Question

Distinguish between a molecular, an ionic, and a net ionic equation.

Short answer

Molecular, ionic, and net ionic equations are three forms to represent a chemical reaction. A molecular equation shows compounds as molecules, an ionic equation shows them as dissociated ions, and a net ionic equation shows only the ions that participate in the reaction, ignoring the 'spectator ions'.

Step-by-step solution

Defining a Molecular Equation

A molecular equation is a balanced chemical equation where the ionic compounds are expressed as molecules instead of component ions. For example, the reaction between sodium chloride (NaCl) and silver nitrate (AgNO3) can be written in a molecular equation as: NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)

Defining an Ionic Equation

An ionic equation is a chemical equation in which electrolytes are written as dissociated ions. Using the same example above, the ionic equation becomes: Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)

Defining a Net Ionic Equation

A net ionic equation focuses only on the particles that are involved in the reaction. It ignores ions which do not take part in the reaction, known as 'spectator ions'. In our example, the sodium Na+(aq) and nitrate NO3-(aq) ions are the spectator ions. Therefore, the net ionic equation is: Ag+(aq) + Cl-(aq) → AgCl(s)

Key concepts

Molecular Equation

A molecular equation is a way to represent a chemical reaction using the complete formulas of the compounds involved. This equation is written to show all participating molecules, even if the substances are actually in the form of ions in a solution.
For example, when you combine sodium chloride (\(\text{NaCl}\)) and silver nitrate (\(\text{AgNO}_3\)), a molecular equation describes this reaction as:

• \(\text{NaCl(aq) + AgNO}_3\text{(aq) → AgCl(s) + NaNO}_3\text{(aq)}\)

This equation shows both the reactants and products as neutral compounds and doesn't reflect the ionic nature of these compounds when they are dissolved in water. Molecular equations are helpful for getting a clear and overall picture of the process occurring, but they do not provide detailed insight into what happens at the microscopic ionic level.

Ionic Equation

An ionic equation takes the molecular equation a step further by showing the actual species that exist in solution. This means it breaks down the soluble ionic compounds into their respective ions. This way, you can see more clearly how these ions interact during the reaction.
Continuing with our previous example, the ionic equation for the reaction of sodium chloride and silver nitrate is:

• \(\text{Na}^+\text{(aq) + Cl}^-\text{(aq) + Ag}^+\text{(aq) + NO}_3^-\text{(aq) → AgCl(s) + Na}^+\text{(aq) + NO}_3^-\text{(aq)}\)

Here, the sodium and nitrate ions are shown as separate from their compounds, reflecting how they exist in the solution. This level of detail allows you to analyze which ions actually change during the reaction and which remain unchanged, known as the spectator ions.

Net Ionic Equation

A net ionic equation simplifies the ionic equation further by removing spectator ions, which don't participate in the actual chemical change. Spectator ions are present in the solution but do not influence the formation of the product.
In the example of sodium chloride and silver nitrate, the ions not involved in forming the solid silver chloride (\(\text{AgCl}\)) are the sodium (\(\text{Na}^+\)) and nitrate (\(\text{NO}_3^-\)) ions. Removing these gives the net ionic equation:

• \(\text{Ag}^+\text{(aq) + Cl}^-\text{(aq) → AgCl(s)}\)

The net ionic equation focuses only on the ions and molecules that play a direct role in the chemical transformation. This gives a clear, concise depiction of the actual chemistry occurring, emphasizing the formation of new compounds in the reaction.