Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 359

14.1.2.9 Chemical Reactions
A fundamental tool in any environmental engineer's toolbox is a basic understanding of chemical
reactions, reaction rates, and physical reactions; however, before discussing chemical reactivity, it
is important that we review the basics of electron distribution and chemical and physical changes
and to discuss the important role heat plays in both chemical and physical reactions. Simply stated,
electron distribution around the nucleus is key to understanding chemical reactivity. Only electrons
are involved in chemical changes; the nuclei of atoms are not altered in any way during chemical
reactions. Electrons are arranged around the nucleus in a definite pattern or series of shells. In gen-
eral, only the outer shell, or valence , electrons (i.e., the ones farthest from the nucleus) are affected
during chemical change. The valence number or valence of an element indicates the number of
electrons involved in forming a compound and the number of electrons it tends to gain or lose when
combining with other elements.
Note: Positive valence indicates giving up electrons. Negative valence indicates accepting electrons.
• If the valence electrons are shared with other atoms, then a covalent bond is formed when
the compound is produced.
• If the valence electrons are donated to another atom, then an ionic bond is formed when
the compound is produced.
• A hydrogen bond occurs when an atom of hydrogen is attracted by rather strong forces to
two or atoms instead of only one, so that it is considered to act as a bond between them. If
an atom gains or loses one or more valence electrons, it becomes an ion (charged particle).
• Cations are positively charged particles.
• Anions are negatively charged particles.
In physical and chemical changes, recall that a chemical change is the change physical sub-
stances undergo when they become new or different substances. To identify a chemical change look
for observable signs such as color change, light production, smoke, bubbling or fizzing, and pres-
ence of heat. A physical change occurs when objects undergo a change that does not change their
chemical nature. A physical change involves a change in physical properties. Physical properties can
be observed without changing the type of matter. Examples of physical properties include texture,
size, shape, color, odor, mass, volume, density, and weight.
An endothermic reaction is a chemical reaction that absorbs energy and where the energy con-
tent of the products is greater than that of the reactant heat taken in by the system. An exothermic
reaction is a chemical reaction that gives out energy and where the energy content of the products
is less than that of the reactants; heat is given out from the system.
14.1.2.9.1 Types of Chemical Reactions
Chemical reactions are of fundamental importance throughout chemistry and related technologies.
Although experienced chemists can sometimes predict the reactions that will occur in a new chemi-
cal system, they may overlook some alternatives. Further, they are usually unable to make reliable
predictions when the chemistry is unfamiliar to them.
Key Point: There are few tools available to assist in predicting chemical reactions, and none at all
for predicting the novel reactions that are of greatest interest. For more information on predicting
chemical reactions, see the work by Irikura and Johnson (2000).
Just as it is convenient to classify elements as gaseous and nongaseous, it is convenient to classify
chemical reactions. There are so many chemical reactions that it is helpful to classify them into four
general types. For example, there are over 450 named reactions listed in the Merck Index. The four
general types of chemical reaction are