- Crude oil is a mixture of compounds called hydrocarbons.
- Many useful materials can be produced from crude oil.
- It can be seperates into different fractions using fractional distillations, and some of these can be used as fuels
- Crude oil forms naturally over millions of years from the remains of living things. Most of the compounds in crude oil are hydrocarbons.
- These are compounds that contain hydrogen and carbon atoms only, joined together by chemical bonds called covalent bonds.
- There are different types of hydrocarbon, but most of the ones in crude oil are alkanes
- The alkanes are a family of hydrocrabons that share the same general formula this is: CnH2n+2
- The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms plus two. For example methane is CH4 and ethane is C2H6
Alkane molecules can be represented by displayed formulas. In a displayed formula, each atom is shown as its symbol (C or H) and each covalent bond by a straight line. Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined together by single bonds/ This makes them relativley unreactive, apart from burning or combustion, which is their reaction with oxygen in the air.
- Distillation is a process that can be used to seperate a pure liquid from a mixture of liquids. 4
- It works when the liquids have different boiling points.
- Distillation is commonly used to seperate ethanol from water.
Distillation process to seperate ethanol from water:
- The mixture is heated in a flask.
- Ethanol has a lower boiling point than water so it exaporates first.
- The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
- The thermometer shows the boiling point of the pure ethanol liquid.
- When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.
- Heating > Evaporating > Cooling > Condensing
- Fractional distillation is different from distillation in that it seperates a mixture into a number of different parts, called fractions.
- A tall column is fitted about the mixture, with several condensers coming off at different heights.
- The column is hot at the bottom and cool at the top.
- Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.
- The crude oil is evaporated and its vapours condense at different temperatures in the frationating column.
- Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
As you go up the fractionating column the hydrogarbons have:
- Lower boiling points
- Lower viscosity (they flow more easily)
- Higher flammabiliy (they ignite more easily)
- This means that the general hydrocrabons with small molecules make better fuels than hydrocarbons with large molecules
Fractional Distillation 2
Combustion of Fuels
Several waste products are released when fuels burn. These do not just disappear and they can harm the environment by contributing to global warming, global dimming and acid rain.
Fuels burn when they react with oxygen in the air. If there is plenty of air, complete combustion happens. Coal is mostly cabron. During complete combustion, carbon is oxidised to carbon dioxide:
Carbon + Oxygen > Carbon Dioxide.
Carbon is a green house gas. Increasing concentration of it in the atmosphere contribue to global warming.
Hydrocarbon fuels contain carbon and hydrogen. During combustion, hydrogen is oxidised to water (remember that water, H2O, is an oxide of hydrogen)
Hydrocabron + oxgen > carbon dioxide +water
Combustion of Fuels 2
If there is insufficent air for complete combustion, imcomplete combustion (also called partial combustion) happens .
Hydrogen is still oxidised to water, but carbon monoxide forms instead of carbon dioxide. Carbon monoxide is a toxic gas, so adeduate ventilation is important when burning fuels.
Solid particles (particulates) are also released. These contain cabron and are seen as soot or smoke. Particulartes cause global dimming. They reduce the amowunt of sunlight reaching the earth's surface.
- Carbon Dioxide dissolves in water in the atmosphere to form a weakly acidic solution.
- This means the rainwater is naturally slightly acidic.
- However, some of the products from burning fuels make rainwater more acidic than normal. This is acid rain.
- Acid rain reacts with metals and rocks such as limestone, causing damage to building and statues.
- Acid rain damages the waxy layer on the leaves of trees. This makes it more difficult for trees to absorb minerals they need for healthy growth and may die.
- Acid rain also makes rivers and lakes to acidic for some aquatic life to survive.
Coal and most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide:
- Sulfur + Oxguen > Sulfur Dioxide
- This gas dissolves in water to form an acidic solution. It is a cause of acid rain.
Sulfur can be removed from fuels before they are used. 'Low Sulfur' petrol and diesel are widely avaliable at filling stations to use in vehciles.
In power stations, sulfur dioxide can be removed from the waste gases before they are released from chimneys. The waste gases are treated with powdered limestone.
The sulfur dioxide reacts with it to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.
Oxides of Nitrogen
At the high temperatures found in an engine or furnace, nitrogen and oxygen from the air can react together. They produce various oxides of nitrogenm often called NOx these also cause acid rain.
Coal and oil are non-renemable resources, They take so long to form that they cannot be replaced once they have all been used up. This means that these fossil fuels are likely to become more expensive as they begin to run out, Petrol, diesel and other fuels prodcued from crude oil make a rang of harmful substances when burned e.g:
- Carbon Dioxide
- Cabron Monoxide
- Water Vapour
- Particulates (solid particles)
- Sulfur Dioxide
- Oxides of Nitrogen
Biofuels are fuels produced from plant material. They have some advantages and disadvantage compared to fossil fuels
Biodiesel is made from rapeseed oil and other plant oils. It can be used in diesel-powered vehicles without needing any modifications to the engine.
- Ethanol, C2H50H, is not a hydrocrabon because it contains oxygen as well as hydrogen and cabron. It is a liquid fuel that burns well.
- Bioethanol is made by fermenting sugars from sugar cane, wheat and other plants. It cannot be used on its own unless the engine is modified.
- Modern petrol engines can use petrol containing up to 10 percent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol.
- There are ethical issues surrounding the use of biofuels. For example, crops that could be used to feed people are used to provide the raw materials for biofuels instead.
- This could cause food shortages or increases in the price of food.
- Human resources- More people are needed to produce buifuels than are needed to produce petrol and diesel
- Increased income- For farmers
- Lower fuel prices- Biofuels limit the demand for fossil fuels, helping to reduce increases in fuel prices.