The initial products of the fractional distillation of oil often undergo cracking. This can be carried out in a number of ways. State the major reason for choosing each of the following techniques.

Catalytic cracking:

Thermal cracking:

Steam cracking:

Explain how these differ from homogeneous catalysts.

Identify one disadvantage of using heterogeneous catalysts.

Many of the compounds produced by cracking are used in the manufacture of addition polymers. State the essential structural feature of these compounds and explain its importance.

The polymers often have other substances added to modify their properties. One group of additives are plasticizers. State how plasticizers modify the physical properties of polyvinyl chloride and explain at the molecular level how this is achieved.

Catalytic cracking:

used to produce moderate length alkanes (for fuels) / lower temperature / lower energy consumption / more control of product;

Thermal cracking:

used to crack very long chain starting material;

Steam cracking:

used to produce low molar mass alkenes (for petrochemicals);

heterogeneous catalysts in a different phase to the reactants / homogeneous catalysts in the same phase as reactants;

easily poisoned / efficiency decreases over time / forms clumps / only effective on surface / require high surface area;

carbon-carbon double bond;

breaks allowing addition reaction / allows monomers/molecules to join together/polymerize;

make the polymer more flexible;

fits between/increases separation between polymer chains / allow polymer chains to slide past each other more easily / weaken intermolecular attraction;

Few candidates appear to have any knowledge of the different cracking techniques, though more appeared familiar with issues relating to catalysts. Quite a number of candidates were aware carbon-carbon double bonds were needed for addition polymerization, though the nature and effect of plasticizers was less well known

Few candidates appear to have any knowledge of the different cracking techniques, though more appeared familiar with issues relating to catalysts. Quite a number of candidates were aware carbon-carbon double bonds were needed for addition polymerization, though the nature and effect of plasticizers was less well known

Few candidates appear to have any knowledge of the different cracking techniques, though more appeared familiar with issues relating to catalysts. Quite a number of candidates were aware carbon-carbon double bonds were needed for addition polymerization, though the nature and effect of plasticizers was less well known

Few candidates appear to have any knowledge of the different cracking techniques, though more appeared familiar with issues relating to catalysts. Quite a number of candidates were aware carbon-carbon double bonds were needed for addition polymerization, though the nature and effect of plasticizers was less well known

Few candidates appear to have any knowledge of the different cracking techniques, though more appeared familiar with issues relating to catalysts. Quite a number of candidates were aware carbon-carbon double bonds were needed for addition polymerization, though the nature and effect of plasticizers was less well known