Alkanes Flashcards
(26 cards)
What is the initiation stage of reacting chlorine with methane
Essential condition = UV light to break Cl-Cl bond, as it is a weak bond
Cl2=2Cl:
: represents a single electron, meant to be one dot
Bond has broken in homolytic fission, each atom gets one electron from the covalent bond
Define free radical
A reactive species which possesses an unpaired electron
Give the overall equation of the formation of chloromethane
CH4+ Cl2= CH3Cl+ HCl
Methane gas reacts with chlorine gas to form chloromethane and hydrochloric acid
Describe stage 1 of propagation for the formation of chloromethane
Chlorine free radicals are very reactive and will remove a H from the methane leaving a methyl free radical and hydrochloric acid
CH4+ Cl:=HCl + :CH3
Describe stage 2 of propagation for the formation of chloromethane
Methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical
;CH3+ Cl2=CH3Cl+ Cl
When the Cl free radical is regenerated it reacts with several more alkanes in a chain reaction
Describe termination stage of forming chloromethane
Collision of two free radicals does not generate further free radicals,chain is terminated
:CH3 + :Cl= CH3Cl
:Cl+:Cl= Cl2
:CH3 + :CH3 = CH3CH3
Describe the process of fractional distillation of crude oil
Oil is preheated
Then passed into column
Fractions condense at different heights
Temp of the column decreases as you go upwards
Separation depends on boiling points
Boiling points depend on size of molecules
The larger the molecule the larger the van der waals forces
Similar molecules ( size bp mass) condense together
Small molecules condense at the top at lower temperatures
Big molecules condense at the bottom at higher temperatures
Give the two main purposes of cracking of hydrocarbons
- Shorter, more useful chains are produced , especially petrol
- Some products are alkenes which are more reactive than alkanes
Describe the process of thermal cracking
Heating alkanes up to a high temperature, 700-1200K under high pressure up to 7000kpa
The carbon carbon bonds break in a way such that one electron from the pair in the covalent bond goes to each carbon atom
This means initially 2 shorter chains are produced which end in a carbon atom with an unpaired electron
These fragments are called free radicals, which are highly reactive intermediates and react in a way so they form a variety of short molecules
Not enough hydrogen atoms to produce two alkanes so an alkene is produced
Many carbon carbon bonds break and the bond does not necassarily break in the middle
Thermal cracking produces a high proportion of alkenes
Describe the process of catalytic cracking
This takes place at a lower temperature, approximatley 720K and a lower pressure
- Using a zeolite catalyst consisting of silicon dioxide and aluminum oxide
- Zeolites have a honeycomb structure with an enoromous surface area, they are also acidic
- This form of cracking is used to mainly produce motor fuels
- Products are mostly branched alkanes, cyclohexanes and aromatic compounds
- Products obtained by cracking are separated by fractional distillation
- Products are mostly gases, have chain lengths below C5, mixture decolourises bromine solution, test for carbon carbon double bond showing product consists of alkenes
Describe combustion of alkanes
Shorter chain alkanes burn completely in an excess of oxygen to give carbon dioxide and water
Very exothermic, longer the carbon chain the greater the heat output
Write the equation of propane being combusted in oxygen
as an example
Balance the carbons and hydrogens as you write it down
Then count oxygens and balance them last
C3H8 + O2 = 3CO2 + 4H2O
10 oxygens on right hand side so
C3H8 + 5O2= 3CO2 + 4H2O
Uneven number of oxygens
Use a half number infront of the O2
Describe the trend in combustion of alkanes
Hydrocarbons become harder to ignite as the molecules get bigger, because bigger molecules dont vapourise so easily
Bigger molecules have more electrons so have more van deer waals attractions which makes it harder for them to break away from their neighbours and form a gas
Bigger the hydrocarbon, the more of a yelllow sootier flame
Hydrocarbons usually burn with a blue clean flame
Describe incomplete combustion of alkanes
In a limited amount of oxygen, carbon monoxide is formed
Yellow flame is observed
This happens most often with longer chained hydrocarbons
Produces less energy per mole than in complete combustion
How does NOx pollute the atmosphere?
equations
Nitrogen oxides such as NO, NO2 and N2O4 are produced when there is enough energy in the air for the N2 bond to break
This happens in petrol engines with high temps, sparks ingnite the fuels
N2+ O2= 2NO
N2+ 2O2= 2NO2
Describe the environmental impact of combusting nitrogen
NO is toxic and can form acidic gas NO2
NO2 is toxic and acidic and can form acid rain
Describe combustion of hydrocarbons containing sulfur
Sulfur containing impurities are found in petroleum fractions which produce SO2 when burned
S+O2= SO2
CH3SH+ 3O2 = SO2 + CO2 + 2H2O
SO2 dissolves in atmospheric water and forms acid rain
How does flu gas desulfurisation work give eq
This is where SO2 is removed from the waste gases from furnaces (e.g coal fired power stations) by flue gas desulfurisation
Gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction
SO2+ CaO = CaSO3
Calcium sulfite is formed which can be used to make calcium sulfate for plasterboard
Why is carbon monoxide a pollutant
Toxic, binds to heamoglobin in the blood causing internal suffocation
Formed during incomplete combustion
Why are unburnt hydrocarbons a pollutant?
This is when not all fuels burn in the engine, this contributes to the formation of smog which causes health problems
Why is soot considered to be a pollutant? ( carbon particulates)
Causes global dimming and respitory problems, exacerbates asthma and can cause cancer
Describe how CO2 is a pollutant to the atmosphere
Greenhouse gas, levels are rising rapildy which is causing climate change
Describe the function of catalytic converters
-They remove CO, NOx, and unburned hydrocarbons from exhaust gases, reducing their output in the gas mixture
-Catalytic converter is a honeycomb made of ceramic material coated with catalyst metals such as platinum and rhodium
- Honeycomb shape provides a large surface area, little of these metals can be used
2CO+ 2NO = 2CO2 + N2
C6H18 + 25NO = 8CO2+ 12.5 N2 + 9H2O
What is petroleum?
A mixture containing mainly alkane hydrocarbons
