BTEC Applied Science Unit 3: Energy Content of Fuels
What is a Fuel?
- Definition: A fuel is a substance that releases heat energy when burned.
- Combustion: Chemical potential energy is converted into heat energy.
Units of Energy
- Energy is measured in joules (J).
- 1,000 joules = 1 kilojoule (kJ).
- Kilocalories (kcal): 1 kcal ≈ 4,184 joules or 4.2 kJ.
- Food labels use "calories" to indicate kilocalories.
Fuels in the Specification
- Examples: Petrol, paraffin, food, cooking oil, methanol, ethanol, propanol, butanol, pentanol, and wax.
Experimentation with Fuels
- Liquid Fuels: Burned in a spirit burner.
- Weigh the spirit burner before and after burning to find the mass of fuel burned.
- Solid Fuels: Often involves burning to measure energy content (e.g., burning food under a boiling tube).
- Measure mass before and after burning and the temperature rise.
Hazards of Burning Fuels
- Flammable: Risk of fire.
- Toxicity: Risk of poisoning.
- Explosion Risk: Particularly with gases.
- Incomplete Combustion: Produces dangerous substances like carbon monoxide.
- Pollution: Soot, carbon dioxide (greenhouse gas), sulfur oxides causing acid rain.
Measuring Energy Content
- Method: Compare energy content by measuring heat energy transferred to water.
- Equation: E = mcΔT
- E: Energy in joules.
- m: Mass of water (kg or g).
- c: Specific heat capacity (4,200 J/kg°C or 4.2 J/g°C).
- ΔT: Temperature change (°C).
Example Problem
- 100 ml water rises from 20°C to 34°C.
- Mass: 0.1 kg (100 g)
- ΔT: 14°C
- Energy: 5,880 joules (5.88 kJ)
Calculating Energy per Mole
- Determine energy released per gram and per mole.
- Ethanol:
- 1 mole = 46 grams.
- Example: 1.2 grams of ethanol releases 5.88 kJ.
- Energy per gram: 5.88 kJ / 1.2 g = 4.9 kJ/g.
- Energy per mole: 4.9 kJ/g * 46 g = 225 kJ/mol.*
Discrepancies in Energy Values
- Experimental values often lower than reference values (e.g., ethanol: 225 kJ/mol vs. 1,367 kJ/mol).
- Reasons for discrepancies should be considered.
Pattern in Fuels
- Different fuels have varying numbers of carbon atoms.
- The energy released is related to the structure of the molecules.
Use these notes to review the key concepts of measuring and comparing the energy content of various fuels, understanding the hazards, and performing relevant calculations.