Alkenes | Chemistry for Grade 11 (IGCSE) PDF Download

Catalytic Cracking

• Alkenes are a type of unsaturated hydrocarbons characterized by the presence of carbon-carbon double bonds (C=C).
• Their general formula is C_nH_2n, where "n" represents the number of carbon atoms in the molecule.
• Due to the presence of the double bond (C=C), alkenes can engage in additional bonding with other atoms. This occurs by breaking the double bond and allowing incoming atoms to form single bonds with each carbon atom of the functional group.
• Consequently, each carbon atom in the functional group forms four single bonds instead of one double bond and two single bonds.
• This increased bonding capability makes alkenes significantly more reactive compared to alkanes.

Manufacture of Alkenes

• Although each fraction obtained from the fractional distillation of crude oil serves a purpose, there's an excess production of longer-chain hydrocarbons.
• These longer-chain hydrocarbon molecules undergo further processing to create other products.
• Catalytic cracking is a method employed to convert longer-chain molecules into shorter-chain and more practical hydrocarbons.
• Longer-chain alkanes are cracked to produce shorter-chain alkanes, alkenes, and hydrogen.
• Alkenes find utility in polymer production, while hydrogen is used in ammonia synthesis.
• Kerosene and diesel oil are commonly subjected to cracking to yield petrol, additional alkenes, and hydrogen.
• Cracking entails heating hydrocarbon molecules to temperatures ranging from 600 to 700°C to vaporize them.
• The vapors then traverse over a hot powdered catalyst, typically alumina or silica.
• Through this process, covalent bonds within the molecules are disrupted upon contact with the catalyst surface, leading to thermal decomposition reactions.
• The breaking of molecules occurs randomly, generating a mixture of smaller alkanes and alkenes.
• Higher temperatures and pressures yield a higher proportion of hydrogen and alkenes.

Distinguishing from Alkanes

Distinguishing Between Alkanes & Alkenes

• Alkanes and alkenes possess distinct molecular configurations.
• Alkanes are invariably saturated, while alkenes are unsaturated due to the presence of the C=C double bond.
• The existence of the C=C double bond enables alkenes to engage in reactions that are inaccessible to alkanes.
• A straightforward chemical examination utilizing bromine water facilitates the differentiation between alkanes and alkenes.
  
• Bromine water comprises an orange-tinted bromine solution.
• Interaction of bromine water with an alkane will sustain its orange hue since alkanes lack double carbon bonds (C=C), thereby retaining the bromine in solution.
• In contrast, when bromine water interacts with an alkene, it undergoes decolorization, turning colorless. This is because alkenes possess double carbon bonds (C=C), enabling bromine atoms to add across the C=C double bond, thus eliminating the orange coloration from the solution.
• This chemical process, where bromine adds across the double bond of alkenes, is termed an addition reaction.