Chemical reactions are classified according to what occurs during the reaction. If a solid forms, the reaction is classed as a "precipitation" reaction. When an acid (substance that can release H+ ions into solution) is mixed with a base (a substance that accepts H+ ions from solution), the combined process is termed an acid-base reaction. The general statement about an acid-base reaction is that the products are a salt and water. Other reactions may form gases or simply more water molecules. One common reaction is the "decomposition" reaction where a single substance is decomposed into other simpler substances or elements. A combination reaction occurs when elemental materials react to form one or more compounds.
If chemicals are mixed and a new substance is not formed, no reaction has taken place. Thus one might ask, "How do I know a reaction occurred?"
First, the student of chemistry must learn the solubility rules for compounds. Knowledge of these rules will help to determine if a solid phase forms.
The basic rules are as follows:
The question arises, "How do these rules work for reactions?"
For example, two solutions are made, one containing calcium
nitrate and a second solution containing potassium sulfate.
The solubilities rules #2 for calcium nitrate and #4 for
potassium sulfate tell us that both chemicals dissolve in
water.
When the two solutions are mixed, one must again look at the
rules in order to predict what is to be expected. The reactant
side of the reaction is thus:
Recall that in order for a reaction to take place, new substances
must be formed. Thus Ca2+ cannot stay bonded to
NO3- and K+ cannot stay with
SO42-.
We also know that cations (+ charged species) will not bond with
other cations and anions (- charged species) will not bond with
other anions.
We are then left with only the following possible combinations of
products.
Now we look back at the solubility rules. If both compounds we
predicts as products are soluble then no reaction takes
place.
Rule #2 says that KNO3 is soluble. Rule #4 says that
CaSO4 is insoluble. The formation of an insoluble
compound means a precipitate will form since the substance does
not dissolve in water. We predict that a reaction does occur and
a solid forms in the container.
This same procedure is used to analyze all reactions.
Try the following:
sodium hydroxide is added to zinc chloride
You will first need to translate the English names into the
symbolic forms. Then use the rules above to see if the substance
dissolve. If one or more do not dissolve then a reaction is
unlikely.
Another series of reactions involve the production of a gas as
one of the products. Recognizing reactions where the formation of
gaseous products occurs takes a bit more practice.
Common gases which form are carbon dioxide (CO2),
hydrogen (H2), oxygen (O2), chlorine (Cl2),
fluorine (F2), nitric oxide (NO2) and
nitrous oxide(N2O).
The release of chlorine (Cl2) and fluorine
(F2) generally occur only in decomposition reactions
such as:
Energy must be used to cause this reaction to occur. Electricity
is used to force this reaction to the right. The process is
called "electrolysis".
If a gas forms then a reaction occurs.
Note that the water which forms as a product is shown as being n
the (l) liquid state. Acid-Base reactions actually produce more
water in the container than was there before the reaction.
One characteristic to look for in many reactions is the existence
of H+ ions and OH- ions in the reactants.
If the reactants can supply these two ions, water will form.
H+ and OH- will always combine to form
water.
These paragraphs only touch lightly on the subject of reactions.
There are many types of reactions, each classified according to
what occurs. The best way to learn to identify them is through
practice. Here are some reactions on which to practice.
1) All chlorides, bromides, and iodides, except those of Ag,
Hg22+, and Pb2+, are soluble.
2) All nitrates (NO3-), acetates
(CH3CO2-), chlorates
(ClO3
3) All ammonium (NH4 compounds are
soluble.
4) All sulfates, except those of Ca2+,
Sr2+, Ba2+, Pb2+,
Hg22+, and Ag+, are
soluble.
5) All carbonates (CO32-), chromates
(CrO42-), oxalates
(C2O42-), and phosphates
(PO43-), except those of the
Group 1 elements and ammonium, are insoluble.
6) All sulfides (S2-), except those of the
Group 1 elements and NH4+, are
insoluble.
7) All hydroxides (OH-) and oxides (O2-),
except those of the Group 1 and Group 2 elements, are
insoluble.
calcium chloride is added to lithium carbonate
silver nitrate is added to potassium bromide