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Metathesis Reactions


Ionic compounds consist of two parts: a positive ion (cation) and a negative ion (anion). The cation comes first, both in the name and in the formula for an ionic compound. For example:

sodium chlorideNaClNa+Cl-
sodium carbonateNa2CO3Na+CO32-
calcium chlorideCaCl2Ca2+Cl-
calcium carbonateCaCO3Ca2+CO32-

The superscript in each ion represents the charge, or oxidation state of the ion. The total charge must be zero for any compound. Since sodium ion has a charge of +1 and carbonate ion has a charge of -2, it takes 2 sodium ions to balance the charge of a carbonate ion. We include this information as the subscript in the formula: Na2CO3, 2 Na+ for each CO32-. What about the subscript in CO32-? Well, it means there are 3 oxygen atoms for each carbon atom in the ion. But it considerably simplifies things if we view the carbonate ion as a whole rather than as parts. In the simplest reactions we will study, the carbonate ion is never broken up, so it makes sense to treat it as a single entity rather than the sum of its parts.

But if you don't recognize carbonate ion, you won't be able to take advantage of this great simplification and you will be hopelessly confused the rest of the term. Consider the compound NH4NO3. This is a simple ionic compound. Every chemist knows that it has two parts, a cation and an anion. Every chemist recognizes the two parts. But to a beginner there appear to be 4 parts. To a beginner the compound seems more complicated than it has to be because he does not recognize the parts. So your first task for this project is to learn the basic parts we will be using all semester:

Positive Ions
Hydrogen ionH+
Ammonium ionNH4+
Sodium ionNa+
Aluminum ionAl3+
Potassium ionK+
Calcium ionCa2+
Iron (II) ionFe2+
Iron (III) ionFe3+
Copper ionCu2+
Silver ionAg+
Lead (II) ionPb2+

Negative Ions
Nitrate ionNO3-
Chloride ionCl-
Sulfate ionSO42-
Hydroxide ionOH-
Oxide ionO2-
Sulfide ionS2-
Carbonate ionCO32-
There are 18 names for you to learn. There is no getting around this. If you don't learn them you will be lost when we discuss chemistry and after all, this is a chemistry class. From these 11 cations and 7 anions, 77 compounds can be formed. And metathesis reactions occur when two compounds come into contact with each other and simply swap their first and last names. This usually happens in aqueous (water) solution. When an ionic compound dissolves in water, the cations and anions separate and float around on their own. It may happen that a cation from one compound will find an anion from another compound and stick to it. If this happens, a new compound forms and we say a methathesis reaction has occurred. Consider the reaction:

CaCl2 (aq) + Na2CO3 (aq) ----> 2 NaCl(aq) + CaCO 3(s)

1 mole of calcium chloride + 1 mole of sodium carbonate yields 2 moles of sodium chloride + 1 mole of calcium carbonate

In the beginning, there were calcium ions, chloride ions, sodium ions, and carbonate ions, all floating around in the solution. But when a calcium ion finds a carbonate ion, they stick together forming an insoluble precipitate, calcium carbonate. The sodium ions and chloride ions remain in solution.

Why is it 2 NaCl and not Na2Cl2? Well, the ions from the tables are Na+ and Cl-, so we know the compound must be NaCl. The 2 is placed out front to balance the reaction, that is to make sure that the number of each kind of atom is the same on both sides. If there is no number out front of a formula, it is assumed to be 1. We call the number out front the stoichiometric coefficient, which, though a mouthful, is shorter than saying the little number in front of each formula in a balanced chemical equation.

And what is this mole thing? It's just the unit for the stoichiometric coefficient. Aren't you glad you asked? Don't worry, the more you use words like mole and stoichiometric coefficient, the more you will grow to understand them.

What about the reaction:

2 NaCl(aq) + CaCO 3(s) -----> CaCl2 (aq) + Na2CO3 (aq)

Isn't this also a balanced reaction? For reactions in aqueous solution, the reaction always proceeds in the direction that produces a solid precipitate, that is, a compound which is not soluble in water. How do we know what compounds are soluble in water? You guessed it, another table:

NameSoluble Compounds
Chloridesall except lead, silver
Sulfatesall except lead, silver, calcium
NameInsoluble Compounds
Hydroxidesall except hydrogen, sodium, potassium, ammonium, calcium
Oxidesall except hydrogen, sodium, potassium, ammonium
Sulfidesall except hydrogen, sodium, potassium, ammonium
Carbonatesall except hydrogen, sodium, potassium, ammonium

So you have three tables to learn. When you are finished you will be able to predict in some detail an enormous number (over 5,000) possible chemical reactions. Try your hand at these for practice:

Will sodium chloride and silver nitrate react in aqueous solution? If so what is the balanced reaction equation? If not, why not?

Both sodium chloride and silver nitrate are soluble in water. If I swap the names I get silver chloride and sodium nitrate. Silver chloride is insoluble and sodium nitrate is soluble. So I know if I mix a solution of sodium chloride and a solution of silver nitrate, an insoluble precipitate of silver chloride will form and sodium nitrate will remain in solution. The balanced equation is

NaCl(aq) + AgNO3(aq) -----> AgCl(s) + NaNO3(aq)

Will sodium chloride and potassium nitrate react in aqueous solution? If so what is the balanced reaction equation? If not, why not?

Both are soluble in water. The products, sodium nitrate and potassium chloride are also soluble in water. The solution just has sodium ions, potassium ions, chloride ions, and nitrate ions floating around separately. None of the ions will stick together to form a precipitate so no reaction occurs.

Will lead sulfide and calcium carbonate react in aqueous solution? If so what is the balanced reaction equation? If not, why not?

Neither of these compounds are soluble in water. If you put them in water, both will settle to the bottom but will not react.

Other Pages of Chemical Reactions

Books on Reserve

Criteria for Success

This project is evaluated by written quiz. You will be given the names of two compounds and asked the following questions:

Will these two chemicals react in aqueous solution. If so, what is the balanced reaction equation? If not, why not?

If you miss any part of this examination, you fail. You may, however, try again (only once per day) until you pass.

A practice quiz is available online. While it covers the same material as the Metathesis Quiz, the format is different to allow for computer scoring. For example, instead of asking:


The online practice quiz would ask:

1 How many moles of sodium chloride will react with one mole of silver nitrate in aqueous solution?

2 What is the chemical formula for silver nitrate?
(A) SiNO3
(B) SiNO2
(C) AgNO3
(D) AgNO2

The answers would be 1 and C, respectively.