Equations: Complete Molecular, Complete Ionic and Net Ionic

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I. Complete Molecular Equations

These equations can also be called 'complete formula equations' or simply 'formula equations.' There is no standard name.

This type of equation shows the full formula for each substance involved (or the full name of each substance), without reference to a substance being ionic or molecular. Many times, problems of this type will start out with an equation in words. Here are two examples:

barium chloride solution reacts with sodium sulfate solution to make solid barium sulfate and aqueous sodium chloride

aqueous solutions of hydrochloric acid and sodium hydroxide react to produce sodium chloride and water

The wording in problems like the above can vary somewhat. The word 'aqueous' could be used as well as 'precipitate.' Sometimes aqueous is used for the reactants, but assumed for the products. Sometimes aqueous is never used and the question writer simply assumes you know everything is happening in aqueous solution. There is no standard way to arrange a chemical equation using the names of the substances. (However, when using formulas, the examples used tend be done in a similiar style world-wide.)

I started out with names for the complete molecular equations because your first answer in a given problem is often to translate the names into a complete molecular equation equation. This means that you must be able to (1) write correct chemical formulas from the names and (2) balance chemical equations.

Here are the word equations above, repeated using formulas:

BaCl2(aq) + Na2SO4(aq) ---> BaSO4(s) + 2NaCl(aq)

HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l)

Only full formulas (never words or ions) are involved in a complete molecular equation. Ions (never words) will be used for the complete ionic equation and the net ionic equation, which will follow just below. You can use words for an ionic equation (either complete or net), it just isn't usually done.

By the way, some sources (and teachers) use 'total' rather than 'complete.'


II. Complete Ionic Equations

To transform a complete molecular equation to a complete ionic equation, you need to know the difference between an ionic compound and a molecular compound. The basics are as follows:

Ionic compounds are between metals and nonmetals or between metals and polyatomic ions. Examples are sodium chloride (NaCl), magnesium nitrate [Mg(NO3)2] and ammonium sulfate [(NH4)2SO4]. When an ionic substance is dissolved in aqueous solution, it ALWAYS ionizes and the ions always have a charge. Remember to never split apart a polyatomic ion.

Sometimes a student wonders if a compound between two polyatomics, say NH4NO3, is an ionic substance. Yes, it is. It has an ionic bond between the two polyatomics and thus qualifies as an ionic substance. It ionizes into NH4+ and NO3¯.

Molecular compounds involve only nonmetals. Examples include sulfur trioxide (SO3), carbon dioxide (CO2), water (H2O) and glucose (C6H12O6). Molecular substances never ionize in solution, they exist as complete molecules and they NEVER have a charge. By the way, molecular compounds are also called covalent compounds. All the bonds in molecules like the examples above are covalent. Not one ionic bond exists in a covalent compound.

There is one more thing you need to know:

ionic substances which are insoluble are always written as the full formula, never as ions

You might think this strange, but keep in mind that an insoluble substance never dissolves, so it never has a chance to ionize. An ionic substance only breaks down into separate ions when it is in aqueous solution.

In order to know which substances are soluble and which are insoluble, you must know your solubility chart. There are many of them available across the Internet.

Above were two complete molecular equations. Just below, I have written then as complete ionic equations:

Ba2+(aq) + 2Cl¯(aq) + 2Na+(aq) + SO42¯(aq) ---> BaSO4(s) + 2Na+(aq) + 2Cl¯(aq)

H+(aq) + Cl¯(aq) + Na+(aq) + OH¯(aq) ---> Na+(aq) + Cl¯(aq) + H2O(l)

Please note that barium sulfate is an insoluble substance and that water is a molecular substance.

Sometimes, the word 'complete' is deleted and the text (or teacher) assumes you know that 'ionic equation' is NOT the same as "net ionic equation.' It is best to assume 'complete ionic equation' is what is meant by 'ionic equation.' In other words, the word 'net' is always included when one means to identify the 'net ionic equation.'

Here is an example of a mistake students sometimes make:

Ba2+(aq) + Cl¯(aq) + Na+(aq) + SO42¯(aq) ---> BaSO4(s) + Na+(aq) + Cl¯(aq)

The mistake is to not show the proper coefficients when writing the complete ionic equation from the molecular equation. For example, when BaCl2 ionizes, it forms one barium ion and two chloride ions in solution, not one of each.

Here is a Yahoo Answers Q&A showing this mistake.


III. The Spectator Ion

The spectator ion is an important idea. Here is the definition:

spectator ions are present on the reactant side and on the product side in exactly the same form

In both of the example equations above, the sodium ion and the chloride ion are the spectator ions.

Here's why identifying the spectator ions is important:

to go from a complete ionic equation to a net ionic equation, all spectator ions are eliminated from the equation

The problem students face is that the ability to identify the spectator ions is bound up in knowing (1) how to write correct formulas, (2) how ionic substances ionize in solution and (3) which substances are soluble and which are insoluble.

That can be a problem because there are many different bits of information to know before being able to answer the types of questions that are the subject of this tutorial. In addition, some of the bits you need to know wind up getting taught after covering this area. Cute!


IV. Net Ionic Equations

Using my two example equations, when we strike out the spectator ions, we are left with the following net ionic equations:

Ba2+(aq) + SO42¯(aq) ---> BaSO4(s)

H+(aq) + OH¯(aq) ---> H2O(l)

Remember, in a net ionic equation, all spectator ions are completely removed. For all intents and purposes, they are not considered part of the reaction.


One more example, then some final comments:

Write the complete molecular, complete ionic and net ionic equations for this reaction:

solutions of sodium chloride and silver nitrate react to for a precipitate of silver chloride and aqueous sodium nitrate

Answer:

complete molecular: NaCl(aq) + AgNO3(aq) ---> AgCl(s) + NaNO3(aq)

You know AgCl is insoluble from using a solubility chart.

complete ionic: Na+(aq) + Cl¯(aq) + Ag+(aq) + NO3¯(aq) ---> AgCl(s) + Na+(aq) + NO3¯(aq)

In this example, the sodium ion and the nitrate ion are the spectator ions. They occur on both sides of the arrow in exactly the same form.

net ionic: Ag+(aq) + Cl¯(aq) ---> AgCl(s)

A slightly old school method to indicate a precipitate is to use a downwards-pointing arrow:

net ionic: Ag+(aq) + Cl¯(aq) ---> AgCl↓

An upwards-pointing arrow is used to indicate a gas is formed. Hydrogen gas, for example, would be H2↑ rather than H2(g)


V. Last Comments

There are three major (and one minor) areas where net ionic equations are used. A major use is in the area called reduction oxidation (short name = redox). I have an entire section on redox (and it will probably be covered later on in your course.) Some of the equations I will use below are redox equations, but they will not be identified as such. The equations I use will be fairly simple and can be balanced by sight and will not require the techniques taught in the redox section.

The two other major types that you will see in the problems below are these:

(1) double replacement reactions (also called double displacement, also called metathesis). Yes, you should know all three names.
(2) acid base neutralizations.

The double replacement reactions are where you will need a solubility chart. Acid base neutralizations will result in water being formed from hydrogen ion and hydroxide ion.

Finally, although there are a number of single replacement reactions that can be written in net ionic form, I'm calling it a minor area. The main reason for single replacement being minor is that it is used much less as a source for net ionic type problems. A very large majority tend to be the double replacement and the neutralization reactions.

Here are two (unbalanced) single replacement examples:

Al(s) + HBr(aq) ---> AlBr3(aq) + H2

Br2(l) + NaI(aq) ---> I2(s) + NaBr(aq)

I'll make them the first two problems in the problems numbered 1 to 10 (see #1-10 link below).

Here are two (unbalanced) double replacement examples:

Pb(NO3)2(aq) + Na2S(aq) --->

ammonium phosphate + calcium chloride --->

The answers to these two are at #21 and #22 (see #11-25 link below). Try them on your own before looking at the answers and see how well you do.


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