Thermal cracking of alkanes usually

Step 1: Analyze the question We need the statement that correctly describes thermal cracking of alkanes.

Step 2: The concept Cracking is the breakdown of large hydrocarbon molecules into smaller, more useful ones. In thermal cracking, high temperature (about 450-750°C) and high pressure are used, often without a catalyst.

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)

Step 3: Check each option

• A. Exothermic process: Wrong. Cracking breaks strong C-C bonds, so it absorbs a lot of heat. It is endothermic.
• B. Produces only small alkanes: Wrong. It produces a mixture of smaller alkanes and alkenes (such as ethene and propene), and sometimes hydrogen.
• C. Involves decomposition: Correct. A large molecule is broken down by heat into smaller molecules. This is thermal decomposition.
• D. Requires hydrogen: Wrong. Hydrogen is needed in hydrocracking, not in ordinary thermal cracking. Thermal cracking needs only heat.

Step 4: Answer

Tip: Remember "cracking = breaking by heat". If you see "decomposition" or "breakdown" with cracking, it is usually the right choice. Also remember that cracking gives alkenes too, so any option saying "only alkanes" is a trap.