Combustion of hydrocarbons (RP-1 - kerosene, methane) and hydrogen

5.3.6 · Chemistry › Combustion Chemistry (Propulsion Bridge)

Subtopic of Combustion Chemistry (Propulsion Bridge) — rocket fuels kaise jalte hain, kya produce karte hain, aur kyun kuch fuels zyada "kick" dete hain.

The Big Picture (WHY this matters)

1. The General Combustion Reaction

$(x + \frac{y}{4})$ coefficient derive kaise karte hain? Atoms ko ek element at a time balance karo.

Carbon: left side par $x$ carbons hain → $C O_{2}$ ke $x$ molecules chahiye. ✓ (Kyun? Har C ko kahin jaana hai; sirf $C O_{2}$ mein C hota hai.)
Hydrogen: $y$ hydrogens → $\frac{y}{2}$ molecules of $H_{2} O$ chahiye (har water mein 2 H hote hain). ✓
Oxygen: right side par O kitna chahiye count karo: $from C O_{2} 2 x + from H_{2} O \frac{y}{2}$ atoms $= 2 x + \frac{y}{2}$ O atoms. Har $O_{2}$ 2 atoms supply karta hai, toh $O_{2}$ ki sankhya $= \frac{2 x + y /2}{2} = x + \frac{y}{4}$ . ✓

Recall Aage padhne se pehle quick check karo

Propane $C_{3} H_{8}$ ke liye, kitne $O_{2}$ molecules honge? $x = 3, y = 8 \Rightarrow 3 + 8/4 = 5 O_{2}$ . Products: $3 C O_{2} + 4 H_{2} O$ .

2. The Three Propulsion Fuels

Figure — Combustion of hydrocarbons (RP-1 - kerosene, methane) and hydrogen

3. Energy Released — Enthalpy of Combustion

Yeh formula kyun kaam karta hai (Hess's law): Enthalpy ek state function hai — yeh sirf start aur end states par depend karta hai, path par nahi. Toh hum imagine kar sakte hain ki har reactant ko elements mein tod diya (reactants ka $- Δ H_{f}$ lagega) aur elements se products banaye (products ka $Δ H_{f}$ milega). Jodne par boxed formula milta hai.

4. Stoichiometric Oxidizer-to-Fuel Ratio (O/F)

Worked: methane/oxygen. $n_{O_{2}} = 2$ , $M_{O_{2}} = 32$ ; $n_{f u e l} = 1$ , $M_{C H_{4}} = 16$ . $O/F = \frac{2 \times 32}{1 \times 16} = \frac{64}{16} = 4.0$ Yeh step kyun? $C H_{4} + 2 O_{2}$ se mole ratio seedha coefficients se padh lo; moles ko mass mein convert karo molar masses ke saath.

5. Common Mistakes (Steel-man + Fix)

6. Active-Recall Flashcards

Methane combustion ki balanced equation

C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O

Hydrogen combustion ki balanced equation

2 H_{2} + O_{2} \to 2 H_{2} O

RP-1 ko kaunse molecule se model kiya jaata hai?

Dodecane

C_{12} H_{26}

General

C_{x} H_{y}

ke liye

O_{2}

coefficient

x + y /4

H₂/O₂ sabse high exhaust velocity kyun deta hai?

Kam exhaust molar mass

M

, aur

v_{e} \propto T_{c} / M

Δ H_{c}

ke liye Hess's-law formula

\sum Δ H_{f} (products) - \sum Δ H_{f} (reactants)

Methane ka

Δ H_{c}

(gas water)?

lagbhag

- 802

kJ/mol

Incomplete (fuel-rich) combustion ke products

C O

, soot (C), unburnt

H_{2}

CH₄/O₂ ke liye stoichiometric mass O/F

4.0

Rockets mein gaseous water ka

Δ H_{f}

kyun use karte hain?

Exhaust hot hota hai, water vapor mein rehti hai

H₂ se kam

v_{e}

ke baavajood RP-1 kyun choose karte hain?

High density → chhote, halke tanks

Recall Feynman: ek 12-saal ke bacche ko samjhao

Ek rocket aise hai jaise ek balloon jise tum chhoddo — haawa peeche se nikalti hai aur use aage push karti hai. Lekin haawa ki jagah, ek rocket super hot gas banata hai fuel jaala ke. Jaalaane ka matlab hai fuel oxygen pakad leta hai aur naye cheez ban jaate hain: methane aur kerosene carbon dioxide aur steam mein badal jaate hain; hydrogen sirf steam (paani!) mein badal jaata hai. Jaalne se bahut ZYADA heat nikalta hai, jo gas ko peeche se bahut fast push karta hai. Halki gas (jaise hydrogen ki steam) sabse fast uda jaati hai, isliye hydrogen rockets sabse speedy hote hain — lekin hydrogen itna "fula hua" hota hai ki tumhe giant tank chahiye, toh kabhi kabhi heavy, zyada squished-together kerosene zyada samajhdaari wali choice hoti hai.

Connections

Rocket Equation (Tsiolkovsky) — $v_{e}$ feeds $Δ v = v_{e} ln (m_{0} / m_{f})$
Hess's Law and Enthalpy of Formation — $Δ H_{c}$ ki basis
Specific Impulse $I_{sp}$ — $T_{c} / M$ se driven performance metric
Stoichiometry and Limiting Reagents — balancing & O/F ratio
Cryogenic Propellants — liquid $H_{2}$ , liquid $O_{2}$ storage
Incomplete Combustion and Soot Formation — fuel-rich operation