Lecture Notes: Cracking and Alkenes

Introduction

• Focus on cracking and its conditions.
• Understanding alkenes and their test.
• Learning to balance chemical equations for cracking.

Hydrocarbons

• Crude oil is a mixture of hydrocarbons.
• Hydrocarbons called alkanes:
  • General formula: ( C_nH_{2n+2} )
  • Only single covalent bonds between carbon atoms.
• Long chain hydrocarbons:
  • Not very flammable.
  • Not good fuels._

Cracking

• Converts long chain hydrocarbons to shorter ones.
• Process:
  • Breaks down long chain alkanes into smaller, useful molecules.
  • Produces shorter chain alkanes used for fuels (e.g., in cars).
  • Also produces alkenes.

Conditions for Cracking

1. Catalytic Cracking
   • Involves high temperature and a catalyst.
   • Catalyst speeds up the reaction.
   • Specific catalyst name not required.
2. Steam Cracking
   • Uses high temperature and steam.

Alkenes

• Short chain hydrocarbons produced by cracking.
• Example: Ethene
• Characteristics:
  • Double covalent bond between two carbon atoms.
  • More reactive than alkanes.

Uses of Alkenes

• Make chemicals called polymers.
• Starting material for other useful chemicals.

Test for Alkenes

• Use bromine water (orange).
• Shake alkene with bromine water:
  • Bromine water turns colorless.
  • Important to say colorless, not clear.

Balancing Equations for Cracking

• Ensure equal carbon and hydrogen atoms on both sides.
• Example:
  • Left: 25 C, 52 H
  • Right: 20 C, 42 H
  • Missing molecule: 5 C, 10 H

Practice Problem

• Left: 40 C, 82 H
• Right: 7 C, 14 H
• First product: 33 C, 68 H

Additional Resources

• For more practice, refer to the exercises in the workbook linked above.