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Atoms bond to form molecules because the energy of the resulting atomic
assembly is lower than the energy of the atoms in isolation. When two
or more molecules meet, it is often the case
that by changing their partners - by rearranging their bonding patterns
- they can form more stable structures. The simplest example and one which
is learnt in elementary chemistry is the reaction between H and
H2. Such reactions rarely take place 'spontaneously'. There
is usually some resistance to the rearrangement - an energy 'hill' that
has to be climbed by the reacting molecules as shown on the left.
The energy hill surmounted in the reaction
of A and BC to form AB and C
But once the molecules are 'over the barrier', the new chemical bonds form
rapidly. Some of the most important types of chemical reactions are collected
in Table below.
| Reaction Type |
Example |
Description |
|
Electrophillic
|
H+ + C2H4 = CH2CH3+
|
Protonation of an ethylene molecule |
| Nucleophillic |
CH3Br + Cl- = CH3Cl + Br-
|
A substitution reaction: in this case a chloride ion (Cl-)
substitutes for a bromide ion (Br-) |
| Free radical |
O. + O2 = .O3 |
An oxygen atom (O.) attacks and oxygen molecule (O2)
to form an ozone radical (.O3) |
| Oxidation |
2Fe(s) + 3/2 O2 (g)
= Fe2O3(s) |
Iron (Fe) combines with Oxgygen (O2) to form iron oxide
(Fe2O3). The (s) and (g) designations correspond
to the solid and gaseous states of the species. |
| Reduction |
3C(s) + 2Fe2O3(s) =
3CO2(g) + 4Fe(s) |
Carbon reacts with iron oxide to produce carbon dioxide and metallic
iron. |
Reactions very often give out energy (although they may
absorb it in some cases); the classical example is combustion where energy
rich molecules react with oxygen to give low energy molecules - carbon
dioxide and water, the basis of more modern fuels. There is a fascinating
class of reactions that result in the emission of light - the field of
photochemistry. And chemical reactions may also be used to create electric
currents. indeed, batteries are nothing more than devices for changing
chemical into electrical energy. And all these processes work in reverse.
Heat, light and electricity may be used to stimulate and drive chemical
reactivity. Indeed, the relationship between chemical reactivity and these
physical phenomenon is one of the most fascinating ways in which the science
of chemistry has developed in the last hundred years. We are constantly
surrounded by chemical reactions. Our bodies rely on an enormous range of
reactions to keep us alive, moving, breathing and thinking. Chemical reactions
are developed and exploited by industry to make new products. At the heart
of all these processes are molecules in which the atoms are rearranging
and finding new partners. We have now established the ground rules for
interactions between atoms.We have seen why they bind to each other and
have learned something of the factors which limit and control the structures
of the resulting atomic assemblies and of the way molecules interact.
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