C12 - Alkanes Flashcards
What are alkanes?
Alkanes are a homologous series of hydrocarbons that are straight or branched chains with the general formula Cn H2n+2
Alkanes contain only single carbon–carbon bonds and so are saturated
What happens to the boiling point as chain length increases and why?
The boiling point of straight-chain alkanes increases with chain length due to increasing van der Waals forces between molecules, as the length of the chain increases, so does its surface area, more electrons can be induced to create temporary dipoles and so the van der Waals forces are stronger
Why does branched chains lead to a lower boiling point?
Branched-chain alkanes have lower boiling points compared to straight alkanes with the same number of carbons, because the chains cannot pack as closely together. There are fewer points of contact between molecules so the van der Waals forces are weaker
What is the solubility of alkanes?
Alkanes are insoluble in water because hydrogen bonding in water is much stronger than Van der Waal forces.
Alkanes dissolve in non-polar solvents
What is the reactivity of alkanes?
They are relatively unreactive
However, they do undergo combustion and will react with halogens under suitable conditions to form halogenoalkanes
How does fractional distillation work?
The fractional distillation column has a temperature gradient, being hotter at the bottom
Crude oil is heated so it is a gas and pumped into the column at the bottom
Alkanes with a high boiling point, longer chains, condense nearer the bottom, alkanes with a lower boiling point, shorter chains, remain as gases and pass up the column
The trays at each level are fitted with a bubble cap which forces the hot vapour to mix with the liquid that has condensed
The condensed fractions are then piped off
Alkanes with very low boiling points are collected as a gas at the top of the column
What is crude oil?
Crude oil is composed of a mixture of straight and branched chains
What is cracking?
Cracking involves breaking C-C bonds in alkanes creating shorter chains from longer chains
What is thermal cracking?
Thermal cracking, takes place at high pressure and high temperature and produces a high percentage of alkenes
What is catalytic cracking?
Catalytic cracking, takes place at a slight pressure, high temperature and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons
Why is catalytic cracking better than thermal cracking?
Catalytic cracking produces a higher proportion of branched alkanes, which burn more easily than straight chains, and therefore a more important component of petrol. The lower pressure and temperature means its cheaper
What is the economic significance of cracking?
The demand for lower boiling point fractions is greater than the proportion found in crude oil and there is a surplus of longer chains so cracking provides economic value
What happens when alkanes undergo combustion containing sulfur?
It leads to sulfur dioxide being produced which causes air pollution
What happens in the internal combustion engine?
The internal combustion engine produces a number of pollutants including nitrogen oxides, carbon monoxide, carbon and unburned hydrocarbons
How can the gaseous pollutants from internal combustion engines be removed?
Can be removed by using catalytic converters which make less harmful products by using chemical reactions to reduce and remove pollutants
