Thermochemistry of cracking In general refinery practice, the cracking of large hydrocarbons into smaller molecules is thermally what kind of reaction?

Introduction / Context:
Cracking breaks carbon–carbon bonds to form smaller hydrocarbons (LPG, gasoline-range, etc.). Knowing whether cracking absorbs or releases heat guides reactor design, heat balance, and furnace duty planning.

Given Data / Assumptions:

• Typical thermal/catalytic cracking conditions are considered.
• No unusual oxidative or hydrogenolysis side reactions are implied.
• Focus is on net thermal effect of cracking itself.

Concept / Approach:
Breaking C–C bonds requires energy input; overall, cracking is endothermic. Industrial units supply heat via furnace coils (thermal) or maintain high reactor temperature with catalyst/regenerator heat balance (e.g., FCC uses hot regenerated catalyst to supply endothermic reaction heat).

Step-by-Step Solution:

Verification / Alternative check:
FCC heat balance explicitly shows regenerator exotherm (coke burn) supplying riser endotherm; thermal crackers rely on fired heaters.

Why Other Options Are Wrong:

• Exothermic: Opposite of the core reaction’s heat effect.
• Favoured at very low temperature: Cracking requires high temperature to proceed at practical rates.
• None of these: Incorrect because one option is correct.

Common Pitfalls:
Confusing overall unit heat balance (which includes exothermic coke burn) with the intrinsic endothermicity of cracking.

Final Answer:
An endothermic reaction