Dr. Boyle's Reaction Control

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:
1) All chlorides, bromides, and iodides, except those of Ag, Hg22+, and Pb2+, are soluble.
2) All nitrates (NO3-), acetates (CH3CO2-), chlorates (ClO3), and perchlorates (ClO4) are soluble.
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.

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:

Ca(NO3)2(aq) + K2SO4(aq) --->

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.

CaSO4 & KNO3

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
calcium chloride is added to lithium carbonate
silver nitrate is added to potassium bromide

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:

2NaCl(s) ---> 2Na(m) + Cl2(g)

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.

Zn(m) + 2HCl(aq) ---> ZnCl2(aq) + H2(g)
Acid-Base reactions involve the mixing of a substance classified as an acid with a substance classified as a base and having the substances in that mixture undergo change. New substances MUST be produced. Here are some examples.

CaCO3 + HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

HNO3(aq) + KOH(aq) --> KNO3(aq) + H2(l)

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.

SrCO3(s) + H2SO4(aq) --> SrSO4(s) + CO2(g) + H2O(l)

C12H22O11 ---> 12C(s) + 11H2O(l)

CsCl(aq) + MnCl2(aq) ---> MnCl2(aq) + CsCl(aq)

MgSO4(aq) + Pb(NO3)2(aq) --> PBSO4(aq) + Mg(NO3)2(aq)

HNO2(aq) + LiOH(aq) ---> LiNO2(aq) + H2O(l)