topic 6b- alkanes Flashcards
6.1. what is the general formula of an alkane?
Cn H2n+2
(have single carbon bonds)
6.2. what is a cycloalkane?
a saturated hydrocarbon
name 3 ways in which we can obtain alkane fuels?
- fractional distillation
- cracking
- reforming
what happens as the molecular mass of alkane’s increases?
- boiling point INCREASES
- viscosity INCREASES
- Denser
- harder to ignite
why is the boiling point of branched chain alkanes lower than that of unbranched chain alkanes?
MORE points of contact= MORE london forces= HIGHER BP
what is crude oil?
a mixture of hydrocarbons
describe how crude oil can be separated by fractional distillation
- the mixture is vapourised (at around 350c) and fed into the fractioning column
- vapours rise, cool and condense
- products are siphoned off for different uses
why are gases at the top of the fractioning column? and the use of gases?
SHORT carbon chain= LOW BP means they rise higher up the column
use= liquefied petroleum gas, camping gas (e.g. methane)
name a use for:
- petrol
- naptha
- kerosene
- gas oil
- mineral oil
petrol= cars
naptha= manafacturing of chemicals
kerosene= for aircrafts
gas oil= central heating, diesel
mineral oil= lubricant
why is bitumen at the base of the fractioning column? and its use?
LONG carbon chain= HIGH BP
- harder to rise up the fractioning column and condenses easier at base
use= paving roads
what is cracking?
compounds collected from the fractioning column that require further breaking down to form smaller, more useful molecules
what is reforming?
processing of straight chain hydrocarbons into branched chain alkanes and cyclic hydrocarbons for efficient combustion
what is catalytic cracking?
uses zeolite catalyst at a slight pressure ana a high temperature (about 450 degrees)
what is thermal cracking?
prouduces a high proportion of alkanes and alkenes
high temperatures of around 1200K and pressures around 7000 kpa
give an example of a cracking equation
C10H22 -> C8H18 + C2H4
or C12H26 -> C10H22 + C2H4
give the general equation of complete combustion
fuel + oxygen -> CO2 + H2O
what is incomplete combustion?
some of the atoms in the fuel are not fully oxidised
incomplete combustion
give the equation for the formation of carbon
C3H8 + 4O2 -> C + 2CO2 + 4H2O
INCOMPLETE COMBUSTION
give the equation for the formation of CO
C3H8 + 4O2 -> 2CO + CO2 + 4H2O
what is knocking?
a type of reforming
-alkanes explode to their own accord (likely to occur with straight chain alkanes)
name the products formed during the combustion of alkane fuels
- carbon monoxide
- oxides of nitrogen and sulfur
- carbon particulates
- unburned hydrocarbons.
describe why carbon monoxide is a bad pollutant
toxic gas that can cause death
=prevents transport of O2 to the body
drawbacks of nitrogen and sulfur oxides
- can produce acid rain
- nitrogen monoxide + O2 -> nitrogen dioxide (acidic can dissolve in H2O)
- ## Acid rain= 2NO2 + H2O -> HNO2 + HNO3SO2 + H2O -> H2SO3 (sulfurous acid)
as well as sulfur trioxide and sulfuric acid
drawbacks of carbon particulates
- formed from incomplete combustion -> seen as smoke or soot
- can discolour buildings
- can cause breathing difficulties
drawback of unburnt hydrocarbons
- released into air unchanged
- can be carcinogenic (may be cancerous)
6.13. understand how the use of a catalytic converter solves some problems caused by pollutants
- uses a rhodium catalyst to convert harmful products into more stable products (H2O or CO2)
what environmental problem does catalytic converters not solve?
- DOES NOT SOLVE CO2
- potential to lead to global warming
equation for catalytic converters and:
-unburnt hydrocarbon
-nitrogen oxide
- CO + O2 -> CO2
- unburnt hydrocarbon: C8H18 + 12O2 -> 8O2 + 9H2O
- nitrogen oxide: 2NO + 2CO -> N2 + 2CO2
6.14. what is a biofuel
(understand the use of alternative fuels, including biodiesel and alcohols derived
from renewable sources such as plants, in terms of a comparison with non-renewable fossil fuels)
fuels made from living matter over a short period of time
what is a biodiesel?
made by refining renewable fats and oils (e.g. vegetable oil)
what is bioethanol?
made by the fermentation of sugar from crops
what is biogas?
produced by the breakdown of organic waste matter
define carbon neutral
the amount of CO2 released during combustion is the same as absorbed by the life of the organism
(land use, yield, manafacture/ transport, carbon neutrality)
compare biofuel and natural gas
biofuel=
- land use -a lot of land needed (replaces land that is used to grow food )
- yield - low, gradually increasing
- manafacture/ transport- no exploration or drilling costs, substantial cost in growing, processing and transport
- carbon neutrality- much closer to being carbon neutral
6.15. what is a radical?
a species with an unpaired electron and is represented in mechanisms by a single dot
6.15. how is a radical formed?
homolytic fission of a covalent bond and results in the formation of radicals
6.16. understand the reactions of alkanes with:
i oxygen in air (combustion)
general equation: fuel + oxygen -> CO2 + H2O
- test for limewater: shows that CO2 is present
6.16. ii- halogens, in terms of the mechanism of radical substitution through initiation,
propagation and termination steps
Overall reaction= CH4 + Cl2 -> CH3Cl + HCl
1. Initiation= always produce radicals
Cl2 -> 2Cl’ (‘=dot)
2. propagation= radicals on both sides of equation
CH4 + Cl’ -> CH3’ + HCl
CH3’ + Cl2 -> CH3Cl + Cl’
3. termination= radicals on reactants side of equation ONLY
Cl’ + CH3’ -> CH3Cl
CH3’ + CH3’ -> CH3CH3
Cl’ + Cl’ -> Cl2
6.17. understand the limitations of the use of radical substitution reactions in the synthesis of organic molecules, in terms of further substitution reactions and the formation of a mixture of products
- LOW yield
- a number of products are produced
(as shown through further substitution reactions)
CH3Cl + Cl2 -> CH2Cl2 + HCl
CH2Cl2 + Cl2 -> CHCl3 + HCl trichloromethane is also formed here
CHCl3 + Cl2 -> CCl4 + HCl - can take place at any point along the carbon chain
