Which type of reaction is illustrated by the following equation? C12H26 → 5C 2H4 + C 2H6

Analyze the question: A large alkane, C12H26 (dodecane), breaks down into smaller molecules: ethene (C2H4, an alkene) and ethane (C2H6, an alkane). We need to name this type of reaction.

(Check the balance: 5 C2H4 + C2H6 gives C = 10 + 2 = 12 and H = 20 + 6 = 26, which matches C12H26. The equation is balanced.)

Concept: Large hydrocarbon molecules can be broken into smaller, more useful ones by heat (thermal cracking) or by heat with a catalyst (catalytic cracking). This is called cracking. It is how the petroleum industry makes useful products like ethene and petrol from heavy fractions such as kerosene and diesel.

Evaluating the options:

• A. Addition: Small unsaturated molecules combine with another reagent to give a single product. Here one molecule splits into several, so this is not addition.
• B. Cracking: One large alkane breaks into smaller alkanes and alkenes. This fits.
• C. Hydrogenation: This adds H₂ across a double bond. The reactant here has no double bond and no hydrogen is added.
• D. Polymerization: Small monomers join to form a large molecule. This is the opposite of what happens here.

Answer: The reaction breaks a long-chain alkane into smaller molecules, including an alkene, so it is cracking, which is option B.

Exam tip: Remember "Crack = Cut." If a big hydrocarbon goes in and smaller hydrocarbons (often including an alkene) come out, think cracking. If small molecules join into a big one, think polymerization.