Cracking and Alkenes

Introduction

• Cracking is the process of breaking down long-chain hydrocarbons (alkanes) into shorter, more useful molecules.
• Alkanes have the general formula ( C_nH_{2n+2} ) and contain only single covalent bonds between carbon atoms.
• Demand for short-chain hydrocarbons is high due to their use as fuels._

Cracking

• Purpose: To convert long-chain hydrocarbons into shorter-chain hydrocarbons and alkenes.
• Types of Cracking:
  1. Catalytic Cracking
     • Uses high temperature and a catalyst.
     • Catalyst speeds up the reaction.
     • Specific catalyst name is not required for exams.
  2. Steam Cracking
     • Uses high temperature and steam.

Alkenes

• Alkenes are produced during cracking.
• They are characterized by a double covalent bond between two carbon atoms.
• Example: Ethene (( C_2H_4 )).
• Uses:
  • Production of polymers.
  • Starting material for other chemicals.
• Reactivity:
  • More reactive than alkanes.

Test for Alkenes

• Bromine Water Test:
  • Bromine water is orange.
  • Shaking bromine water with an alkene turns it colorless.
  • Important note: Do not say the bromine water turns "clear."

Balancing Cracking Equations

• Essential to balance the number of carbon and hydrogen atoms on both sides of the equation.
• Example:
  • Starting with 25 carbon and 52 hydrogen atoms.
  • Resulting in products with 20 carbon and 42 hydrogen atoms.
  • The other molecule must have 5 carbon and 10 hydrogen atoms.
• Practice problem:
  • Given: 40 carbon and 82 hydrogen on the left.
  • Known product: 7 carbon and 14 hydrogen.
  • First product should have 33 carbon and 68 hydrogen.

Additional Resources

• More questions and exercises can be found in the vision workbook available online.