Chapter 4 - Crude Oil and Fuels Flashcards
What is fractions?
Crude oil straight from the ground is not much use, there are too many substances in it, all with different boiling points. Before we can use crude oil, we must separate into different substances with similar boiling points, these are known as fractions.
What is crude oil?
Crude oil is a dark smelly liquid, it is a mixture of lots of different chemical compounds.
What is distillation?
Because the properties of substances don’t change when they are mixed,we can separate the mixture of substances in crude oil by using distillation. It separates liquid with different boiling points.
What is mixture?
A mixture contain one or two elements or compounds that are not chemically combined together.
What is hydrocarbon?
Hydrocarbon is a compound containing only hydrogen and carbon, nearly all the compounds in crude oil are hydrocarbons. Most of the hydrocarbons in crude oil are alkanes.
Write down some alkane molecules;
Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12
Define saturated hydrocarbon and write down the general formula for alkane molecules
Hydrocarbon containing as many hydrogen atoms as possible in each molecule is called saturated hydrocarbon.
General formula for alkane molecules;
Cn H(2n+2)
Where n is number of carbon
How are the properties of each fraction affected by the size of molecule in it?
Hydrocarbon molecules can be very different. Some of them are very small with relatively low carbon atoms in short chains. These short chains hydrocarbons tend to most useful as they burn well and they are very flammable.
Properties affected by size of molecules are given below;
Short chain Long chain
Low boiling point High boiling point
High volatility Low volatility
Low viscosity High viscosity
High flammability Low flammability with smoke
How do we separate crude oil into fractions?
We separate out crude oil into hydrocarbons with similar boiling points. These are known as fractions. We call the process fractional distillation.
Each hydrocarbon fraction molecules contains similar number of carbon atoms but each of them boils at different temperature level, this is because of the different sizes of their molecule.
Crude oil is fed near the bottom of the fractionating columns as hot vapour. The column is kept very hot at the bottom of the while cooler at the top. The temperature decreases going up the column. The gases condense when they reach the temperature of their boiling point.so the different fractions are collected as liquid at different temperature levels. Crude oil enters the fractionating column and fractions are collected in a continuous process.
What happen when we burn fuels?
The lighter fraction from crude oil are very useful as fuel. When hydrocarbons burn in plenty of air, they release energy and 2 new substances are formed, carbon dioxide and water.
For example;
Propane + Oxygen → carbon dioxide + water
C₃H₈ + 5O₂ → 3CO₂ +4H₂O
Carbon and hydrogen in the fuel are oxidised completely when they burn like this. Oxidised means adding oxygen in chemical reaction in which oxides are formed.
Discuss the pollution from fuels
1.All fossil fuels - oil, coal, natural gas and hydrocarbon fuels produce carbon dioxide and water when they burn in plenty of air. But these fuels also contain some other substances. Impurities containing sulfur cause us major problems. When we burn fuels, sulfur react with oxygen and it forms a gas known as sulfur dioxide. This gas is poisonous and acidic. So this is very bad for environment as it is a cause of acid rain. It can also cause engine corrosion.
2. When there is not enough oxygen inside the engine, we get incomplete combustion. Instead of all carbon in the fuel turning into carbon dioxide, carbon monoxide CO also formed.
Carbon monoxide is poisonous gas. Your red blood cells pick up this gas and carry around in your blood instead of oxygen so even small amount of carbon monoxide is very bad for your health.
3. The high temperature inside the engine allows nitrogen and oxygen to react together. These are poisonous and can trigger some people’s asthma. It can also acid rain.
4. Diesel engines burn hydrocarbon with much bigger molecules. When these big molecules react with oxygen, they don’t always burn completely. Solid particles of carbon and unburnt hydrocarbons are produced. These tiny solid particulates carried into the air. Scientists think that these particulates can be very dangerous for lungs as they damage the lung cells and even cause cancer.
