermal cracking?
High temperatures up to 1000 degrees C
High pressure up to 70atm
What are the products made from thermal cracking?
Alkenes
Give an example of what alkenes are used to make
Polymers
Describe the conditions for catalytic cracking
High temperature of about 450 degrees C
Low-pressure 1-2atm
Zeolite catalyst
What are the products made from catalytic cracking?
Motor fuels
Aromatic hydrocarbons
Why is zeolite used?
Zeolite has a honeycomb structure and an enormous surface area
Why is a catalyst used?
Catalysts cut costs because lower conditions can be used(temperature/pressure) saving energy
The reaction is speeded up which saves time
What are the economic reasons for cracking?
- products of cracking are more valuable
- shorter alkane chains are in more demand than longer alkane chains
Describe how the zeolite catalyst works
- A zeolite catalyst has sites which remove a hydrogen from the alkane along with the 2 electrons in the bond
- This leaves a positive charge on the carbon atom(carbocation)
What makes alkanes great fuels?
Burning a small amount releases a large amount of energy
What is complete combustion?
Complete combustion occurs when alkanes are burnt with plenty of oxygen
What are the products of complete combustion?
Carbon dioxide
Water
What is the equation for the combustion of propane?
C3H8(g) + 5O2 -> 3CO2(g) + 4H2O(g)
What is incomplete combustion?
Incomplete combustion occurs when alkanes burn with a limited supply of oxygen
What are the products of incomplete combustion?
Carbon(soot)
carbon monoxide
What are the problems caused by burning crude oil?
Carbon monoxide(poisonous gas) is produced
Nitrogen oxides are formed
Acid rain(sulfuric acid,nitrogen oxides)
Photochemical smog(nitrogen oxides, greenhouse gases)
Particulates
Greenhouse gases
Give examples of greenhouse gases
Water vapour
Carbon dioxide
Methane
What is the equation for nitrogen oxide?
N2(g) + O2(g) -> 2NO(g)
What is the equation for sulfuric acid?
SO2 + H2O -> H2SO3 + 0.5O2 -> H2SO4
What problems can particulates cause?
Cancer
Asthma
How does acid rain affect the environment?
Vegetation and trees are destroyed
Buildings and statues are corroded
Marine life is killed because of the acidic conditions
How can sulfur be removed from flues?
Calcium oxide or limestone are now used in flues to absorb sulfur dioxide
What salt is formed when the sulfur reacts with the calcium compounds?
Calcium sulfate
What is the process of removing sulfur from flues called?
Flue gas desulfurisation
What is the greenhouse effect?
The greenhouse effect is the absorption of infrared energy by greenhouse gases and trapping them in the Earth’s atmosphere keeping it warm
What is global warming?
Global warming is the increase in the Earth’s temperature ,due to what most scientists believe to be caused by the increase in the level of carbon dioxide in the atmosphere
why is infra-red radiation important?
It is important as it heats up the Earth or else it would be too cold and life couldn’t be sustained
Why does the concentration of water vapour in the atmosphere stay roughly the same?
This is because of the equilibrium that exists between water and water vapour
Why are catalytic converters important?
They remove unburnt hydrocarbons and oxides of nitrogen from the exhaust
Describe catalytic converters
- ceramic material coated with platinum and rhodium metals which act as catalysts
- honeycomb structure which provides an enormous surface area
What is the equation involving carbon monoxide and nitrogen oxide passing through a catalytic converter?
2CO(g) + 2NO(g) -> N2(g) + 2CO2(g)
What is the equation involving a hydrocarbon and nitrogen oxide passing through a catalytic converter?
hydrocarbon + nitrogen oxide -> nitrogen + carbon dioxide + water
e.g. C8H18 + 25NO ->12.5N2 + 8CO2 + 9H2O
What would happen to the amount of water vapour in the atmosphere if the temperature increased?
The amount of water vapour would increase resulting in an increase of the greenhouse effect
What is a haloalkane?
An alkane with halogen atoms
What type of bond is there between a carbon and a halogen?
A polar bond
Why is the bond between carbon and a halogen polar?
Halogens are more electronegative than carbon
Where is the delta positive charge in a carbon-halogen bond?
On the carbon
What is the delta positive carbon atom susceptible to?
Nucleophiles
What is a nucleophile?
A nucleophile is an electron pair donor
Which are the only nucleophiles to react with haloalkanes?
OH-
CN-
NH3
What happens to the bond between a carbon-halogen as you go down group 7?
The bond becomes less polar as the electronegativity decreases
What are the main intermolecular forces in haloalkanes?
Dipole-dipole attractions
van der Waals forces
Why are haloalkanes not soluble in water?
They are not polar enough
What happens to the boiling point as you go down the halogen group?
The boiling point increases
Why do haloalkanes have higher boiling point than similar chained alkanes?
They have higher relative molecular mass
They are more polar
What is a free radical?
A particle with an unpaired electron
When are free radicals formed?
When a covalent bond splits
In what two ways can a covalent bond break?
Homolytic fission
Heterolytic fission
What is homolytic fission?
The covalent bonds breaks equally and each atom/molecule gets one electron
What is heterolytic fission?
The covalent bond breaks unequally and one atom /molecule gets both electrons and the other none
How can a free radical in a mechanism be shown?
Using a dot
What property does the unpaired electron give to the atom/molecule?
It makes them very reactive
In what types of reactions do halogens react with alkanes?
Photochemical reactions
What are photochemical reactions?
Reactions started by UV light
What is a free radical substitution reaction?
A reaction where a hydrogen is replaced by a halogen
What is a reaction mechanism?
A series of steps that show what happens in a chemical reaction
What is photodissocation?
When the sun provides enough energy to break a bond
What are the three steps in free radical substitution?
Initiation
Propagation
Termination
What happens during the initiation reaction?
Free radicals are formed
e.g. Cl2 –> 2Cl(free radicals)
The bond splits equally
What happens during the propagation reaction?
free radicals are used up and created in a chain reaction
CH4 + Cl(radical) –> HCl + CH3(radical)
CH3(radical) +Cl2–> CH3Cl + Cl(radical)
All propagation steps have a free radical in the reactants and products
What happens in the termination reaction?
When 2 free radicals collide, the chain reaction is terminated as the radicals make a stable molecule
CH3Cl(radical) + Cl(radical) –> CH3Cl
What are CFCs?
Chlorofluorocarbons that contain no hydrogen
Hydrogen atoms are replaced by chlorine and flourine atoms
Why were CFCs used for refrigerators, air conditions and aerosols?
They have low reactivity, low volatility and non-toxic
What is the ozone layer?
A layer in the upper atmosphere that filters out much of the Sun’s harmful UV radiation
What is the equation that shows the formation and depletion of the ozone layer?
O + O2 O3
Reversible reaction
Why is there a constant amount of ozone in the atmosphere?
The rate of formation = the rate of depletion
What is the formula for the ozone formation?
O2 + UV –> O(radical) + O(radical)
What is the formula for the ozone depletion?
O3 +UV –> O2 + O
Radicals from what can affect the breakdown of the ozone layer?
Thunderstorms
Aircraft
CFCs
NOx
How can one regenerated chlorine atom destroy thousands of ozone molecules?
Chlorine free radicals catalyse the decomposition of the ozone as they provide an alternative route with a lower activation energy
Why were CFCs banned?
They were destroying the ozone layer
What did chemists develop as a safer alternative to CFCs?
HFCs- hydrofluorocarbon
They contain no chlorine
What problems can UV radiation cause?
Skin cancer
Sunburn
What is a nucleophilic substitution reaction?
A reaction where a nucleophile reacts with a polar molecule by kicking out the functional group and taking its place
What do curly arrows in a nucleophilic substitution reaction show?
The movement of electron pairs
How does a nucleophilic substitution reaction work?
- The lone pair of electrons on the nucleophile attack the delta positive carbon and the carbon-halogen bond breaks
- The halogen leaves taking both electrons with it
- A new bond forms between the carbon and nucleophile
What is formed when haloalkanes react with hydroxides?
Alcohols
for example, bromoethane can react to form ethanol
What is nucleophilic substitution sometimes called?
Hydrolysis
What is hydrolysis?
Splitting a molecule apart by reacting it with water
What is formed when cyanide reacts with haloalkanes?
Nitrile
What is formed when haloalkanes react with ammonia?
Amines
What factor decides the reactivity in haloalkanes?
The bond enthalpy
Which haloalkane bnd has the highest bond enthalphy?
C-F bond
they undergo nucleophilic substitution more slowly
What happens to the speed of the substitution as the bond enthalphy decreases?
The substitution is faster as it is easier to break the bonds
what would happen if you warm a haloalkane with excess ethanoic ammonia?
The ammonia swaps places with the halogen
What can the amine group in the product of the reaction do?
The amine group has a lone pair of electrons and can act as a nucleophile
What other types of reaction can haloalkanes also undergo?
Elimination reactions
What happens if you warm a haloalkane with hydroxide ions in ethanol?
An alkene is formed
How does the elimination reaction with ethanol and haloalkane work?
e.g. CH3CHBrCH3 + OH- –> CH2CHCH3 + H2O + Br-
OH- acts as a base and takes a proton, H+, from a carbon and forms water
The carbon has a spare electron and forms a double bond with the next carbon
To form the bond, the carbon has to let go of the Br which becomes a Br- ion
In aqueous conditions, what type of reaction happens between haloalkanes and hydroxides?
Nucleophilic substitution
OH- acts as a nucleophile
In anhydrous conditions, what type of reaction happens between haloalkanes and hydroxides?
Elimination
OH- acts as a base
