Civil Engineering Reference
In-Depth Information
upon the temperature needed to make the substances transform into the higher
state, i.e. the substance's boiling point:
Classification of volatility for organic substances
Type
Boiling point
VOC: Volatile organic compounds
Above 250°C
SVOC: Semi-volatile organic compounds
250-380°C
POM: Particle-bound organic compounds
Below 380°C
There are also chemical reactions which emit energy. When water is mixed with
unslaked lime (CaO) slaked lime (Ca(OH)
2
) is formed by the release of a great
deal of heat. If slaked lime is then burned, unslaked lime will form and water will
be given off in the form of steam. The energy supply in this reaction is exactly the
same as the amount of energy released in the first reaction.
Each substance has a given energy content, known as the element's cohesive
energy. If the energy content in the original substances of a chemical reaction is
greater than the energy content of the resultant substances, then energy is
released, mainly in the form of warmth. This is called an exothermic reaction. In
an endothermic reaction, energy must be supplied to the reaction. Exothermic
reactions usually occur in nature; endothermic reactions are usual in all forms of
industrial processes.
It is not only energy in the form of warmth that can stimulate chemical reac-
tions: radioactivity, electricity and light can also have an effect. Sunlight is an
example of light that can initiate a number of chemical processes in different
materials. One of the most important rules in chemistry is: 'Within a chemical
reaction the sum of the mass energy is constant'.
Other conditions for chemical processes
Other factors also affect the reactions process. The solidifying process of chalk
(CaCO
3
) is an example:
Ca(OH)
2
+ H
2
O + CO
2
r
CaCO
3
+ 2H
2
O
(6)
Note that the solidifying is reduced with lower temperature; it can also be accel-
erated with larger amounts of carbon dioxide. A higher concentration of carbon
dioxide accelerates the chemical reaction, even if not all of it is used in the reac-
tion or is part of the final product.
The size of the particles also plays a part. The finer the particles and the greater
the surface of the materials, the quicker the reaction is. Fine cements therefore
