alkanes Flashcards
what are alkanes
alkanes are saturated hydrocarbons - they contain only carbon -carbon and carbon - hydrogen singe binds
what are alkanes used for
alkanes are the least reactive organic compounds
they are used for:
- fuels
- lubricants
- starting materials for a range of other compounds - this means they are very important to industry
what is the source of alkanes
the main source for alkanes is crude oil
what is the general formula for alkanes
the general formula for chain alkanes is CnH2n+2
what are the different structure of alkanes
hydrocarbons may be unbranched chains , branched chains or rings
what are unbranched alkanes
unbranched chains are often called straight chains but the C-C-C angles is 109.5 degrees.
This means that the chains are not actually straight
In an unbranched alkane, each carbon has two hydrocarbons except the end carbons which have one extra
what is the general formula for ring alkanes
ring alknes have the general molecular formula of Cn H2n
what is the polarity of alkanes
alkanes are almost non - polar because the electronegativity of both carbon and hydrogen 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
what is the boiling point of alkanes
due to the increasing intermolecular forces, the boiling points of alkanes increase as the chain length increases
the shorter chains are gases at room temperature
Pentane, with five carbons, is a liquid with a low boiling point of 309K (36C)
At a chain length of about 18 carbons, the alkanes become solids at room temperature
what is the trend of melting points in branched chains
alkanes with branched chains have lower melting points than straight chain alkanes the the same number of carbon atoms
This is because they cannot pack together as closely as branched chains and so the Van der Waals forces are not effective
what s the solubility of alkanes
Alkanes are insoluble in water. This is because water molecules are held together by hydrogen bonds which are much stronger than he Van der Waal’s forces that act between alkane molecules.
However, alkanes do mix with other relatively non -polar liquids
how do alkanes react
alkanes are relatively unreactive, they have strong carbon - carbon and carbon - hydrogen bonds
Thy do not react with acids, bases, oxidising agents, and reducing agents
what happens when alkanes are burned in oxygen
when they burn in plentiful supply of oxygen, they form carbon dioxide and water
OR
in a restricted supply of oxygen, they form carbon monoxide and carbon )
why are crude oils important
crude oil is at the present the world’s main source of organic chemicals
how are fossil fuels formed
fossil fuels are formed by the breakdown of plant and animal remains at the high pressures and temperatures deep below the Earth’s surface
because it forms slowly, it effectively non - renewable
what is crude oil
crude oils are a mixture mostly of alkanes , both unbranded and branched
crude oils from different sources have different compositions
what is fractional distillation
to convert crude oil into useful products you have to separate the mixture
This is done by heating it and collecting the fractions that boil over different ranges of temperatures
what are the different fractions composed of
each fraction is composed of:
- hydrocarbons of similar chain lengths and therefore similar properties
how do you carry out fractional distillation/ how does it work
METHOD:
- the crude oil is first heated in a furnace ( at 350C)
- a mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom
- the vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool ( at a lower temperature than their boiling point). Then they condense to liquid
- the mixture of liquids that condenses on 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
what is the thick residue that collects at the base of the fractional tower called
the thick residue that collects at the base of the tower is called tar or bitumen. It can be used for road surfacing but, as suppy often exceeds demand, this fraction is often further processed to give more valuable products
what is cracking
longer chain fractions are not as useful and therefore of lower value economically
Most crude oil has more longer chain fractions than is wanted and not of the naphtha fraction ( which has a shorter chain and are not in high demand)
The shorter chain products are economically more valuable than the longer chain material
to meet the demand for the shorter chain hydrocarbons, many if the longer chains are broken into shorter lengths (cracked)
what are the products of thermal cracking
shorter more useful chains are produced especially petrol
some of the products of the long-chain alkanes are the more reactive alkenes
what are alkenes used for
alkenes are used as chemical feed stock (which means they supply industries with the starting materials to make different product) and are converted into a huge range of other compounds e.g. polyethene and a range of everyday materials
what are the different ways in which alkanes are cracked
since alkanes are very unreactive and harsh conditions are required to break them down. There are a number of different ways of carrying out cracking:
- Thermal cracking
- Catalytic cracking
what is thermal cracking
thermal cracking involves heating alkanes to a high temperature, 700K -1200K (1000C) under high pressure, up to 7000kPa (70atm)
the C-C bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom
These fragments are called free radicals
Free radicals are highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules
how do the C - C bonds in alkanes break
any number of C - C may break and the chain does not necessarily break in the middle
-Hydrogen may also be produced