Question

In a combustion process, the "Theoretical Adiabatic Flame Temperature" is the maximum temperature the products can reach. This temperature is lower in practice because

• A. real reactions are always endothermic
• B. part of the energy is consumed by the dissociation of product molecules
• C. the heat capacity of the products decreases at high temperatures
• D. the nitrogen in the air acts as a catalyst, absorbing energy

Hint:

At high temperatures, dissociation acts as a natural chemical buffer.
The heat absorbed by endothermic dissociation limits the maximum temperature in gas turbines, rockets, and internal combustion engines, keeping it below theoretical values.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks why the actual measured maximum flame temperature in real-world combustion is always significantly lower than the theoretically calculated adiabatic flame temperature.

Step 2: Detailed Explanation:

• Theoretical Adiabatic Flame Temperature Definition:
This is calculated by assuming complete combustion, zero heat loss to the surroundings, and that all released chemical energy goes entirely into raising the temperature of the products.

• The Phenomenon of Dissociation:
At extremely high temperatures (typically above $1600 \text{ K}$ to $2000 \text{ K}$), the main combustion products ($CO_2$ and $H_2O$) are no longer completely stable.
They undergo reversible, endothermic dissociation reactions into simpler molecules and radicals: \[ CO_2 \rightleftharpoons CO + \frac{1}{2} O_2 \quad (\Delta H > 0) \] \[ H_2O \rightleftharpoons H_2 + \frac{1}{2} O_2 \quad (\Delta H > 0) \] \[ H_2O \rightleftharpoons OH + \frac{1}{2} H_2 \quad (\Delta H > 0) \]
• Effect on Flame Temperature:
These chemical dissociation reactions are highly endothermic (they absorb heat).
Consequently, some of the thermal energy that would have raised the temperature of the gas is instead consumed to break the chemical bonds of the products.
This limits the peak temperature achievable by the flame.

• Evaluating other options:
- Option (A) is incorrect as combustion is highly exothermic.
- Option (C) is incorrect because specific heat capacities of gases actually *increase* with temperature, which also helps lower the temperature, but dissociation is the primary thermodynamic limit at peak temperatures.
- Option (D) is incorrect; nitrogen is inert in terms of catalysis, although it acts as a thermal diluent.

Step 3: Final Answer:
In practice, the flame temperature is lower because part of the energy is consumed by the dissociation of product molecules.