3.2 Alkanes
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General formula of alkanes
CₙH₂ₙ₊₂
Shape around C atoms in alkanes
Tetrahedral (all have 4 bond pairs)
General formula of cyclic alkanes
CₙH₂ₙ
Are alkanes polar?
No, they contain no polar bonds
Structure + bonding of alkanes
• Simple molecular • van der Waals’ forces between molecules
Are alkanes soluble in water?
No. Hydrogen bonds between water molecules are much stronger than the van der Waals’ forces in alkanes
Effect of increasing chain length on boiling point of alkanes
As chain length increases b.p increases.
Why does b.p of alkanes increase with chain length?
• More electrons in molecules • So stronger van der Waals’ forces • More energy needed to overcome these
Effect of increasing number of branches on boiling point of alkanes
As number of branches increases, b.p decreses
Why does b.p of alkanes decrease with increasing number of branches?
• Fewer points of contact between molecules • Weaker van der Waals’ forces • Less energy needed to overcome these
Two types of cracking
Catalytic and thermal
Conditions for thermal cracking
• very high temperature • very high pressure
Products of thermal cracking
Alkanes and a high percentage of alkenes
Uses of the products of thermal cracking
Making polymers
Conditions for catalytic cracking
• High temperature • Slight pressure • Zeolite catalyst
Products of catalytic cracking
Cycloalkanes, branched alkanes, and aromatic hydrocarbons
Uses of the products of catalytic cracking
Motor fuels
Products of complete combustion of hydrocarbons
Carbon dioxide and water
Products of incomplete combustion of hydrocarbons
Carbon monoxide and water
When does incomplete combustion occur?
In a limited supply of oxygen
Products of further incomplete combustion
Solid carbon and water
When does further incomplete combustion occur?
In a very limited supply of oxygen
Effect of unburned hydrocarbons
Forms ozone in the lower atmosphere, which can cause respiratory problems
Ozone
O₃