Lessons 22–23 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
22
Specific heat of combustion of fuel
Textbook: pp. 65–66
GoalKnow the specific heat of combustion q and use Q = qm; explain why heat is released in burning.
When fuels (wood, coal, natural gas, petrol) burn, heat is released. The reason: energy is needed to split molecules into atoms, and when atoms combine into new molecules energy is released. When the carbon of a fuel combines with oxygen of the air to form CO₂, more energy is released than was spent, so the process gives heat. The heat released when 1 kg of fuel burns completely is the specific heat of combustion q, in J/kg. For a fuel of mass m burning completely Q = q·m. Values from the book: petrol 46 MJ/kg, kerosene 42, natural gas 44, coal 29, alcohol 29, dry wood 10 MJ/kg. Burning fuel is dangerous: do it only under adult supervision in a ventilated place; the combustion products (CO₂, CO) affect the atmosphere.
Worked examples
Burning 2 kg of coal (q = 29 MJ/kg): Q = qm = 29·2 = 58 MJ = 5.8·10⁷ J.
How much natural gas (q = 44 MJ/kg) gives the same heat as 11 kg of dry wood (q = 10 MJ/kg)? q₁m₁ = q₂m₂, m₂ = 10·11/44 = 2.5 kg: 4.4 times less gas than wood is needed.
Class activity
Table game with fuel cards (petrol, gas, coal, wood) and masses; groups compute Q and rank the fuels by heat produced. Calculation only; no fuel is burned.
Practice
1
What is the specific heat of combustion and what is its unit?
The heat released when 1 kg of fuel burns completely, J/kg.
2
How many MJ of heat are released when 3 kg of petrol (q = 46 MJ/kg) burns?
138
3
How many kg of kerosene (q = 42 MJ/kg) must burn to release 210 MJ?
5
4
What does “q of natural gas = 44 MJ/kg” mean? Why is heat released in burning?
Burning 1 kg of gas completely releases 44 MJ; forming the new molecules (CO₂, H₂O) releases more energy than was spent.