Catalytic Cracking

HKDSE Tip

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  Purpose:

  Break larger hydrocarbons into smaller ones by heating in the absence of air with a catalyst.
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  Cracking:

  Cracking is a chemical reaction because covalent bonds in the hydrocarbon are broken to form smaller molecules.
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  Absence of air:

  Air is excluded so paraffin vapour cracks instead of burning with oxygen, which would form carbon dioxide.
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  Catalyst:

  Broken unglazed porcelain or aluminium oxide is used. It allows cracking to occur at a lower temperature.
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  Heating:

  Do not heat the mineral wool directly. Heat the catalyst so paraffin vapour from the wool passes over it and cracks.
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  Collection:

  The gaseous product is collected by displacement of water in an inverted test tube.
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  First gas:

  Do not collect the first few cm³ of gas. It is mainly air already in the apparatus, not the cracking product.
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  Safety:

  Remove delivery tube before heating stops to prevent suck-back.