4.1.2 Alkanes Flashcards
What is an alkane?
- A saturated hydrocarbon containing C-H bonds only
What is the general formula of an alkane?
- CnH₂n+₂
Are alkane bonds polar? Why/why not?
- Nonpolar because carbon and hydrogen have similar electronegativities
What is the shape and angle of an alkane?
- Tetrahedral
- 109.5°
Describe the sigma (σ) bond in alkanes.
- The sigma bond is a covalent bond formed by the direct overlap of the electron clouds of the bonding atoms.
What type of intermolecular forces do alkanes have? Why?
- London forces (induced dipole-dipole interactions)
- Because the bonds are nonpolar
What happens to the boiling point as alkane chain length increases? Why?
- The boiling point increases because there is more surface area and more induced dipole-dipole interactions
- Therefore, more energy is required to overcome the attraction
Does a branched molecule have lower or higher boiling point compared to an equivalent straight chain? Why?
- A branched molecule has a lower boiling point because:
- It has less surface area
- Therefore, fewer induced dipole-dipole interactions
Are alkanes soluble in water? Why?
- Insoluble because hydrogen bonds in water are stronger than the London forces of attraction in alkanes
How reactive are alkanes?
- Very unreactive
What reactions will alkanes undergo?
- Combustion
- Reaction with halogens
What type of reaction is combustion?
- Oxidation reaction
What is complete combustion?
- Combustion that occurs with a plentiful supply of air
What are the products of complete combustion when alkanes are used?
- Carbon dioxide
- Water
What is the colour of the bunsen burner flame during complete combustion?
- Blue flame
What is incomplete combustion and what products are formed in the case of alkanes?
- Combustion in a limited supply of oxygen
- Products: water, carbon dioxide, carbon monoxide
Write an equation for the complete combustion of propane.
- C₃H₈ + 5O₂ -> 3CO₂ + 4H₂O
Which type of hydrocarbons are most likely to undergo incomplete combustion?
- Longer chains
What is the environmental impact of carbon monoxide?
- Toxic/poisonous
What is the environmental impact of soot (carbon)?
- Asthma
- Cancer
- Global dimming
How are halogenoalkanes formed from alkanes?
- Radical substitution
In the presence of what does alkane react with halogens?
- UV light
What are the three stages of free radical substitution?
- Initiation - breaking halogen bond to form free radicals
- Propagation - chain reaction where products form but free radical remains
- Termination - free radicals combine to form stable products
Write equations for the reaction of CH₄ with Cl₂ to form CH₃Cl.
- Initiation: Cl₂ -> 2Cl· (in presence of UV light)
-
Propagation: Cl· + CH₄ -> HCl + ·CH₃
·CH₃ + Cl₂ -> CH₃Cl + Cl· -
Termination:
·CH₃ + Cl· -> CH₃Cl
2Cl· -> Cl₂
·CH₃ + ·CH₃ -> C₂H₆
