Alkanes Flashcards
General formula alkanes
CnH2n+2
Alkanes
Contain single carbon-carbon bonds only
How many bonding pairs of electrons is each carbon atoms in an alkane surrounded by
4
What is the shape and bond angle around a carbon atom in an alkane
Tetrahedral
109.5
Structural isomers
Compounds with the same molecular formula but a different structural formula
Cycloalkanes have…
Two less hydrogen atoms than the open chain alkane and are names according to the largest ring
Explain the polarity of alkanes
C and H- similar electronegativities- non-polar bonds
Neighbouring molecules attracted to each other by VDWs- weak intermolecular forces
Why are alkanes not soluble in water
Water molecules held together by hydrogen bonds which are stronger than VDWs in alkanes
What is the correlation between length of carbon chain and boiling point of the alkabe
As length or carbon chain increases, boiling point of alkane increases
Why does boiling point increase with length of carbon chain
More electrons- VDWs stronger- require more energy to break
What is the correlation between number of branches and boiling point
As number of branches increase boiling point decreases
Why does boiling point decreases as more branches are added
Fewer points of contact between molecules- VDWs weaker- require less energy to break
Crude oil
A mixture consisting mainly of alkane hydrocarbons- main source of fuels and petrochemicals
Fractional distillation
The separation of the components of a liquid into fractions which differ in boiling point
Each fraction is a group of…
Compounds that have similar boiling points and are removed at the same level
What happens to boiling points as you go up the column
They decrease
Process of fractional distillation
- crude oil vaporised- vapour introduced at bottom of column
- vapour rises and creates temp gradient
- alkanes have different boiling points so condense at different levels- fractions collected
- hydrocarbons with lowest boiling points don’t condense and are drawn off as gases at top of tower
- largest hydrocarbons don’t vaporise and are collected at base of tower as thick residue
what is the issue with crude oil
demand for shorter chain hydrocarbons doesn’t match abundancies in crude oil
how are the issues with crude oil solved
longer, less useful alkanes converted into more useful molecules through cracking
two types of cracking
thermal and catalytic
what conditions is thermal cracking carried out under
very high temp and pressure
what does thermal cracking produce
alkanes and a high percentage of alkenes
why can thermal cracking create a mixture of products
c-c bonds break at different positions in the chain
what can the products of thermal cracking be used for
to make polymers
under what conditions is catalytic cracking carried out
high temp, slight pressure and in presence of a zeolite
what does catalytic cracking produce
cycloalkanes, branched alkanes and aromatic hydrocarbons
what are the products of catalytic cracking used for
motor fuels
what is obtained from alkane fuels when they’re combusted
energy
what do the products formed in the combustion of an alkane depend on
the supply of oxygen
what products are formed in complete combustion
carbon dioxide and water
what products form during incomplete combustion
carbon monoxide and water
when does incomplete combustion occur
in a limited supply of oxygen
what products form during further incomplete combustion
solid carbon (soot) and water
when does further incomplete combustion occur
in an even more limited supply of oxygen
what does the combustion of hydrocarbon fuels produce
pollutants
what is the effect of unburned hydrocarbons
low level ozone (respiratory problems)
how do unburned hydrocarbons produce low level ozone
reactions with NO gases
what is the effect of carbon dioxide
global warming
what is carbon dioxide produced from
combustion of fuels
what effect does carbon monoxide have
toxic gas
how is carbon monoxide produced
incomplete combustion of fuels in limited supply of oxygen
effect of carbon
particles exacerbate asthma
how is carbon produced
incomplete combustion of fuels in a very limited supply of oxygen
effect of nitrogen oxides
acid rain and photochemical smog
how are nitrogen oxides produced
nitrogen and oxygen from the air react at high temperatures in engines
effect of sulphur dioxide
acid rain
how is sulphur dioxide produced
sulphur from fuel impurities reacts with oxygen in the air
catalytic converters
fitted to cars to remove CO, NO and unburned hydrocarbons
contain honeycomb structure coated with Pt/Pd/Rh metals
honeycomb structure- large surface area
how are NO and CO removed
react to produce less polluting products eg carbon dioxide and nitrogen
flue gas desulfarisation
chimneys/flues coated with CaO or CaCO3 which absorb and react with SO2 produced
what is halogenation
reacting an alkane with a halogen
why are alkanes unreactive generally
non-polar bonds
lots of strong covalent bonds need to be broken
what conditions are needed for halogenation
uv light
what type of reaction is halogenation
substitution- one atom replaced by other
what is formed when alkanes with 3 or more carbons react with a halogen
a mixture of position isomers
what happens if an alkane is reacted with an excess halogen
each hydrogen atom can be replaced by halogen atom
What is the mechanism for halogenation of alkanes
Free radical substitution
What is a radical
A species with an unpaired electron
What are the 3 stages of free radical substitution
Initiation
Propagation
Termination
What is initiation
Formation of radicals
What is propagation
Formation of products
what is termination
removal of radicals
how is the unpaired electron of a radical shown
by a dot
