Alkanes are primarily used as fuels because their combustion reactions are highly exothermic. When they burn, they release large amounts of heat energy. However, the combustion of fossil fuels is a major source of air pollution and greenhouse gases, leading to significant environmental challenges.
🔑 Key Principle
Exothermic combustion reactions release valuable energy but produce pollutants like CO, soot, NOx, and SO2. Catalytic converters reduce toxic car exhaust emissions, while biofuels provide a renewable alternative that is theoretically carbon neutral but practically limited by production and transport emissions.
1. Complete vs. Incomplete Combustion
Depending on the availability of oxygen, alkanes burn in two distinct ways: complete combustion and incomplete combustion.
A combustion reaction that occurs in an excess of oxygen, fully oxidising the hydrocarbon to carbon dioxide and water.
Complete combustion releases the maximum amount of energy because all carbon and hydrogen atoms are fully oxidised. An example is the complete combustion of propane: \[ \text{C}_3\text{H}_8\text{(g)} + 5\text{O}_2\text{(g)} \rightarrow 3\text{CO}_2\text{(g)} + 4\text{H}_2\text{O(g)} \]
Incomplete Combustion
When the oxygen supply is limited, incomplete combustion occurs. Under these conditions, the carbon is not fully oxidised, resulting in the formation of carbon monoxide (CO) and/or solid carbon particulates (soot), alongside water:
- Carbon Monoxide formation: \[ \text{C}_3\text{H}_8\text{(g)} + 3.5\text{O}_2\text{(g)} \rightarrow 3\text{CO(g)} + 4\text{H}_2\text{O(g)} \]
- Soot (carbon) formation: \[ \text{C}_3\text{H}_8\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow 3\text{C(s)} + 4\text{H}_2\text{O(g)} \]
When balancing combustion equations, always follow this order: first balance carbon, then balance hydrogen, and finally balance oxygen. If balancing oxygen requires a fractional coefficient, such as 3.5 or 7/2, this is fully acceptable at A-Level.
2. Environmental Pollutants and Their Effects
The burning of alkane fuels releases several harmful pollutants into the atmosphere. You must know the origin and environmental impact of each of these substances:
| Pollutant | Origin / Formation | Environmental / Health Impact |
|---|---|---|
| Carbon Monoxide (CO) | Incomplete combustion of hydrocarbons in engines. | Toxic gas. It binds irreversibly to hemoglobin in red blood cells, reducing oxygen transport. |
| Soot / Particulates (C) | Incomplete combustion of hydrocarbons. | Causes respiratory problems, exacerbates asthma, and contributes to global dimming. |
| Nitrogen Oxides (NOx) | Reaction of N2 and O2 from air due to high temperatures and pressures in engines. | Causes acid rain (forms HNO3) and contributes to photochemical smog. |
| Sulfur Dioxide (SO2) | Combustion of sulfur impurities present in fuel: S + O2 → SO2. | Causes acid rain (forms H2SO3 and H2SO4), which damages buildings and aquatic life. |
Sulfate Removal (Flue Gas Desulfurisation)
To prevent sulfur dioxide from entering the atmosphere, power stations remove it from waste gases. Flue gas desulfurisation involves reacting sulfur dioxide with calcium oxide (lime) or calcium carbonate (limestone) to produce calcium sulfite, which can be oxidised to form gypsum (calcium sulfate, used in plasterboard): \[ \text{CaO(s)} + \text{SO}_2\text{(g)} \rightarrow \text{CaSO}_3\text{(s)} \] \[ \text{CaCO}_3\text{(s)} + \text{SO}_2\text{(g)} \rightarrow \text{CaSO}_3\text{(s)} + \text{CO}_2\text{(g)} \]
3. Catalytic Converters
Internal combustion engines in cars are equipped with catalytic converters to reduce the emission of harmful pollutants. The converter consists of a ceramic honeycomb support coated with a thin layer of transition metals, specifically platinum (Pt), palladium (Pd), and rhodium (Rh).
The honeycomb structure provides a very large surface area to maximise the rate of reaction. The metals catalyse reactions that convert toxic exhaust gases into less harmful substances:
- Removal of Carbon Monoxide and Nitrogen Monoxide: \[ 2\text{CO(g)} + 2\text{NO(g)} \rightarrow 2\text{CO}_2\text{(g)} + \text{N}_2\text{(g)} \]
- Removal of Unburnt Hydrocarbons and Nitrogen Monoxide: \[ \text{C}_8\text{H}_{18}\text{(g)} + 25\text{NO(g)} \rightarrow 8\text{CO}_2\text{(g)} + 9\text{H}_2\text{O(g)} + 12.5\text{N}_2\text{(g)} \]
4. Biofuels and Carbon Neutrality
Due to the environmental impacts of burning fossil fuels, renewable alternatives like biofuels have been developed. Common biofuels include bioethanol (produced by fermenting plant sugars) and biodiesel (produced from plant oils).
An activity or process in which there is no net transfer of carbon dioxide to the atmosphere over its entire lifecycle.
Theoretical Carbon Neutrality
Bioethanol is theoretically carbon neutral because the carbon dioxide released during its combustion and production equals the carbon dioxide absorbed by the plants via photosynthesis during growth. This can be shown by balancing equations:
- Photosynthesis (CO2 absorption): \[ 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \] (Takes in 6 moles of \(\text{CO}_2\))
- Fermentation (during bioethanol production): \[ \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 \] (Releases 2 moles of \(\text{CO}_2\))
- Combustion (burning bioethanol): \[ 2\text{C}_2\text{H}_5\text{OH} + 6\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O} \] (Releases 4 moles of \(\text{CO}_2\))
Over the entire cycle, the net release of carbon dioxide is: \[ \text{Net }\text{CO}_2\text{ change} = -6 \text{ (absorbed)} + 2 \text{ (fermentation)} + 4 \text{ (combustion)} = 0 \]
Practical Limitations and Energy Transport
In practice, biofuels are not fully carbon neutral. This is due to several real-world factors:
- Farming and Harvesting: Fossil fuels are burned to run farm machinery, such as tractors and harvesters.
- Fertilisers: The manufacture of chemical fertilisers is highly energy-intensive and releases carbon dioxide.
- Processing: Refining, fermenting, and distilling bioethanol requires thermal energy, which is often generated by burning fossil fuels.
- Transport: Biofuels must be transported from farms to refineries and distribution networks, which consumes fossil fuels.
- Energy Density and Transport Limitations: Biofuels generally have a lower energy density than fossil fuels, meaning larger volumes must be transported, increasing carbon emissions from transport vehicle fleets.
In written exam questions, do not state that biofuels are carbon neutral without qualification. You must clarify that carbon neutrality is a theoretical concept. In practice, the energy required for cultivation, harvesting, processing, and transport relies on fossil fuels, releasing carbon dioxide and making the net footprint positive.
- Write a balanced equation for the incomplete combustion of pentane (\(\text{C}_5\text{H}_{12}\)) to produce carbon monoxide and water.
- Write the chemical equation for the reaction occurring in a catalytic converter between carbon monoxide and nitrogen monoxide. State the role of the ceramic honeycomb.
Solution:
Part 1: Incomplete Combustion
- First, write the reactants and products: \[ \text{C}_5\text{H}_{12} + \text{O}_2 \rightarrow \text{CO} + \text{H}_2\text{O} \]
- Balance carbon (5 carbons on left → 5 CO on right): \[ \text{C}_5\text{H}_{12} + \text{O}_2 \rightarrow 5\text{CO} + \text{H}_2\text{O} \]
- Balance hydrogen (12 hydrogens on left → 6 H2O on right): \[ \text{C}_5\text{H}_{12} + \text{O}_2 \rightarrow 5\text{CO} + 6\text{H}_2\text{O} \]
- Balance oxygen (5 + 6 = 11 oxygens on right → 5.5 O2 on left): \[ \text{C}_5\text{H}_{12}\text{(l)} + 5.5\text{O}_2\text{(g)} \rightarrow 5\text{CO(g)} + 6\text{H}_2\text{O(g)} \]
Part 2: Catalytic Converter
- The balanced equation is: \[ 2\text{CO(g)} + 2\text{NO(g)} \rightarrow 2\text{CO}_2\text{(g)} + \text{N}_2\text{(g)} \]
- The ceramic honeycomb structure provides a high surface area. This maximises the rate of reaction by allowing more reactant molecules to contact the catalytic metal surface at any given moment, while keeping the amount of expensive catalyst metals (Pt, Pd, Rh) to a minimum.
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