CR - Net Ionic Equations (Lesson)

Net Ionic Equations

In order for us to explore chemical reactions more closely, we need to look at them in a different way. The balanced equations we have written in the past are not the complete picture of what is going on in the reaction. These balanced equations are called molecular reactions. In these types of equations, all the substances are written as molecules or compounds.

For example, NaCl _(aq) + AgNO_3(aq) $LaTeX: \longrightarrow$ $\longrightarrow$ AgCl_(s) + NaNO_3(aq) is called a "molecular equation" even though there are no molecules. The reason is because the formulas are shown with no charges even though the compounds contain ions with charges.

The reality is that often reactions take place in solution (symbolized by aq for aqueous), allowing the chemicals to dissolve (if they are able). Remember that in order for a reaction to occur, the particles must collide. When chemicals are in solution, the particles are more free to move around than they would be in another state and therefore more likely to collide in a successful reaction.

So, if a compound is dissolved in water to form a solution, the ions separate. If we were to look at an ionic compound that is dissolved in water, we would not see the ions bound together. Instead, we would see individual ions surrounded by water molecules. Note that most of the time these water molecules act as the vessel and do not actually participate in the reaction. When an aqueous chemical is involved in a chemical reaction, it is more accurate to write ions instead of the compound. The reactants from the equation above would more accurately look like this:

Na⁺(aq) + Cl^-(aq) + Ag⁺(aq) + NO₃^-(aq)

The products would be:

Na⁺ (aq) + NO₃^-(aq) + AgCl(s)

When regular chemical reactions are written using ions, we call them total ionic equations or TIE.

Na⁺(aq) + Cl^-(aq) + Ag⁺(aq) + NO₃^-(aq) $LaTeX: \longrightarrow$ $\longrightarrow$ Na⁺ (aq) + NO₃^-(aq) + AgCl(s)

The Following Substances are More Accurately Written As Ions:

soluble salts
strong acids
strong bases

What Should Be Written As Ions?

So far you have learned that salts that are dissolved are written as ions. What else should be written this way? Strong acids and bases should also be written as ions. The term strong refers to the extent that the chemical breaks into the ions. Strong acids and bases break completely, or almost completely, into their ions. Molecular compounds are always written as individual molecules in overall ionic equations.

The easiest way to remember which acids and bases should be written as ions is to memorize the list of strong acids and bases.

Strong Bases=7
All group 1 metal hydroxides as well as Ba from group 2

The overall ionic equation for the sodium chloride reaction with silver nitrate would be represented as:

Na⁺(aq) + Cl^-(aq) + Ag⁺(aq) + NO₃^-(aq) $LaTeX: \longrightarrow$ $\longrightarrow$ Na⁺(aq) + Cl^-(aq) + AgCl (s)

How do we know that the silver chloride should be written as a solid? A solubility table is needed to predict which products are soluble as opposed to ones that will form a solid (precipitate).

A table for solubility guidelines is a good tool to have on hand until you have the basics committed to memory.

Do Not Write the Following As Ions:

insoluble salts
solid elements
weak acids
weak bases
gases
water
organic compounds

The simulation below illustrates the spectators remaining in solution as precipitants form in a double displacement reaction, allowing you to see why we don't include these ions in the NIE.

Think about this example:

Solutions of lead II nitrate & potassium iodide react

You Try It: Net Ionic Equations

See how you do with these examples! The following questions should help to determine if formula units should be written as separate ions or as uncharged molecules in an overall ionic equation. You do not need to worry about balancing these equations.

Does the substance dissolve in water? If no, write the molecular formula. If yes... continue.
If the substance dissolves in water, does it dissociate into either a salt or a strong base? If yes, write the ionic form of the formula, if no, write the molecular formula.
If the substance dissolves in water, does it ionize into a strong acid? If yes, then write the ionic form of the formula, if on, write the molecular formula.

Click on the plus sign for each example to check your answers and see how you did!

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