6. Alkanes Flashcards
What are some uses of alkanes?
Fuels
lubricants
Starting materials for other compounds
What kind of hydrocarbons are alkanes?
Saturated hydrocarbons
What is the main source of alkanes?
Crude oil
What is the general formula for alkanes?
CnH2n+2
What shapes can alkanes take?
Unbranched and branched chains or rings
What general formula do ring alkanes have?
CnH2n (the end hydrogen a are not required)
Unbranched chains are often called ‘straight’ chains but the C-C-C angle is…
109.5 degrees
When naming alkanes find the longest unbranched…
Chain (for meth-/eth-/prop-/but- etc) (remember, you can go round corners for longest chain)
Which alkanes have no isomers?
Methane, ethane and propane
What is the polarity of the alkanes like?
Alkanes are almost non-polar because the electronegativity of carbon and hydrogenated so similar.
What intermolecular forces act between alkanes?
van der Waals forces, the larger the molecule the stronger the van der Waals forces
What are the boiling points of alkanes like?
The larger the molecule the stronger the van der Waals forces, the higher the boiling point.
Which alkanes are gases?
The shorter chain alkanes e.g 1-4 carbons
Which alkanes are liquids?
Alkanes with carbons 5-17
Which alkanes are solids?
Alkanes with 18+ carbons
Are alkanes soluble in water?
No, the hydrogen bonds in water are stronger than the van der Waals forces between the alkanes. They do mix with non-polar liquids.
How reactive are alkanes?
Alkanes are relatively unreactive. They do not react with acids, bases oxidising agents and reducing agents but they do burn in oxygen to form carbon dioxide and water (or in incomplete combustion to form carbon monoxide or carbon).
What is the worlds main source of organic chemicals?
Crude oil
Is crude oil renewable?
No
What is crude oil a mixture of?
Mostly alkanes.
How do we convert crude oil into useful products?
We have to separate the mixture by heating it and collecting fractions.
What does each fraction contain?
A mixture of hydrocarbons of similar chain length and therefore properties.
What is the process of separating the alkanes in crude oil called? Where is it done?
Fractional distillation
A fractionating tower
Give a short explanation of fractional distillation
The crude oil is heated in a furnace.
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. Then they condense into a liquid.
Each collected tray is piped off.
Where do shorter chain hydrocarbons condense in a fractionating tower?
In trays nearer to the top of the tower where it is cooler because they have a lower boiling point.
What collects at the base of a fractionating tower?
A thick residue called tar or bitumen. It is used for road surfacing but this fraction is often further processed to give more valuable products.
Which fraction is in high demand?
The naphtha fraction for petrol and the chemical industry
Most crude oil has more longer chain fractions than is wanted and not enough of the….
Naphtha fraction
What is done to meet the demand for shorter chain hydrocarbons? What is this called?
Many of the longer chain fractions are broken into shorter lengths. This is called cracking.
What are the two useful results of cracking long chain hydrocarbons?
- Shorter, more useful, chains are produced, especially petrol.
- Some of the products are alkenes, which are more reactive than alkanes.
What is petrol a mixture of?
Mainly alkanes containing between four and twelve carbon atoms.
Which is the most important alkene? Why is it the most important?
Ethene. It is the starting material for polythene and other materials.
What are alkenes used for?
As starting materials to make new materials. E.g polymers, paint, drugs.
Is it easy to break down alkanes?
Alkanes are very unreactive and harsh conditions are required to break them down.
Name the two types of cracking.
Thermal cracking
Catalytic cracking
What are the conditions for catalytic cracking?
Heating alkanes up to a high temperature (950K)
Put them under high pressure (7000kPa)
In thermal cracking how do the carbon-carbon bonds break?
In a way that one electron from the pair in the covalent bond goes to each carbon atom so two shorter chains are produced each ending in a carbon atom with an unpaired electron as an intermediate. This is a free radical (highly reactive). They combine in a number of ways to form a variety of shorter chain molecules.
What is also produced in thermal cracking?
As there Are not enough hydrogen atoms to produce two alkanes, one of the new chains must have a carbon-carbon double bond-an alkene.
Hydrogen can also be produced.
What does thermal cracking produce a large amount of?
Alkenes - to avoid more decomposition the alkanes are not kept in these conditions for long (about a second).
What are the conditions for catalytic cracking?
Lower temperature (720K) Lower pressure (but more than atmospheric) Using a zeolite catalyst
Hat does the zeolite catalyst consist of?
Silicon dioxide and aluminium oxide
What structure do zeolite catalysts have?
A honeycomb structure with an enormous surface area. They are also acidic.
What is catalytic cracking mainly used to produce?
Motor fuels.
What does catalytic cracking mainly produce in terms of alkanes etc?
Alkanes, cycloalkanes, aromatic compounds and some alkenes.
Products obtained from cracking are separated by…
Fractional distillation.
The complete combustion of short chain alkanes produces water and carbon dioxide. Does this reaction give out, or take in heat?
These combustion reactions give out heat. They have a large negative enthalpy of combustion and the more carbons present the greater the heat output. They are therefore important as fuels.
Give some examples of alkanes which are fuels.
Methane, propane (camPing gas), butane, petrol and paraffin.
In a limited supply of oxygen what is formed in the combustion of an alkane?
The poisonous gas, carbon monoxide (CO)
What is it called when something is not completely burnt?
Incomplete combustion
What is produced when you burn an alkane in very little oxygen?
Carbon (soot). On Bunsen, flame is yellow and black sooty deposit appears on the apparatus. This often happens with longer chain hydrocarbons which need more oxygen to burn.
What polluting products can be produced by all hydrocarbon-based fuels when they burn?
- Carbon monoxide
- Nitrogen oxides
- Sulfur dioxide
- Particulates
- Unburnt hydrocarbons
- Carbon dioxide
- Water vapour
How are nitrogen oxides produced?
When there is enough energy for N2 and O2 in the air to combine:
N2 (g) + O2 (g) —–> 2NO (g)
How is nitric acid formed from the nitrogen oxides given off in the combustion of alkanes?
The oxides produces may react with water vapour and oxygen in the air to form nitric acid. They are therefore contributors to acid rain and photochemical smog.
How is sulfur dioxide produced?
It is produced form the sulfur-containing impurities present in the crude oil. The oxide combines with water vapour and oxygen in the air to form sulfuric acid:
SO2 + H2O —> H2SO3 + 1/2 O2 —> H2SO4
How do particulates damage health?
They can exacerbate asthma and cause cancer
Why are unburnt hydrocarbons bad for the environment?
They are significant greenhouse gases. They contribute to photochemical smog.
Why is carbon dioxide a concern for the environment?
It is a possible cause (it is a greenhouse gas) of the increase in the earths temperature and consequent climate change.
Why is water vapour a concern for the environment?
It is a greenhouse gas
What is used to absorb sulfur dioxide in chimneys? What does this produce? What is the process called?
Calcium oxide (CaO) or limestone (CaCO3). This produces CaSO4 which is used as a plaster. This process is called flue gas desulfurisation.
Ow is sulfur in fuels less of a problem now?
Sulfur is removed form petrol
What are all new cars with petrol engines now equipped with?
Catalytic converters
What is the catalytic converter?
It is a honeycomb (large surface area - small amount of catalyst goes a long way) made of ceramic material coated with platinum and rhodium metals. These are the catalysts.
Give the equations for the reactions at the catalytic converter.
- 2CO (g) + 2NO (g) —> N2 (g) + 2CO2 (g)
2. Hydrocarbons + nitrogen oxide —>nitrogen+carbon dioxide+water
How does carbon dioxide heat up the earths atmosphere? Why is is important?
It traps infra-red radiation. Without carbon dioxide and other greenhouse gases the earth would be too cold.
Since the industrial revolution more fossil fuels have been burnt and more CO2 released. Scientist think this has been the main cause of global warming as the amounts of other greenhouse gases has remained similar to previous years.
Give two greenhouse gases except carbon dioxide.
Water vapour and methane
What could happen if the temperature of the atmosphere rises?
There will be more water vapour in the air and so more greenhouse warming. However, this could be offset by greater cloud formation and clouds reflect solar radiation.
What are activities that produce no carbon dioxide emissions referred to as?
Carbon neutral
Changes in sun spot activity could also be responsible for…
increased CO2 levels.
