CHEMISTRY | EXTRA REFERENCE | EBOOK
DOWNLOAD HSEB NOTES ON REACTION TYPE | CHEMISTRY
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Chemical Reactions
Chemical
reactions are also
known as chemical
changes.This refers to the changes in the structure of molecules. Such reactions can result in molecules at taching to each other to form larger molecules, molecules
breaking apart to form two or more
smaller molecules, or
rearrangements of atoms
within molecules. Chemical reactions usually involve the making or breaking of
chemical bonds. A chemical reaction does not involve or change the nucleus of
the atom in any way. Only the electron
clouds around the
atoms, involved in
the reaction, interact. A chemical reaction
can result in:
- molecules attaching to each other to form larger molecules
- molecules breaking apart to form two or more smaller molecules
- rearrangements of atoms within molecules
A chemical reaction almost always involves a change in
energy, conveniently measured in terms
of heat. The
energy diference between
the "before" and "after" states of a chemical reaction can be
calculated theoretically using tables of data (or
a computer). For
example, consider the
reaction CH4 + 2O2 ?
CO2 + 2H2O (combustion of methane in
oxygen). By calculating the amounts of energy required
to break all
the bonds on
the left ("before") and
right ("af-ter") sides of the equation,
we can calculate
the energy diference
between the reactants and the
products. This is
referred to as
∆H, where ∆
(Delta) means diference, and H
stands for enthalpy,
a measure of
energy which is
equal to the heat transferred
at constant pressure.
∆H is usually
given in units
of kJ (thousands of joules) or in kcal (kilocalories). If ∆H is negative for the reaction, then energy has been released. This type of reaction is referred to as
exothermic (literally,
outside heat, or
throwing of heat).
An exothermic reaction
is morefavourable and thus
more likely to
occur. Our example
reaction is exothermic, which we already know from everyday experience, since
burning gas in air givesof heat. A reaction may
have a positive ∆H. This means
that, to proceed,
the reac- tion requires an
input of energy
from outside. This type of reaction
is called endothermic
(literally, inside heat,
or absorbing heat).
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