Q.What is pyrolysis (cracking)? Distinguish thermal cracking from catalytic cracking and explain why cracking is important in the petroleum industry.

Pyrolysis, called cracking in the petroleum industry, is the thermal decomposition of a higher alkane into a mixture of smaller alkanes and alkenes, achieved by heating it strongly (typically $400$–$700\,°\text{C}$) in the absence of air so that combustion does not compete. In thermal cracking, only heat and high pressure are used, and the carbon-carbon bonds break homolytically by a free-radical mechanism, giving a relatively unselective mixture that is comparatively rich in alkenes (especially ethene and propene). In catalytic cracking, an acidic catalyst -- typically a zeolite -- is used at a lower temperature, and the mechanism instead proceeds through carbocation intermediates, which favour skeletal rearrangement to more branched products; catalytic cracking is therefore preferred industrially for producing motor fuel, because branched alkanes burn more smoothly (higher octane rating, less 'knocking') than straight-chain ones. Cracking matters enormously in petroleum refining because the fractional distillation of crude oil produces far more of the heavier, high-boiling fractions (diesel, kerosene, heavy gas oil) than there is market demand for, while demand for petrol and light alkenes (ethene, propene, the building …

Define cracking by its conditions (heat, no air) and general outcome (bond-breaking into smaller fragments), then contrast the two named variants by mechanism (radical vs carbocation) an …

Confusing cracking with combustion (cracking needs the ABSENCE of air/oxygen, unlike burning); thinking thermal and catalytic cracking give identical products, when in fact …