Combustion, Cracking & Alkenes
Complete Combustion
Hydrocarbon fuels release energy when they burn. When there is a plentiful supply of oxygen, complete combustion occurs. The products are carbon dioxide and water, and energy is given out — the reaction is exothermic.
For example, the complete combustion of methane:
$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$
Complete combustion of a hydrocarbon produces carbon dioxide (\(CO_2\)) and water (\(H_2O\)). Energy is released (exothermic). This only happens when there is plenty of oxygen.
Incomplete Combustion
When the oxygen supply is limited, incomplete combustion occurs. Instead of all the carbon atoms forming carbon dioxide, some form carbon monoxide (\(CO\)) or carbon (soot). Water is still produced.
Incomplete combustion is a problem because:
- Carbon monoxide is a toxic gas. It is colourless and odourless, so it cannot be detected without a detector. It binds to haemoglobin in red blood cells, preventing them from carrying oxygen. This can be fatal.
- Soot (carbon particles) causes breathing problems, contributes to smog, and blackens buildings.
Gas boilers, fires and cookers can produce carbon monoxide if they are faulty or poorly ventilated. Because carbon monoxide is colourless and has no smell, people may not realise it is present. Carbon monoxide detectors should be fitted in homes with gas appliances. Regular servicing ensures complete combustion and reduces the risk of carbon monoxide poisoning.
Pollution from Burning Fuels
Burning hydrocarbon fuels causes several environmental problems beyond carbon monoxide and soot:
Some hydrocarbon fuels contain sulfur impurities. When these fuels burn, the sulfur reacts with oxygen to produce sulfur dioxide (\(SO_2\)). Sulfur dioxide dissolves in rainwater to form acid rain, which damages buildings, kills aquatic life in lakes and rivers, and harms trees and other vegetation.
Inside car engines and power stations, the temperatures are extremely high. At these temperatures, nitrogen and oxygen from the air react together to form oxides of nitrogen (\(NO_x\)). These gases also contribute to acid rain and can cause breathing problems. Note that the nitrogen comes from the air, not from the fuel.
Burning fossil fuels produces: \(CO_2\) (greenhouse gas, climate change), \(CO\) (toxic, binds to haemoglobin), soot (breathing problems), \(SO_2\) (acid rain from sulfur impurities), and \(NO_x\) (acid rain, formed at high temperatures from \(N_2\) and \(O_2\) in air).
Hydrogen as an Alternative Fuel
The specification asks you to evaluate hydrogen as an alternative to petrol for cars:
- Advantages: The only product of burning hydrogen is water — no \(CO_2\), \(CO\), soot or \(SO_2\). It does not contribute to climate change or acid rain at the point of use.
- Disadvantages: Hydrogen is difficult to store (it must be kept at very high pressure or very low temperature). It is highly flammable and explosive. Most hydrogen is currently produced from fossil fuels, which still releases \(CO_2\). Hydrogen fuel cells and filling stations are expensive and not widely available.
Cracking
Cracking is the process of breaking down larger, less useful hydrocarbon molecules into smaller, more useful ones. It is carried out because the demand for short-chain hydrocarbons (like petrol and gases) is much greater than the supply from fractional distillation alone. There is a surplus of long-chain fractions (like fuel oil and bitumen).
Cracking is necessary because the supply of long-chain hydrocarbons from crude oil exceeds demand, while demand for short-chain hydrocarbons exceeds supply. Cracking produces shorter alkanes (saturated, for fuels) and alkenes (unsaturated, used to make polymers).
Alkenes are unsaturated hydrocarbons — they contain at least one carbon–carbon double bond (\(C=C\)). This makes them more reactive than alkanes. Alkenes are used as the starting material for making polymers (plastics) through polymerisation.
Cracking involves heating long-chain hydrocarbons to high temperatures and/or passing them over a catalyst. In the school practical, paraffin oil (a long-chain alkane mixture) is vaporised and passed over heated broken pottery (a catalyst). The products include shorter alkanes and alkenes. Alkenes can be tested for using bromine water — it turns from orange to colourless when an alkene is added, because the double bond reacts with bromine.