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:
| Name | Formula | Cation | Anion |
|---|---|---|---|
| sodium chloride | NaCl | Na+ | Cl- |
| sodium carbonate | Na2CO3 | Na+ | CO32- |
| calcium chloride | CaCl2 | Ca2+ | Cl- |
| calcium carbonate | CaCO3 | Ca2+ | 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
| Name | Formula |
|---|---|
| Hydrogen ion | H+ |
| Ammonium ion | NH4+ |
| Sodium ion | Na+ |
| Aluminum ion | Al3+ |
| Potassium ion | K+ |
| Calcium ion | Ca2+ |
| Iron (II) ion | Fe2+ |
| Iron (III) ion | Fe3+ |
| Copper ion | Cu2+ |
| Silver ion | Ag+ |
| Lead (II) ion | Pb2+ |
Negative Ions
| Name | Formula |
|---|---|
| Nitrate ion | NO3- |
| Chloride ion | Cl- |
| Sulfate ion | SO42- |
| Hydroxide ion | OH- |
| Oxide ion | O2- |
| Sulfide ion | S2- |
| Carbonate ion | CO32- |
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:
| Name | Soluble Compounds |
|---|---|
| Nitrates | all |
| Chlorides | all except lead, silver |
| Sulfates | all except lead, silver, calcium |
| Name | Insoluble Compounds |
| Hydroxides | all except hydrogen, sodium, potassium, ammonium, calcium |
| Oxides | all except hydrogen, sodium, potassium, ammonium |
| Sulfides | all except hydrogen, sodium, potassium, ammonium |
| Carbonates | all 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.
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:
Will sodium chloride and silver nitrate react in aqueous solution? If so what is the balanced reaction equation? If not, why not?
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.