Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.2.2Physical Chemistry

Modification of Alkanes by Cracking — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 70 of 102

📚 AQA AS Chemistry Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read

Part 1 of 5Why Crack?

Cracking breaks long-chain hydrocarbons into shorter, more useful molecules.

Supply and demand
  • Supply and demand Crude oil contains too many long chains and not enough short ones, which are in greater demand as fuels.
Alkenes
  • Alkenes Cracking produces alkenes, which are reactive and are the starting materials for polymers and many other chemicals.
Bond breaking
  • Bond breaking Carbon–carbon bonds in large molecules are broken, so the products have shorter chains.
Hydrogen balance
  • Hydrogen balance A cracking reaction gives at least one alkene along with a shorter alkane, so the total atoms are conserved.

Part 2 of 5Thermal and Catalytic Cracking

Thermal and Catalytic Cracking
Thermal cracking
  • Thermal cracking Uses high temperature and pressure without a catalyst to produce a high proportion of alkenes.
Catalytic cracking
  • Catalytic cracking Uses a zeolite catalyst at lower pressure, saving energy and producing motor-fuel-quality molecules.
Zeolite
  • Zeolite A crystalline aluminosilicate with a porous structure that provides a large surface for the reaction.
Fuel quality
  • Fuel quality Branched and aromatic products burn more smoothly in engines than straight-chain alkanes of the same size.
TypeConditionsMain products
ThermalHigh pressure (up to about 7000 kPa) and temperature (about 700 to 1200 K)Mainly alkenes, sometimes with alkanes
CatalyticSlight pressure, temperature about 720 K, zeolite catalystBranched and cyclic alkanes and aromatic hydrocarbons, used as motor fuels

Part 3 of 5Writing Cracking Equations

1
Count the carbon and hydrogen atoms in the large alkane, which must equal the totals in the products.
2
Choose a simple alkene, such as ethene (C₂H₄) or propene (C₃H₆), as one of the products.
3
Subtract the alkene from the large alkane to find the formula of the remaining molecule.
4
Check that the remaining molecule fits CₙH₂ₙ₊₂ for an alkane, or CₙH₂ₙ for another alkene.
5
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ shows decane cracking into octane and ethene.

Part 4 of 5Worked Example

Question

  • Dodecane, C₁₂H₂₆, is cracked to give octane, C₈H₁₈, and ethene only.
  • Write a balanced equation.

  1. Dodecane has 12 carbons and 26 hydrogens.
  2. Octane accounts for 8 carbons and 18 hydrogens, leaving 4 carbons and 8 hydrogens.
  3. Ethene is C₂H₄, so two molecules of ethene account for C₄H₈.

Answer C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄

Part 5 of 5Exam Checklist & Tips

3.2.2 – Key Points for the Exam

  • Cracking The breaking of long-chain hydrocarbons into shorter-chain alkanes and alkenes.
  • Thermal cracking Cracking at high temperature and pressure that gives mainly alkenes.
  • Catalytic cracking Cracking using a zeolite catalyst at lower pressure, giving branched, cyclic and aromatic hydrocarbons.
  • Zeolite A porous aluminosilicate used as a catalyst for cracking.

Common Mistakes

  • Writing a cracking equation that does not balance because carbon or hydrogen atoms are missing.
  • Stating that cracking always produces only alkanes.
  • Confusing the conditionsthermal needs high pressure, catalytic needs only slight pressure.
  • Describing cracking as a physical process when it is a chemical reaction.

Last updated:

\n