2 Cracking

Economic reasons for cracking • The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions. • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked. • The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.) Cracking: conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds High Mr alkanes  smaller Mr alkanes+ alkenes + (hydrogen) This is a chemical process involving the splitting of strong covalent bonds so requires high temperatures. C. There are two main types of cracking: thermal and catalytic. They need different conditions and are used to produce different products produces mostly alkenes e.g. ethene used for making polymers and ethanol sometimes produces hydrogen used in the Haber Process and in margarine manufacture Thermal Cracking Conditions: High Pressure (7000 kPa) High Temperature (400°C to 900°C) Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission. C8H18  C6H14 + C2H4 C12H26  C10H22 + C2H4. d Conditions: Low pressure High Temperature (450°C) Zeolite Catalyst Catalytic Cracking Produces branched and cyclic alkanes and Aromatic hydrocarbons Used for making motor fuels Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number. Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used