Civil Engineering Reference
In-Depth Information
stances react together in certain circumstances, and the different molecular com-
binations that result always have the same proportion of elements as the original
substances.
A chemical combination between iron (Fe) and sulphur (S) making ferric sul-
phide (FeS) will be as a result of their atom weights consisting of:
56 g Fe + 32 g S = 88 g FeS
(3)
If we begin with 60 g Fe, there will be 4 g Fe left over after the reaction has taken
place. In the production of polymers, the remaining products from the reaction
are called residual monomers. These by-products usually follow the plastics in
the process as a sort of parasite, even though they are not chemically bound to
them. This physical combination is very unreliable and can lead to problematic
emissions in the indoor climate.
It is possible to calculate how much of each of the different elements is need-
ed to produce a particular substance. In the same way we can, for example, cal-
culate how much carbon dioxide (CO
2
) is released when limestone is heated up:
CaCO
3
r
CaO + CO
2
(4)
CaCO
2
has the following weight, through adding the relative atomic weights:
40 + 12 + 16 + 16 + 16 = 100 g
(5)
CaO is 40 + 16 = 56 g
CO
2
is 12 + 16 + 16 = 44 g
This means that 44 g of CO
2
are given off when 100 g of limestone is burned.
Supply of energy and release of energy in chemical
reactions
The conditions governing how a chemical reaction takes place are decided by the
physical state of the substances. There are three different states: the
solid
state
which is characterized by solid form, defined size and strong molecular cohe-
sion; the gaseous state which has no form and very weak molecular cohesion;
and the
liquid
state, which is somewhere between the two other states.
When heated most substances go from the solid state, through the liquid state
and to the
gaseous
state. In a few cases there is no transitional liquid state, and the
substance goes direct from the solid to the gaseous state. As the molecular cohe-
sion is weakened in the higher states, we can assume that the majority of chem-
ical reactions need a supply of heat. The amount of energy supplied is dependent
