Alkanes Flashcards
Saturated
a compound which contains only carbon-carbon single bonds
Hydrocarbon
A compound consisting of carbon and hydrogen ONLY
General molecular formula of alkanes
CnH2n+2
what are the different states of alkanes
alkanes 1-4 = gases
5-17 = liquids
18+ = solids
why can different alkanes in crude oil be separated by fractional distillation
Different alkanes have different boiling points due to their different chain lengths
structural isomers
Compounds with the same molecular formula but different structural formulae
where does isomerism start from in alkanes
butane
Physical properties of alkanes
alkanes are only slightly soluble in water
There is a gradual change in the physical properties as Mr increases e.g BP and MP increase
State and explain the trend in boiling points from methane to octane
The boiling points increases from methane to octane. There are more electrons per molecule from methane to octane, so the strength of the van der walls forces increases and more energy is needed to break the stronger intermolecular forces.
what is the trend in melting points and boiling points of alkanes with increasing chain length
The melting points and boiling points of the alkanes increases as chain length increases. Larger molecules have larger electron clouds meaning stronger Van der Waals forces.
What is the effect of branching in the carbon chain on the boiling points of alkanes
As the molecule becomes more branched, the boiling point decreases.
There is less contact between molecules with a smaller surface area, therefore there are fewer van der waals forces between these molecules compared to molecules with a larger surface area.
why does 2,2-dimethylpropane have a lower boiling point than pentane
2,2-dimethylpropane is a spherical molecule so has a smaller surface area than pentane.
There is less contact between molecules of 2,2-dimtheylpropane therefore there are fewer wan der waals forces between molecules of 2,2-dimethylpropane.
Less energy is needed to break the weaker intermolecular forces in 2,2-dimethylpropane.
why does butane have a higher boiling point than 2 - methylpropane
Butane is a linear molecule so has a greater surface area than 2-methylpropane.
There is more contact between molecules of butane therefore there are more van der waals forces between molecules of butane.
More energy is needed to break the stronger intermolecular forces in butane
What happens when alkanes burn in a plentiful supply of air
Carbon dioxide and water and formed. The reaction is exothermic
When does incomplete combustion take place
When the fuel burns in an insufficient amount of oxygen/air. Carbon monoxide and carbon soot are produced
What are alkanes used as
fuels as they produce large amounts of heat energy when burned
What are the main environmental problems arising from the complete combustion of alkane fuels
CO2 is produced when fuels are burned.
CO2 is a greenhouse gas linked to global warming
Coastal flooding, rising sea levels, change in weather patterns
Problems caused by incomplete combustion of alkane fuels
CO is formed.
CO is a toxic gas which combines with haemoglobin in red blood cells, preventing oxygen from binding with haemoglobin. This reduces the amount of oxygen in the bloodstream
Carbon soot is formed which causes breathing problems and damages the lungs
What are the 3 different incomplete combustion reactions
alkane + oxygen = CO + water
alkane + oxygen = C + water
alkane + oxygen = C + CO + water
why are liquid fuels preferred to gaseous fuels
they are easier to store and transport
Gaseous fuels need to be cooled to a very low temp to be liquefied
Liquids occupy a smaller volume than gases so more fuel can be carried
Disadvantages of using hydrogen fuel
It is a gas and needs to be stored under pressure in large heavy containers
It needs to be cooled to a very low temp to be liquefied
what is a hazard associated with alkanes
they are flammable
what is the principal source of alkanes
crude oil
In a fractionating column where is it the hottest
At the bottom
what are the conditions for free radical substitution
Ultraviolet radiation
Why is ultraviolet radiation necessary for free radical substitution
The ultraviolet radiation provides the energy to break the Cl-Cl bonds producing chlorine free radicals. The reaction is initiated by chlorine free radicals
Why are free radicals needed to attack the alkane
The molecules have no significant polarity in their strong C-H bonds
Why does propagation lead to a chain reaction
Because a chlorine free radical is regenerated by the propagation reactions each time a molecule of product is formed
What happens in termination step
2 free radicals react with each other in the termination step. The reaction ends because the product is a stable species and no free radicals are formed
State 2 limitations of using free radical substitution in organic synthesis
Further substitution takes place so a mixture of products is formed
There could be substitution at different positions along the chain
why do C-H bonds have no significant polarity
the electronegativity of C is similar to that of H
What is cracking
Breaking large alkanes/hydrocarbons into smaller hydrocarbon molecules by breaking C-C bonds
What are the economic reasons for cracking
The petroleum fractions with shorter carbon chain such as petrol are more in demand
To make use of excess larger hydrocarbons and to supply demand for shorter
The products of cracking are more valuable than the starting materials
what are the 2 different types of cracking
Thermal cracking and catalytic cracking
What are the conditions for thermal cracking
High temperature and High pressure
What are the conditions for catalytic cracking
moderate pressure
High temperature
Zeolite catalyst
What does thermal cracking produce
Mostly alkenes and sometimes hydrogen
What does catalytic cracking produce
branched and cyclic alkanes and aromatic hydrocarbons and motor fuels
What effect on the environment does carbon soot have
Causes global dimming - reflection of sun’s light
What pollutants does internal combustion engine produce
nitrogen oxides, carbon monoxide, carbon and unburned hydrocarbons
How is sulfur dioxide produced
Sulphur containing impurities are found in petroleum fractions which produce sulphur dioxide when they are burned
What is produced when sulphur dioxide dissolves in atmospheric water
acid rain
How can sulphur dioxide be removed from flue gases
Using calcium oxide or calcium carbonate because they are bases and react with the acidic sulphur dioxide and neutralises it
How are the oxides of nitrogen produced
When nitrogen and oxygen inside a car engine react under high temperature.
what do catalytic convertors remove
CO, NOx and unburned hydrocarbons
What do catalytic convertors do to the harmful gases when they remove them
they convert them into the ‘harmless’ CO2, N2, and H20
What does the combustion of hydrocarbons containing sulphur impurities lead to
Air pollution and acid rain
What are catalytic convertors made of
Platinum, palladium etc….
Describe the separation process of petroleum in order to separate the fractions
Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top
Crude oil enters the fractionating column and is heated strongly so vapours rise
Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column
Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top
The different fractions condense at different heights according to their boiling points
The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases
The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column
