Understanding Points
Reactivity 1.3.1—Reactive metals, non-metals and organic compounds undergo combustion
reactions when heated in oxygen.
Reactivity 1.3.2—Incomplete combustion of organic compounds, especially hydrocarbons, leads to the production of carbon monoxide and carbon.
Reactivity 1.3.3—Fossil fuels include coal, crude oil and natural gas, which have different
advantages and disadvantages.
Reactivity 1.3.4—Biofuels are produced from the biological fixation of carbon over a short period of time through photosynthesis.
Reactivity 1.3.5—A fuel cell can be used to convert chemical energy from a fuel directly to electrical energy.
Fuel: a source of chemical energy
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Any compound that releases energy when oxidized
Combustion
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Specific energy = energy released from the fuelmass of fuel consumed (kJ g-1)
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Energy density: the energy released from burning 1L of the fuel
C(s) + O2(g) → CO2 (g)
Mg (s) + O2 (g) → MgO (s)
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) | Oxygen acts as the oxidizing agent → reduced
The fuel acts as the reducing agent → oxidized |
Incomplete combustion
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When the supply of oxygen is limited
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Produces less heat energy than complete combustion
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Produces CO (g) and C (s) in addition to CO2 (g) and H2O (l)
2C(s) + O2(g) → 2CO(g)
C4H10(g) + 5O2(g) → 2CO2(g) + 5H2O(l) + CO(g) + C(s)
C2H5OH(l) + 2O2(g) → CO2(g) + C(s) + 3H2O(l) | More carbon atoms
→ more oxygen required for combustion
→ higher chance of incomplete combustion |
Fossil fuels
Fuel | Main component | Advantages | Disadvantages |
Coal | Carbon | 1. Large quantity
2. Longer lifespan than oil | 1. Global warming
2. Dangers of mining |
Oil | Alkanes | 1. Ease of transportation
2. Convenient for vehicles | 1. Global warming
2. Uneven distribution |
Natural gas | Methane | 1. Releases more energy per kg than coal or oil
2. Does not cause acid rain | 1. Global warming
2. Risk of explosion |
Biofuels
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Release energy that has been stored by carbon fixation (photosynthesis)
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Gasohol: gasoline + ethanol
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Advantages: renewable, readily available, less polluting
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Disadvantages: widely dispersed, take up land nutrients from soil
Direct combustion of plant material | C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) |
Fermentation | C6H12O6(s) → 2C2H5OH(l) + 2CO2(g) |
Anaerobic decay of organic matter | Products: mixture of CO2(g) + CH4(g) |
Photosynthesis
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6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)
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Endothermic: 2916 kJ mol-1 glucose
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Chlorophyll absorbs light via its extended system of alternating single and double bonds (conjugation)
Fuel cells
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Converts chemical energy directly to electrical energy
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Oxidation at anode(-), reduction at cathode(+)
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Unlike rechargeable batteries, do not need an external electricity source to recharge