Chapter 12 Flashcards
What type of covalent bond are between the atoms in alkanes?
- Sigma bonds (σ-bonds)
What are σ-bonds?
- The result of a head-on overlap of 2 orbitals (either s or p), directly between the bonding atoms
(- Each orbital contains 1 electron, so the σ-bond contains 2 shared electrons)
How do the boiling points of alkanes change?
- Their boiling points increase as the chain length increases
Why do the boiling points of alkanes increase as their chain length increases?
- Larger molecules have a larger surface area, and therefore form more contact points with other molecules
- As a result, there is more surface contact between longer chain alkanes
- The London forces between them are therefore stronger (as there are more of them), and more energy is required to overcome them
- They then have a higher boiling point
How and why does branching affect the boiling point of alkanes?
- It decreases the boiling point
- This is because there are fewer surface points of contact between branched molecules (they cannot be packed as closely together as straight chain alkanes)
- There are therefore fewer London forces
- Less energy is needed to overcome them
- They have a lower boiling point
Why are alkanes very unreactive?
- C-C and C-H σ-bonds have high bond enthalpy (are strong), and so are difficult to break
- C-C bonds are non-polar (they don’t have a dipole, so are less likely to react)
- C and H are similar enough to each other in terms of electronegativity that C-H bonds are also non-polar
What are the 2 types of combustion, and when does each occur?
- Complete combustion: when there is a plentiful supply of oxygen
- Incomplete combustion: in a limited supply of oxygen
List the reactants and products in the complete combustion of alkanes.
- Alkane and oxygen
- Carbon dioxide and water
List the reactants and products in the incomplete combustion of alkanes, and explain why they are formed.
- Alkane and oxygen
- Water is always produced as the hydrogen atoms are always oxidised
- The combustion of the carbon is incomplete, so either carbon monoxide or carbon (as soot) are produced
Why is CO dangerous?
- Carbon monoxide is colourless, odourless and highly toxic
When do alkanes react with halogens, and why?
- In the presence of UV radiation, as it provides the initial energy to start the reaction
What is the reaction of a halogen with an alkane an example of?
- Radical substitution
How many steps are there in radical substitution, and what are their names?
- 3
- Initiation
- Propagation
- Termination
In the example of methane reacting with bromine, what happens in the initiation stage?
- Br-Br -UV-> ·Br + ·Br
(- Homolytic fission)
What happens in the propagation stage of methane’s reaction with bromine?
- ·Br + CH4 -> ·CH3 + HBr
- ·CH3 + Br2 -> CH3Br + ·Br
