| what is ultraviolet light | a high energy form of light present in sunlight |
| what can ultraviolet light provide enough energy to do to bonds within molecules | can provide enough energy to break bonds within molecules |
| what is the effect of ultraviolet light on our skin | it can cause sunburn and accelerate ageing of the skin |
| what does sunblock do to UV light | it contains compounds that reflect the UV so that it does not reach the skin at all
(this is not the same as sunscreen) |
| what are free radicals | atoms that are highly reactive due to the presence of an unpaired electron |
| how are free radicals formed | when UV light breaks covalent bonds, leaving two atoms with unpaired electron |
| what is a substitution reaction | a reaction involving free radicals
eg.
CH4 + Cl2 → CH3Cl + HCl
it is known as substituion as one of the H atoms in methane is substituted for a Cl atom |
| what are the three steps to a free radical reaction | initiation
propagation
termination |
| describe the initiation stage of a free radical reaction involving methane and chlorine | UV light is shone on a mixture of methane and chlorine
there is enough energy in the UV light to break the Cl-Cl bond to produce to Cl atoms
Cl-Cl→Cl˙+Cl˙ |
| what is meant by the notation Cl˙ | chlorine radical |
| describe the propagation step of a free radical reaction involving chlorine and methane | highly reactive chlorine radical react with methane molecules
this produces methyl radicals which then react with chlorine molecules (Cl2)
Cl˙ + CH4 →CH3˙+ HCl
CH3˙ + Cl2 → CH3Cl + Cl˙ |
| describe the termination step of a free radical reaction involving methane and chlorine | two free radicals combine to form stable molecules
Cl˙ + Cl˙ → Cl2
CH3˙ + CH3˙ → C2H6
CH3˙ + Cl˙ → CH3Cl |
| when writing the steps for a free radical reaction between a hydrocarbon and a halogen, what does the initiation step always involve | the halogen molecule breaking up to form two halogen radicals |
| what are free radical scavengers | compounds that combine with free radicals to form stable molecules and prevent chain reactions from occuring |
| where are free radical scavengers added | products including cosmetics, food products and plastics |
