Alkanes Flashcards
1
Q
what are alkanes
A
saturated hydrocarbons containing single, sigma C-C and C-H bonds
2
Q
why are alkanes not very reactive
A
- high bond enthalpy
- low polarity of the sigma bonds
3
Q
what are sigma,σ, bonds
A
direct overlap of s orbitals
4
Q
angle around C atom in alkanes and explanation
A
- 109.5 degrees
- 4 bonding pairs of e- around C repel equally to give a tetrahedral shape
5
Q
how does chain length affect boiling point
A
- longer chain length increases London forces between molecules as theres more surface contact and more electrons to interact
6
Q
why does branching decrease boiling point of alkanes
A
- branched alkanes can’t pack as clodely together as straight chains
- More branching gives less (surface) contact
- fewer/weaker London forces
7
Q
why are alkanes used as fuels
A
- readily undergo combustion
- release a lot of energy due to very exothermic combustion enthalpy changes
8
Q
when does complete combustion occur
A
- when there is sufficient oxygen
- burns with a clean, often blue flame
9
Q
products of complete combustion
A
- CO2 and H2O
- both greenhouse gases known to contribute towards global warming
10
Q
when does incomplete combustion occur
A
when there is limited supply of oxygen and burns with a dirty yellow flame
11
Q
products of incomplete combustion
A
- CO and H2O
12
Q
dangers of CO
A
- colourless, odourless, highly toxic gas
- combines with Hb, blocking transport of O2 to cells
13
Q
what do alkanes need to undergo free radical substitution
A
- halogen
- UV light
14
Q
what does free radical substitution form
A
haloalkanes
15
Q
what are the three steps in free radical substitution
A
- initiation
- propagation
- termination ( multiple reactions can occur)
