Cracking
Alkanes Worksheet
Cracking Fundamentals
1. State what cracking is and explain why it is necessary.
Larger hydrocarbons are broken down into shorter hydrocarbons. It is necessary because there is a higher demand for shorter chain hydrocarbons than there is supply.
2. Copy and complete the table below to compare the two methods of cracking hydrocarbons.
| Thermal Cracking | Catalytic Cracking | |
|---|---|---|
| Conditions | ||
| Main products | ||
| Main use of products |
| Thermal Cracking | Catalytic Cracking | |
|---|---|---|
| Conditions | 450 – 700°C 70 atmospheres |
500°C Moderate pressure Zeolite catalyst |
| Main products | Short alkenes, such as ethene | Branched chain alkanes, rings, aromatic compounds |
| Main use of products | Polymers | Motor fuel |
3. Thermal cracking is used to produce alkenes, such as ethene and propene.
a) State one use of ethene.
- Making poly(ethene)
- Making ethanol
b) State the type of bond breaking that occurs and what type of intermediate this forms.
Homolytic bond fission resulting in the formation of free radicals.
c) The hydrocarbon with 20 carbons is cracked, producing two molecules of ethene and one other molecule. Write the equation for this reaction.
C20H42 → 2C2H4 + C16H34
d) The hydrocarbon with 21 carbons is cracked, producing two molecules of propene, one molecule of ethene and one other molecule.
C21H44 → C2H4 + 2C3H6 + C13H28
4. Catalytic Cracking is used to produce shorter molecules, such as 2,2-dimethylhexane.
a) 2,2-dimethylhexane is an isomer of octane. State which of these two molecules has a higher boiling point and explain why that is the case.
- Octane has a higher boiling point.
- Both molecules are non-polar, so intermolecular forces are temporary dipole-dipole interactions (Van Der Waals forces).
- They have the same molecular formula, so the same number of electrons.
- However, Octane is a straight-chain molecule, whereas 2,2-dimethylhexane is branched.
- Straight chains pack together more closely, resulting in a greater surface area of contact.
- This leads to stronger Van Der Waals forces and a higher boiling point.
b) State the name of the catalyst used in catalytic cracking.
Zeolite
c) State the type of bond breaking that occurs and what type of intermediate this forms.
Heterolytic bond breaking, resulting in the formation of carbocations.
d) The hydrocarbon with 36 hydrogen atoms is cracked, producing 2,2-dimethylhexane as well as propene. Write the equation for the reaction.
2C17H36 → C8H18 + 3C3H6