Alkanes Flashcards
alkanes, type of molecules, general formula
Homologous series of saturated hydrocarbons with the general formula CnH2n+2 (ring alkanes: CnH2n)
saturated definition
contains only single C-C bonds
hydrocarbon definition
a compound that only contains hydrogen and carbon atoms
reactivity of alkanes
• Unreactive, burn, react with halogens
types of forces between alkanes molecules (why)
• Only van der waals forces
o Electronegativity between C-H is very similar so no dipole-dipole attractions or H-bonds
how does increase in chain length affect boiling point
• boiling point increases with increase in chain length due to more points of contact between molecules so more van der waals. Higher Mr so stronger van der waals forces between molecules require lots of energy to break
how does increase in branching affect boiling point
• boiling point decreases with increase in branching as it results in fewer points of contact (unable to pack as closely together) so there are fewer and weaker van der waals forces between forces require less energy to break
fractional distillation definition
a method of separating mixtures of liquids according to their different boiling points
fraction definition
a mixture of hydrocarbons with similar boiling points
fractional distillation steps (5)
- crude oil is vaporised
- vapour enters the bottom of the column which is hot and the bottom and cold at the top
- as vapour rises it cools and condenses
- different molecules condense at different heights due to different boiling points
- the larger the molecule, the higher the boiling point and so the lower down the column it condenses
as carbon chain increases in length: (4)
- more viscous (longer chains become tangled)
- harder to ignite
- less volatile
- higher boiling points
number of carbons in chain to be a gas
1-4
number of carbons in chain to be a liquid
5-17
number of carbons in chain to be a solid
> 17
cracking definition
a process for breaking long-chained saturated hydrocarbons into smaller ones, producing a mixture of saturated and unsaturated products
why do we need to crack (2)
High demand for useful, short hydrocarbon chains but low supply. Excess long hydrocarbons can be cracked into shorter ones
Alkanes are more useful than alkanes as starting materials for further chemical reactions
type of reaction for cracking
thermal decomposition of alkanes breaks C-C bonds
thermal cracking conditions (2)
900°C, 70atm
what does thermal cracking mostly produce
alkenes
catalytic cracking conditions (3)
450°C, 1atm, zeolites catalyst (e.g. Al2O3 with honeycomb structure)
what does catalytic cracking mostly produce (4)
motor fuels, aromatics, cyclic alkanes, branched alkanes
why can alkanes be used as fuels
Can burn readily in oxygen and is highly exothermic
when does complete combustion occur and in what does it produce
a sufficient supply of O2 and produces CO2 and H2O
when does incomplete combustion occur and in what does it produce
insufficient supply of O2 and produces CO/C and H2O
why are some molecules greenhouse gases (3 examples)
Molecules with covalent bonds absorb IR radiation e.g. CO2, CH4, H2O
catalytic converter structure and metal
honeycomb structured with thin layer of platinum, palladium and rhodium
equation for catalytic converter with CO
2CO + 2NO -> 2CO2 + N2
equation for catalytic converter for hydrocarbons (e.g. C8H18)
C8H18 + 25NO -> 8CO2 12.5N2 + 9H2O
CO2 formation, problems and ways to reduce problem
Complete combustion of fuels containing C
Greenhouse gas- global warming
Burn less fossil fuels
CO formation, problems and ways to reduce problem
Incomplete combustion of fuels containing C
Toxic – binds to haemoglobin causes suffocation
Catalytic converter – 2CO + 2NO -> 2CO2 + N2
C (soot) formation, problems and ways to reduce problem
Incomplete combustion of fuels containing C
Global dimming, triggers asthma
Ensure sufficient supply of oxygen when burning
H2O formation, problems
Combustion of fuels containing H Greenhouse gas – global warming but falls as rain
SO2 formation, problems and ways to reduce problem
S impurities in fuels burn with O2
Acid rain Desulfurisation – remove SO2 in flue gas by reacting with CaO base so neutralisation reaction forms CaSO4
NOx formation, problems and ways to reduce problem
N2 and O2 in air react under high temperatures of engines and furnaces
Acid rain and breathing problems
Catalytic converter – 2CO + 2NO -> 2CO2 + N2
unburnt hydrocarbons formation, problems and ways to reduce problem
Incomplete combustion of fuel
Global dimming – reflect back rays of light back into space
Ensure sufficient supply of oxygen C8H18 + 25NO -> 8CO2 12.5N2 + 9H2O
mechanism definition
a sequence of steps showing the path taken by electrons in a reaction
homolytic fission definition
Breaking of a covalent bond with each atom getting one electron from the covalent bond to form two radicals
heterolytic fission definition
one atom gets the bonding pair of electrons from the covalent bond
free radical definition
a species with an unpaired electron (represented as •) which is highly reactive
radical substitution definition
a type of substitution in which a radical replaces a different atom or group of atoms
overall synthesis of chloroalkanes
CH4 + Cl2 CH3Cl + HCl
type of mechanism for synthesis of chloroalkanes
free radical substitution
mechanism steps for synthesis of chloroalkanes
Initiation: Cl2 2Cl• (UV light and >350⁰C)
Propagation: CH4 + Cl• •CH3 + HCl
•CH3 + Cl2 CH3Cl + Cl•
Termination: •CH3 + Cl• CH3Cl
how can further substitution be minimised for synthesis of chloroalkanes
add methane to excess
why is the ozone important
protective layer from UV radiation which can cause mutation and skin cancer
ozone formation overall
O2 + O ↔ O3
ozone formation steps (4)
- O2 2O (UV with high frequency)
- O + O2 O3 (+ heat)
- O3 O2 + O (UV with lower frequency)
- O2 + O O3 (+ heat)
removal of ozone overall
O3 + O 2O2
what can act as catalysts to remove ozone and where do they come from
CFCs are used for refrigerants, aerosols, insecticides etc. and are released through these uses
NO (from lightning or aircraft engines)
removal of ozone with Cl• catalyst steps
Initiation: CFCl3 •CFCl2 + Cl• Propagation: Cl• + O3 ClO• + O2 ClO• + O3 Cl• + 2O2 Termination: 2O3 3O2 `
removal of ozone with NO catalyst steps
N2 + O2 2NO
NO + O3 NO2 + O2
NO2 + O3 NO + 2O2
2O3 3O2
How is distillation different to fractional distillation
Distillation separates all volatile components of a mixture from non-volatile ones