1. Fossil fuels Flashcards
What is the key source of most of the fossil fuels we use to power our everyday lives?
Crude oil.
Crude oil is made up of molecules called what?
Hydrocarbons.
Crude oil contains many useful compounds but isn’t overly functional as a fuel on its own. Instead, the hydrocarbons within crude oil need to be what to produce fuels that we can use?
Separated and refined.
These fuels include petrol, diesel and natural gas.
What is one of the most common fossil fuels that is produced from crude oil?
Petrol.
It’s mainly used as a transport fuel and is what powers the majority of vehicles on roads all over the world. If you’ve ever jumped in a car, on a motorbike or on public transport, then it’s likely that these vehicles required petrol to run properly.
Petrol is made up of a mixture of different hydrocarbons, but its key component is what?
Octane.
To release energy from petrol (or any other fuel), we require a what?
Combustion reaction.
When petrol combusts, it releases a large amount of energy.
What is the combustion reaction for octane?
2C8H18(l)+25O2(g)→16CO2(g)+18H2O(g)
This energy is what makes petrol so useful: a small volume of petrol can provide a large amount of useful energy. In fact, most new cars can travel 100 kilometres with only 8 litres of fuel.
What is the other well-known fuel that is produced from crude oil?
Diesel.
You might see diesel also called petrodiesel, but regardless of what it is called, diesel is always around 75% alkanes with 10–15 carbon atoms. The other 25% of diesel is made up of ring-shaped molecules called aromatic hydrocarbons.
Though the energy content of petrol and diesel is similar, diesel is used to power much larger transport vehicles than petrol. This is because diesel is made up of larger hydrocarbons than petrol, with much longer carbon chains.
The combustion of propane, the key component of LPG, releases a significant amount of energy—just not as much as petrol or diesel. This is because propane molecules are what?
Much smaller than the hydrocarbons of petrol and diesel, so have lower energy density.
The components of LPG can also be found in the Earth’s crust: as natural deposits of what?
Natural gas.
Natural gas contains many different hydrocarbons but is mainly made up of methane (CH4) and smaller amounts of ethane(C2H6) and propane (C3H8).
Natural gas forms alongside crude oil. If a deposit of crude oil is trapped underneath a layer of impermeable rock, then a layer of natural gas tends to form above the crude oil. It can also be found in the tiny spaces between rocks themselves, and in large cracks between layers of rock.
Which fossil fuel trumps all others in terms of the amount of energy it releases?
Coal.
Like natural gas, coal is also used to generate electricity. However, it is used on a much larger scale—coal supplies over one-third of the entire world’s energy. This is because coal is currently very abundant, so it’s cheap to use.
Like crude oil and natural gas, coal is also formed from the remains of long-dead organisms (mainly plants) that have undergone complex changes over time. Coal is essentially a combustible rock made up of large molecules: mainly carbon, hydrogen, nitrogen and sulfur.
The amount of energy released by coal increases with the age of the deposit. Due to the variation in the quality of coal, the energy released from its combustion will also vary.
The highest quality coal is what?
Anthracite, the only fossil fuel with a higher energy density than diesel.
Crude oil is an example of a fossil fuel formed millions, if not billions, of years ago. While it’s not useful as a fuel on its own it does contain useful hydrocarbons of varying sizes.
These hydrocarbons can be extracted from crude oil to form products such as petrol, diesel and LPG. In order to do this, which two main processes are carried out?
Fractional distillation and catalytic cracking.
Fractional distillation is a process that separates the differently sized hydrocarbons in crude oil.
It uses heat and a structure called a fractionating tower to separate the crude oil mixture into different groups of similarly sized hydrocarbons, which we call what?
Fractions.
Crude oil is heated in a furnace set to 400 °C and then moved into the base of the fractionating tower. When it is introduced into the tower, it starts to vaporise. These vapours from the oil rise through the column.
The temperature of the tower decreases with increasing height: the temperature at the bottom of the tower is approximately 350 °C, but only 25 °C at the very top. As the oil vapours rise, they cool according to the temperature of the tower. When they get to a point where the temperature in the tower matches their boiling point, the vapours condense and are captured in specially designed trays.
The large temperature difference in the tower means that each separated fraction consists of alkanes within a very specific mass range.
Lighter alkanes will condense in the cooler regions where?
At the top of the fractionating tower.
What does cracking require?
It requires a reactor, where high pressures and temperatures are generated to break the large hydrocarbons apart.
Now, modern cracking is carried out with the use of a catalyst and therefore, lower temperatures and pressures. This is known as catalytic cracking.
Since cracking is carried out in a reactor, we can vary the temperature, pressure and type of catalyst used to produce different products from large hydrocarbons.
