Calorimetry — Measuring Heat from Fuels

Calorimetry is the science of measuring the heat released or absorbed during a chemical or physical process.

In the context of fuels: it measures how much heat energy a fuel produces when it burns completely.

Calorific value = heat energy released by complete combustion of 1 kg of a fuel.

Unit: kJ/kg (kilojoules per kilogram) or kJ/g

LPG > Petrol > Kerosene > Coal > Charcoal > Wood

1. A known mass of fuel burns in a sealed chamber (bomb calorimeter)
2. Heat released warms a known mass of water surrounding the chamber
3. Temperature rise of water is measured
4. Heat energy = mass of water × specific heat capacity × temperature rise

   Q = mcΔT (where c for water = 4.2 J/g°C)

A good fuel should have:

1. High calorific value — more energy per kg
2. Low ignition temperature — easy to start
3. No harmful products — safe combustion
4. Easy to store and transport — practical
5. Low cost — affordable
6. Smokeless — environmentally friendly

Complete combustion of a hydrocarbon:

Hydrocarbon + O₂ → CO₂ + H₂O + heat energy

Incomplete combustion (insufficient O₂):

Hydrocarbon + O₂ → CO + C (soot) + H₂O (less heat, more pollution)

Not all heat from combustion does useful work. Efficiency:

η = (useful heat output / total heat released) × 100%

Modern gas stoves are ~65% efficient; traditional chulhas are ~10–20% efficient.

• Calorific value = kJ/kg; higher = better fuel
• LPG has highest practical calorific value among common fuels
• Hydrogen has highest calorific value but is impractical due to storage
• Q = mcΔT is the fundamental calorimetry equation
• Complete vs incomplete combustion: CO₂/H₂O vs CO/soot

1. What does "calorific value" mean?
2. Arrange in order: wood, LPG, coal, kerosene (lowest to highest CV).
3. Why is hydrogen not commonly used as a fuel despite its high calorific value?
4. A fuel releases 50 kJ of heat when 1 g burns. What is its calorific value in kJ/kg?
5. Stretch: Why does incomplete combustion produce less heat than complete combustion?