4.10 Delocalisation

Question 1

What is delocalisation?

Answer 1

The sharing of a pair of electrons between three or more atoms.

Question 2

What is important about the structure of O3?

Answer 2

The Lewis structure of 03 suggests one short O=O bond and one longer O-O bond but actually the bond lengths are equal, this can be explained by delocalisation. Instead of the two electrons in the pi bond being localised between two atoms, they are delocalised over the three O atoms. Since the pi bond in ozone is shared between two single bonds rather than just one we conceder that each bond has a half share of it and we talk about the O-O bond order being 1.5

Question 3

What is the bond order of a single bond?

Answer 3

1

Question 4

What is the bond order of a double bond?

Answer 4

2

Question 5

What is the bond order of a triple bond?

Answer 5

3

Question 6

How do you know whether a molecule or ion will have resonance structures?

Answer 6

Whether it is possible to draw exactly equivalent structures where the only difference between them is the position of the double bonds and lone pairs if appropriate.

Question 7

How is the surface of the Earth protected from UV light by O2 and O3 in the atmosphere?

Answer 7

The uV radiation is absorbed by the molecules of O2 and O3 as they undergo dissociation (O-O bond is broken)
O2 —-> 2O.
O3 —-> O. + O2

O2 absorbs higher energy (higher frequency/shorter wavelength) electromagnetic radiation than O3 the bond between the atoms in an O2 molecule is stronger as it is a double bond of bond order 2 but 03 has a delocalised structure where the bond order is 1.5 (weaker bond).

Question 8

What are the steps of the catalysis of ozone depletion by CFCs and NOx?

Answer 8

Chlorofluorocarbons such as CCl2F2 are broken down by absorbing UV radiation in the upper atmosphere
CCl2F2 —–>UV .CClF2 + Cl.
Ozone molecules are easily destroyed by free radicals (species with unpaired electrons) such as nitrogen oxides or chlorine atoms. The free radical can take part in a chain reaction which uses up ozone and regenerates the free radical x.
X. + O3 —–> XO. + O2
XO. + O. —–> O2 + X.
overall reaction: O3 + O. —–> 2O2
The free radial speeds up the decomposition of ozone and is regenerated in these reactions, therefore it acts as a catalyst.