Lesson 4.2 Molecular and Ionic Equations
Suggested Reading:

Zumdahl Chapter 4 Section 4.6
Essential Question:

What are the different ways in which chemical equations can be
expressed?
Learning Objective:

Write balanced molecular (formula), ionic and net ionic equations.
We use chemical equations to help us describe chemical reactions. For a
reaction involving ions, we have a choice of chemical equations, depending
on the kind of information we wish to convey. We can use a molecular
(formula) equation, a complete ionic equation, or a net ionic equation.
To illustrate these different kinds of equations, consider the preparation of
precipitated calcium carbonate, CaCO3. This white, fine powdery compound
is used as a paper filler to brighten and retain ink, as an antacid as in the
trade-named Tums, and as a mild abrasive in toothpastes. One way to
prepare this compound is to react calcium hydroxide, Ca(OH)2, with sodium
carbonate, Na2CO3. Lets look at the different ways to write the equation for
this reaction.
Molecular (Formula) Equations
You could write the equation for this reaction as follows:
We call this a molecular (formula) equation, which is a chemical equation
in which the reactant and products are written as if they were molecular
substances (ionic compounds), Even though they may actually exist in
solution as ions.
The molecular equation is useful because it is explicit about what the
reactants are and what products are obtained. The reaction above tells us
that if you add aqueous solutions of calcium hydroxide and and sodium
carbonate to a reaction vessel solid calcium carbonate forms (precipitates).
After you remove the precipitate you are left with a solution of sodium
hydroxide. The molecular equation describes what you do in the laboratory or
in an industrial process.
Complete Ionic Equations
Although the molecular equation is useful in describing the actual reactant
and product substances, it does not tell you what is happening at the level of
ions. That is, it does not give you an ionic-theory interpretation of the
reaction. Because this kind of information is useful, you often need to rewrite
the molecular equation as an ionic equation. A complete ionic
equation is a chemical equation in which strong
electrolytes are written as separate ions in solution.
Again, consider the reaction of calcium hydroxide and sodium carbonate.
Both are soluble ionic substances and therefore strong electrolytes (when
they dissolve in water they go into solution as ions). Each formula unit of
Ca(OH)2 forms one Ca2+ ion and two OH- ions in solution. Writing Ca2+(aq) +
2OH-(aq) in place of Ca(OH)2(aq) emphasizes that the solution contains
free moving ions. Similarly, each formula unit of Na2CO3 forms two Na+
ions and one CO32- ion in solution, and you would show this by writing
2Na+(aq) + CO32-(aq) in place of Na2CO3(aq). The reactant side of the
equation becomes
This shows that the reaction mixture begins as a solution of four different
kinds of ions.
On the product side, one product is the precipitate CaCO3(s). This is an
ionic compound, but the ions are not dissolved. Rather, the ions are fixed
in particular sites in the crystalline solid. The picture to the left shows a
naturally occurring precipitate of CaCO3 called calcite, which is found in
marine waters. We leave the formula as CaCO3(s) to convey this. NaOH,
on the other hand, is a soluble ionic substance and therefore a strong
electrolyte; it dissolves in water as freely moving ions, which we denote by
writing Na+(aq)+OH-(aq). Thus, the complete ionic equation is:
The purpose of the complete ionic equation is to represent each substance
by its predominant form in the reaction mixture. If the substance is a
soluble ionic compound, it is represented as individual ions. If the
substance is a weak electrolyte, it is present in solution primarily as
molecules, so it is represented by the molecular formula. If the substance
is an insoluble ionic compound, you represent it by the formula of the
compound. The state symbol (aq) is used to denote soluble
substances while (s) is used with insoluble ionic solids
Net Ionic Equations
In the complete ionic equation given above, some ions (OH- and Na+)
appear on both sides of the question. This means that nothing happens to
these ions as the reactions occurs. They are called spectator ions.
Spectator ion: an ion in an ionic equation that does not take part in the
reaction.
You can cancel spectator ions from both sides to express the essential
reaction that occurs. The resulting equation is:
This is the net ionic equation, an ionic equation from which spectator ions
have been canceled. It shows that the reaction that actually occurs at the
ionic level is between calcium ions and carbonate ions to form solid calcium
carbonate.
Putting it All Together
Lets look ate another example with calcium and carbonate. Mixing any
solution of calcium ion with any solution of carbonate ion will give you this
same net reaction from above. For example, calcium nitrate, Ca(NO3)2,
dissolves readily in water to provide a solution of calcium ion. Similarly,
potassium carbonate, K2CO3, dissolves in water to provide a solution of
carbonate ion. When you mix solutions of these two compounds, you obtain a
solution of calcium and carbonate ions, ,which react to form calcium
carbonate. The other product is potassium nitrates, a soluble ionic compound,
and is therefore a strong electrolyte. Lets look and the different types of
equations that can be used to describe this reaction.
The molecular (formula) equation:
The complete ionic equation:
The net ionic equation:
Note that the net ionic equation is identical to the one obtained from the
reaction of Ca(OH)2 and Na2CO3. The essential reaction is the same whether
you mix solutions of calcium hydroxide and sodium carbonate or solutions of
calcium nitrate an potassium carbonate. The generality of the net ionic
equation makes it useful for understanding many of the basic reactions that
take place in the world around us.