OCR AS Chemistry F332: Radiation and Radicals

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Radiation and Radicals
Ways of breaking bonds
All reactions involve the breaking and remaking of bonds.
Breaking bonds is sometimes called bond fission. The way that bonds break has an
important influence on reactions.
In a covalent bond, a pair of electrons is shared between two atoms
E.g. in the HCl molecule
H : Cl
When the bonds break, these electrons get redistributed between the two atoms.
There are two ways this can happen ­ by heterolytic fission or homolytic fission
Heterolytic Fission
Two different substances are formed:
A positively charged cation and a negatively charged anion
Homolytic Fission
Two electrically uncharged radicals are formed

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Heterolytic Fission
In this type of fission, both of the shared electrons go to just one of the atoms
when the bond breaks.
This atom becomes negatively charged, because it has one more electron than it has
The other atom becomes positively charged.
In the case of HCl
H : Cl H+ : Cl -
Homolytic Fission
In this type of fission, one of the two shared electrons goes to each of the atom
when the bond breaks.…read more

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They key feature of a radical is its unpaired electron, which makes it particularly
The unpaired electron is often shown as a dot.
Showing all Showing the Showing none
the outer unpaired of the
electrons electron only electrons
Sometimes the dot is omitted together and a chlorine radical is simply represented by the
symbol for the atom.
Some radicals are a bit more subdued in their reactivity. This allows them to live long
enough to behave as ordinary molecules.…read more

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Radicals are Reactive
Filled outer shell electron shells are more stable than unfilled ones.
Radicals are reactive because they tend to try and fill their outer shells by grabbing an
electron from another atom or molecule.
For example, when a chlorine radical collides with a hydrogen molecule, the chlorine grabs
an electron from the pair of electrons in the bond between the H atoms.
The effect is to make a new bond between the Cl and H atoms.…read more

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Radical Chain Reactions
Like all radical chain reactions, this reaction has three stages:
Initiation, propagation and termination.
Initiation Chlorine radicals are initially formed by the photodissociation of chlorine
Cl2 + hv Cl · + Cl ·
Only a few chlorine radicals are formed, but they are so reactive that they
soon react with something else ­ they initiate the reaction.
Propagation Chlorine radicals react with hydrogen molecules ­ this produces hydrogen
chloride molecules and hydrogen radicals.…read more

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Methane and Chlorine
Apart from their use as fuels, alkanes are generally considered to be unreactive, which they
are with polar or organic solvents. However, alkanes will react with chlorine, and other
halogens, in the presence of light. For example, methane and chlorine do not react at all in
the dark, but in sunlight an explosive reaction occurs to form chloromethane and hydrogen
The reaction is another example of a free radical chain reaction.…read more


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