Alkanes Flashcards
what is the general formula for alkanes
CnH2n+2
what are unbrached chains called
straight chains. but the c-c-c angle is 109.5 degrees so the chains are not actually straight.
what happens in an unbranched alkane
each carbon atom has two hydrogen atoms except the end carbon which have one extra
what is the general formula for ring carbons
ring alkanes have the general molecular formula CnH2n because the end hydrogens are not required
Explain why Alkanes are almost non-polar
Alkanes are almost non-polar because the electronegative of carbon (2.5) and hydrogen (2.1) are so similar.
as a result, the only intermolecular forces between their molecules are weak van der waals forces, and the larger the molecule, the stronger the van der Waals forces
why do Boiling points of alkanes increase as the chain length increases
because the intermolecular forces increase as the chain gets longer
why do Alkanes with branched chains have lower melting points than unbranched alkanes with the same number of carbon atoms
this is because branched chains pack together as closely as unbranched chains an so van der waals forces are not so effective
why are alkanes insoluble in water
alkanes are insoluble in water.
this is because water molecules are held together by hydrogen bonds which are much stronger than the van der Waal’s forces that act between alkane molecules
however, alkanes do mix with other relatively non-polar liquids
why are alkanes relatively unreactive
they have strong carbon-carbon and carbon-hydrogen bonds
they do not react with acids, bases, oxidising agents, and reducing agents
in what way do alkanes react
they burn and they will react with halogens under suitable circumstances
they burn in a plentiful supply of oxygen to form carbon dioxide and water
in a restricted supply of oxygen they form carbon monoxide or carbon
describe the process of fractional distillation
the crude oil is first heated in a furnace
a mixture of liquid and vapour passes into a twoer that is cooler at the top than at the bottom
the vapour pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool.
then they condense to liquid
the mixture of liquids that condenses n each tray is piped off
the shorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler, because they have lower boiling points
the thick residue that collects at the bottom of the base of the tower is called tar.
what can tar be used for
road surfacing, as supply often exceeds demand this fraction is often further processed to give more valuable products
state each fraction in order (crude oil)
natural gases
petrol
naphtha
kerosene
gas oil
mineral oil
bitumen
since fractional distillation is a physical process what happens
fractional distillation is a physical process so no covalent bonds within the molecules are broken.
it is the van der waals forces between the molecules that are broken during vaporisation and reform on condensation
what type of hydrocarbon is economically more valuable
shorter chain hydrocarbons
what are the benefits of cracking
shorter, more useful chains are produced, especially petrol
some of the products are alkenes which are more reactive than alkanes
what are alkenes used as
chemical feedstock ( so they supply industries with the starting materials to make different products
they are converted into polymers and a variety of products from paints to drugs
what is the most important Alkene
ethene
this is because it is the starting material for polyethene and a wide range of other everyday materials
what temperatures are required for thermal cracking
this reaction involves heating alkanes to a high temperature, 700-1200k, under high pressure, up to 7000kPa.
explain how the carbon-carbon double bonds break
carbon-carbon double bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom
so initially two shorter chains are produced, each ending in a carbon atom with an unpaired electron
define free radicals
highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules
what is produced during thermal cracking
hydrogen may be produced
a high proportion of alkenes
how do we avoid too much decomposition
the alkanes are kept in these conditions for a very short time, typically one second
describe catalytic cracking
takes place at a lower temperature ( 720 k)
and a lower pressure ( but more than atmospheric)
using a zeolite catalyst, consisting of silicon dioxide and aluminium ocide
describe zeolites
zeolites have a honeycomb structure with an enormous surface area.
they are also acidic
what does catalytic cracking produce
motor fuels
the products are mostly branched alkanes, cycloalkanes, and aromatic compounds
what happens to the products obtained from cracking
they are separated by fractional distillation
