Standard Grade Chemistry
Summary Notes
Topic 5: Fuels
Learning Outcomes
General
· A fuel is a chemical which burns giving out energy
· Combustion is a reaction of a substance with oxygen, giving out energy
· The test for oxygen is that it relights a glowing splint
· The main components of air are oxygen and nitrogen in the proportion 1:4
· An exothermic reaction is one in which energy is released
· A finite resource is non-renewable. It will run out. Fossil fuels are finite resources.
· Coal, oil and natural gas are fossil fuels formed from ancient living things.
· All fossil fuels are formed from the remains of living things which have been buried underground over millions of years.
· The pollution formed by the burning of fossil fuels includes sulphur dioxide, nitrogen dioxide, soot and unburned hydrocarbons.
· Carbon monoxide and carbon (soot) are formed by incomplete combustion, because of a lack of oxygen.
· Catalytic converters in car exhausts change harmful gases into harmless gases.
· Reducing the fuel-to-air ratio (increasing the air-to-fuel ratio) in car engines means more complete combustion and less pollution.
· The chemical compounds which are found in oil and natural gas are mainly hydrocarbons.
· A hydrocarbon is a compound which contains hydrogen and carbon only.
· The test for carbon dioxide is that it turns lime water milky.
· The test for water is that it freezes at 0oC and boils at 100oC.
· Crude oil is a mixture of compounds.
· These compounds have specific melting and boiling points.
· A fraction is a group of compounds with boiling points within a specific range.
· Fractional distillation is the process used to separate crude oil into fractions.
· Fractional distillation separates crude oil into fractions depending on their boiling points.
· Flammability is the ease of which a fuel catches fire.
· Viscosity is the thickness of a liquid.
· The fractions in order of increasing boiling point are : fuel gases < petrol < naptha < kerosene < gas oil < fuel oil < lubricating oils < bitumen.
· As the boiling points of the fractions increases, their molecular size increases.
· As the molecular size of the fraction increases, their viscosity increases.
· As the molecular size of the fraction increases, their flammability decreases.
· The balanced equation for a chemical reaction has the same number of atoms of each element on each side.
Credit
Crude oil fraction / Number of carbon atoms / Uses of fractionsfuel gases / C1 to C4 / portable stoves and heaters
gasoline / C4 to C10 / petrol for cars
naphtha / C6 to C12 / to make chemicals
kerosene / C9 to C16 / paraffin and fuel for jet aircraft
gas oil / C15 to C25 / diesel fuel for lorries and trains
fuel oil / C20 to C36 / domestic and industrial heating
lubricating oils / C34 to C70 / machinery
bitumen / > C70 / tar for roads and roofs
· The smallest molecules require less energy to separate and therefore evaporate, so have lower boiling points.
· The larger molecules require more energy to separate and therefore evaporate, so have higher boiling points.
Fuels
A fuel is a chemical which burns giving out energy.
The type of chemical reaction which takes place when a fuel is burned is called combustion. Combustion is the reaction of a substance with oxygen, giving out energy. The fuel itself and oxygen are two of the three requirements necessary for a combustion reaction to take place. A source of ignition, often a spark or a little heat, is the third requirement. The three requirements are sometimes said to form the three sides of the ‘Fire Triangle’.
Combustion reactions, where energy is given out during the reaction are called exothermic reactions. Most of the energy released is in the form of heat energy.
A good fuel is one which is cheap and readily available, which burns easily to produce lots of energy and which produces little or no pollution.
A fuel burned in pure oxygen will burn better than the same fuel burned in air.
Air is composed of about 1/5th oxygen and 4/5th nitrogen. Although air contains the oxygen necessary for combustion, the lower percentage of oxygen in air means fewer collisions and a slower reaction. The test for oxygen gas is that it will relight a glowing splint.
Many different fuels are forms of the element carbon or are compounds containing carbon. Charcoal which is commonly used as a fuel on a barbecue, is a form of the element carbon and is a good fuel. Natural gas is the gas we use in our homes, schools and factories and is one of the commonest fuels in use today. Natural gas is 96% methane. Methane is the simplest hydrocarbon and has the chemical formula CH4. Hydrocarbons are compounds which contain hydrogen and carbon only.
When the charcoal burns, the carbon atoms combine with oxygen to form carbon dioxide. Carbon dioxide gives a cloudy/ milky colour when tested with lime water.
Methane can be burned in the laboratory using a Bunsen burner. Because methane contains carbon and hydrogen atoms, the oxygen required for combustion can combine with both types of atom to form two different products.
The carbon atoms combine with oxygen to form carbon dioxide and the hydrogen atoms combine with oxygen to form water.
The lime water turns milky indicating that carbon dioxide has formed, and the colourless liquid collected was found to boil at 100oC and freeze at 0oC. This is the test for water.
Formula Equations
During a chemical reaction the atoms which make up the reactants get rearranged to make the products.
In the combustion of methane, carbon dioxide and water are formed as products.
methane + oxygen à carbon dioxide + water
This word equation can be rewritten as a formula equation.
CH4 + O2 à CO2 + H2O
However, the formula equation does not fully represent what happened in the reaction. It would appear that 2 atoms of hydrogen have been ‘lost’ during the reaction. This is not the case.
Atoms can neither be ‘lost’ or ‘gained’ during a chemical reaction, only changed from one substance to another. Therefore all the atoms which make up the reactants should be found in the products.
The above formula equation is not balanced and must be balanced so that it can truly represent what happens in the reaction.
Fossil Fuels
Natural gas is an example of a fossil fuel and is often found with crude oil, another fossil fuel.
Crude oil is one of the most important sources of fuel in the world today. It is a mixture of many different chemical compounds, mostly hydrocarbons, which are found in fuels such as petrol, diesel and fuel oil.
Crude oil and natural gas are termed fossil fuels because they were formed from things that were living millions of years ago. Coal is a third very important fossil fuel.
Formation Of Crude Oil
Crude oil was formed from the remains of dead sea plants and animals. The dead remains were covered by layers of sediment and over millions of years they were affected by heat, pressure and bacteria and converted into crude oil.
Natural gas and crude oil are finite resources. The word ‘finite’ means that they cannot be replaced and will run out in time. Although there are greater reserves of coal, supplies of all three of these very important fossil fuels are eventually going to run out. This is such a major problem that it has been called the ‘Energy Crisis’.
One thing that could be done is to use energy sources that are not going to run out. These are called renewable sources e.g. wind power, wave power, solar power etc. They are called alternative energy sources, because they are alternatives to the fossil fuels which are running out.
Crude Oil
Crude oil is one of the most important sources of fuels in the world today because it is a mixture of many different chemical compounds, mostly hydrocarbons, which is found in these fuels.
Before these hydrocarbons can be used they have to be separated. This is done in large scale oil refineries, as part of the whole process of refining the crude oil. The separation of crude oil into its useful parts (or fractions) is called fractional distillation.
Other parts of the refining process involve fractions being converted to others and purifying of products.
A fraction collected will be a group of compounds with boiling points within a range of temperatures.
Crude Oil Fractions
At an oil refinery, the crude oil, which is a mixture of hydrocarbons is separated into fractions in a large tower called a fractionating column.
The crude oil is continuously fed into the bottom of the tower and heated.
The components of the mixture evaporate and pass into the tower and rise up. As they rise, they cool at different rates (due to their different boiling points) and are constantly tapped off at different levels where they condense.
The fractions collected through fractional distillation have many different uses depending on their properties. Properties such as flammability and viscosity must be considered.
Flammability is a measure of how easily a substance catches fire or ignites.
Viscosity is a measure of the thickness and stickiness of a liquid. A viscous liquid is one of high viscosity which due to its thickness and stickiness will not flow easily.
Typical fractions obtained through fractional distillation of crude oil are:
Fraction / Number of carbon atoms in typical molecule / Boiling point range (oC) / Flammability / Viscosity & colour / Usesfuel gas (refinery gas) / 1 to 4 / up to 20oC / more
difficult
to
ignite / becoming
thicker
and
darker
in
colour / bottled gas
gasoline / naphtha / 5 to 12 / 20oC to 120oC / petrol,
making chemicals
kerosene
(paraffin) / 10 to 16 / 120oC to 240oC / aviation fuel, paraffin
gas oil / 14 to 30 / 240oC to 350oC / diesel fuel, lubricating oil, central heating fuel
residue / 25+ / 350oC + / waxes, bitumen for roads and roofs
The fraction with the lowest boiling point range is the most easily evaporated. The fraction with the highest boiling point range is the most difficult to evaporate.
It can also be seen from the table that the fractions with the lowest boiling point ranges are the most flammable, the least viscous and the lightest in colour.
It can be noted that the fractions with the bigger molecules have the greater boiling point ranges. It is more difficult for the bigger, heavier molecules to move fast enough and escape from the liquid.
For similar reasons, the bigger molecules also have greater viscosities as the molecules are more likely to get tangled together.
Smaller molecules which are more likely to evaporate are more flammable.
Coal
Coal is still burned extensively as a source of energy, particularly in power stations.
Burning coal to obtain energy is however a rather wasteful way of using it as it has many other uses e.g. as a source of coke for use in blast furnaces in the steel industry or to produce a wide range of chemicals for making disinfectants, insecticides, wood preservatives etc.
Coal like crude oil and natural gas is also a fossil fuel.
Formation of coal
Coal is formed from the remains of dead vegetation (plants and trees). The dead remains were covered by layers of sediment and over millions of years they were affected by heat, pressure and bacteria and converted into coal.
Oxides Of Carbon
When methane is burned, carbon dioxide and water are formed as products. In fact when any hydrocarbon is burned completely only carbon dioxide and water are produced.
hydrocarbon + oxygen à carbon dioxide + water
However, when hydrocarbons burn there is not always a sufficient supply of oxygen to ensure complete combustion. Unless you use a very efficient burner, you will probably get some unburned carbon produced as soot. This can happen with the combustion reactions in a Bunsen burner when it is used with the air hole closed. You may have seen the sooty deposits on the bottom of beakers you have heated. You can also see soot in the flame itself. The sootier the flame, the more unburned carbon is being produced.
Another possible product of the incomplete combustion of carbon, or a fuel containing carbon is carbon monoxide. This can occur in boilers for gas central heating systems if the boiler is not properly ventilated.
The following table compares the properties of carbon dioxide ad carbon monoxide.
carbon dioxide / carbon monoxidechemical formula / CO2 / CO
appearance / colourless, odourless, tasteless gas / colourless, odourless, tasteless gas
pH / slightly acidic / neutral
combustion reaction / does not burn / burns to form CO2
lime water / turns lime water milky / no effect
dangers / can cause suffocation in very large volumes
also thought to contribute to Global Warming through its action as a Greenhouse Gas / very dangerous
extremely poisonous, combines with haemoglobin in the blood stopping oxygen transport, leading to death through oxygen starvation
Uses Of Carbon Dioxide
Carbon dioxide is commonly used as a fire extinguisher. It is very effective for this purpose as it does not burn. The gas is heavier than air which causes it to sink and lie over the fire like a blanket preventing the necessary oxygen reaching the fire thus smothering the fire.
Solutions of carbon dioxide in water have a pleasant taste – soda water. Most fizzy drinks are made by dissolving carbon dioxide in them at high pressure.
Solid carbon dioxide ‘dry ice’ is used for refrigerating ice-cream, meat and soft fruits. Dry ice is also used for producing smoke effects in concerts.
Carbon dioxide is the gas used by green plants in the process known as photosynthesis.
Pollution