A-Level Chemistry OCR Notes

Google rating

4.8

Trustpilot rating

4.7

View All Resources

Alkanes

Alkanes are a homologous series made up of saturated hydrocarbons, containing only carbon and hydrogen atoms joined by sigma bonds
Carbon atoms have four electrons in their outer shell. Therefore, each carbon atom can form four covalent bonds, resulting in tetrahedral geometry
Alkanes are not polar and only weak London forces of attraction occur, as carbon and hydrogen have similar electronegativities.
Boiling point of alkanes increases with chain length, as there is a greater surface area and number of electrons for stronger London forces.
Shorter carbon chain alkanes and more branched alkanes have a smaller surface area for contact between molecules and weaker London forces, resulting in lower boiling points
C-C and C-H σ bonds are strong and non-polar therefore alkanes are not reactive. The electronegativities of C and H are so similar that the C-H bonds are considered non-polar

Combustion of Alkanes

Alkanes can be used as a fuel source
In complete combustion, the alkane burns with a clean blue flame. Water vapour and carbon dioxide are formed (greenhouse gases). E.g. C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)
In incomplete combustion, the alkane burns with a dirty yellow flame. It can produce carbon, carbon monoxide and unburned hydrocarbons as products.
CH4 (g) + O2 (g) → C (s) + 2H2O (g)
CH4 (g) + 1.5O2 (g) → CO (g) + 2H2O (g)

Chlorination of Alkanes

Alkanes react with the halogens, specifically chlorine and bromine, in the presence of UV light to form haloalkanes
Methane reacts with chlorine to form chloromethane and hydrogen chloride:
- CH4 + Cl2 → CH3Cl + HCl
This reaction is a free radical substitution with the steps:
- Initiation- free radicals are formed when exposed to UV
  - Cl2 → 2Cl•
- Propagation- free radicals are used up and created in a chain reaction
  - Cl• + CH4 → •CH3 + HCl
  - •CH3 + Cl2 → CH3Cl + Cl•
Termination- free radicals are removed.
- 2Cl• → Cl2
- 2•CH3 → C2H6
- •CH3 + Cl• → CH3Cl
Radical substitution reactions have limitations:
- Further substitution could occur if another chloride radical collides with the product of propagation
- If the carbon chain is longer, a mixture of products forms as substitution occurs at different positions along the chain

Google rating

4.8

Trustpilot rating

4.7