C.9. Crude Oil and Fuels

• Crude oil is a finite resource found in rocks, it was formed over millions of years from the remains of tiny, ancient sea animals and plants, mainly plankton, that were buried in mud.
• Over time, layer upon layer of rock was laid down on top, creating the conditions (high pressure and temperature, in the absence of oxygen) to make crude oil.
• The crude oil formed is a dark, smelly liquid. It is a mixture of many different carbon compounds. A mixture contains 2 or more elements or compounds that are not chemically combined together.
• Nearly all of the compounds in crude oil are compounds containing only CARBON and HYDROGEN atoms, these compounds are called HYDROCARBONS.
• Crude oil is straight from the ground and has not been processed.
• It is a finite resource and contains a mixture of compounds called HYDROCARBONS.
• Crude oil straight from the ground is not much use because there are too many substances in it, all with different boiling points.

• Mixtures of liquids can be separated using distillation, only if the liquids having different boiling points. 
• Before crude oil can be used, it must be separated into different substances with similar boiling points.- These are known as fractions.

• You describe alkanes as SATURATED HYDROCARBONS. All the carbon-carbon bonds are single covalent bonds, this means that they contain as many hydrogen atoms as possible in each molecule. 
• The general formula of alkane molecules is:

                    C(n)H(2n+2)

• Some hydrocarbons are quite small with few carbon atoms in SMALL CHAINS. 
• These short- chain molecules make up the hydrocarbons that tend to be most useful.
• SMALL CHAINS:
  • make good fuels- they ignite easily and burn well, with less smoky flames.
  • a lower boiling point, less energy can be used to boils it, meaning less expensive to use.
  • higher volatility (tendency to turn into a gas)
  • very runny, lower viscosity (how easily it flows)
  • higher flammability (how easily it burns)
• LONG CHAINS
  • has a higher boiling point, takes longer to boil, more energy is used.
  • lower volatility (the tendency to turn into a gas)
  • thick, high viscosity (how easily it flows)
  • lower flammability (it has a smoky flame)

• Crude oil is separated into hydrocarbons with similar boiling points, called fractions.
• each of these fractions contains molecules with a similar number of carbon atoms
• each of these fractions boils at a different temperature range because of the different sizes of the molecules in it.
• it is carried out in a large column, hot oil enters the bottoms of the column. The higher you go in the column the cooler the temperature is. Different hydrocarbons runoff at different levels.
• The hydrocarbons with the highest boiling temperature are turned to a gas and drips down into a tube. When a hydrocarbon with a lower boiling point is boiled it rises and drips down the higher column. The further you are from the heat source the cooler it is, therefore the hydrocarbons with the lower boiling points rise for longer.

• the lighter fractions from crude oil are very useful as fuels.
• when hydrocarbons burn in plenty of air, they transfer lots of energy to the surroundings.
• when the carbon and hydrogen are burnt, they oxidise. OXIDATION- adding oxygen. 
• the products of the complete combustion of a hydrogen are carbon dioxide.
• limewater is used to test if carbon dioxide is present, the change that will be seen is that the limewater will turn cloudy.
• blue cobalt chloride paper/ white anhydrous copper sulfate can be used to test for the water formed in the complete combustion. On contact with the water, the *********** turns blue.
• all fossil fuels- oil, coal, natural gas- produce carbon dioxide and water when they burn in plenty of air.
• INCOMPLETE COMBUSTION-
  • when there is not enough oxygen (e.g. inside of an engine), there is incomplete combustion
  • instead of all the carbon in the fuel turning into co2, carbon monoxide gas is produced, CO.
  • carbon monoxide is a TOXIC gas, is COLOURLESS and ODOURLESS.
  • your red blood cells pick this up from the atmosphere and carry CO around the body instead of oxygen.

• some of the heavier fractions from the fractional distillation of crude oil are not in high demand, the hydrocarbons in them are made up of large molecules.
• They are thick liquids/solids with boiling points. They are difficult to vaporise and do not burn easily- so they are poor fuels.
• the larger, less useful hydrocarbon molecules can be broken down into smaller, more useful ones, in a process called cracking.
• CRACKING takes places in an oil refinery in steel vessels, called crackers.
• in the cracker, a heavy fraction distilled from crude oil is heated to vaporise the hydrocarbons
• the vapour is either:
  • passed over a hot catalyst, or
  • mixed with steam and heated to a very high temperature.
• the hydrocarbons are cracked as THERMAl DECOMPOSITION reactions take place.
• the larger molecules split apart to form smaller more useful ones. 

• decane, C10H22, is an alkane molecule when it is heated to 500*C with a catalyst, it breaks down. One of the molecules that are produced is PENTANE C5H12, which is used in PETROL.
• PROPENE and ETHENE are also made (propene is C3H6, ethene is C2H4).
• this cracking reaction is an example of THERMAL DECOMPOSITION.
• the pentane shows us that it has 5 carbon atoms, and the fact that it is an ALKANE shows that its molecules have as much hydrogen as possible in them.
• the other molecules in this reaction have names which end in 'ene', this means that the molecule is called an ALKENE. the molecules are unsaturated compared to an ALKANE.
• unsaturated compounds contain at least one DOUBLE BOND between their carbon atoms. C=C.

• carbon atoms have 4 covalent bonds, it means that the unsaturated alkene molecules have 2 fewer hydrogen atoms in their molecules than the saturated alkane molecules with the same number of carbon atoms.
• alkenes are generally more reactive than alkenes, therefore the reaction in bromine water is used as a test to see if an organic compound is unsaturated, like the alkenes with the C=C.
• a positive test for an unsaturated hydrocarbon is that it turns orange bromine water colourless. the alkenes do not react with bromine water, so you can use this test to distinguish between an alkene and an alkane