In this explainer, we will learn how to convert molecular equations to net ionic equations by identifying the charges and states of ionic species.
When writing a chemical formula or chemical equation, we can indicate an aqueous ionic compound like sodium chloride in two different ways. We can write it as a compound, , or as a series of ions, . Both of these expressions represent the same substance: an aqueous solution of sodium chloride made from sodium ions and chloride ions.
However, is not written in terms of its individual ions. Since the ions are bonded together as a solid compound, it is not accurate to represent them as separate ions. The same is true for substances in the liquid and gaseous state. Only substances that form ions when dissolved in aqueous solutions can be represented by individual ions in a chemical equation.
Example 1: Converting Aqueous Copper Sulfate from Chemical Form to Ionic Form
What is the ionic form of the chemical expression ?
Answer
In this question, we have been given the chemical formula for copper sulfate. In order to break down this chemical expression into its individual ions, we need to know what charge the ions have. Often, we can use an element’s group on the periodic table to determine its charge, but we cannot do that for the transition metal and polyatomic ion in this question. Instead, we must recall the ions they form.
Copper ions can have a or a charge, while sulfate ions have a charge. Since the neutral compound contains one ion of each, the charges on the ions must cancel each other out. For this to occur, we must have a copper ion and a sulfate ion. We can write our final answer as follows:
To explore ion notation in a chemical equation, let’s consider the reaction involving sodium chloride and silver nitrate:
Chemists have given names to the types of chemical equations depending on how we portray the ions involved. The reaction above is the molecular equation. Here, the aqueous substances are represented as molecules.
Definition: Molecular Equation
A molecular equation is a chemical equation in which the formulas of the compounds are written as though all substances exist as molecules.
Another type of chemical equation is the ionic equation, which separates substances in aqueous solutions into their individual ions as follows:
Definition: Ionic Equation
An ionic equation is a chemical equation in which the formulas of dissolved aqueous solutions are written as individual ions.
While this form more accurately represents the mix of ions in solution, the presence of so many individual ions can make it harder to visually determine what is occurring in the reaction. Note that in the ionic equation above, is a solid and therefore is not written as individual ions.
In order to better understand the changes that occur among the aqueous ions during a chemical reaction, we can also write the net ionic equation. For the reaction we have been observing, the net ionic equation is as follows:
Net ionic equations include only the ions that change over the course of the reaction. In the reaction above, chloride ions and silver ions combine to form solid silver chloride. It is important when we write a net ionic equation to ensure that the number of atoms and the overall charge are balanced on both sides.
What is excluded from this equation? If we look back at the full ionic equation, we notice that the sodium and nitrate ions are unchanged in the reactants and the products. Since these ions are present but do not take part in the reaction, we call them spectator ions. Ionic equations include spectator ions, while net ionic equations do not.
Definition: Spectator Ion
A spectator ion is an ion that exists as a reactant and product in an ionic equation but does not take part in the chemical reaction.
Example 2: Identifying Spectator Ions in the Chemical Equation for the Reaction of Magnesium with Copper Sulfate
A redox reaction is described by the equation
Including the state symbol, identify the spectator ion in this reaction.
Answer
There are three ions in this equation: magnesium ions, copper ions, and sulfate ions. The spectator ion is the ion that remains unchanged during the reaction.
We can write the ionic equation for this reaction as follows:
From this ionic equation we can see the following:
- Magnesium begins as a solid and ends as an ion.
- Copper begins as an ion and ends as a solid.
- Sulfate remains an ion the entire reaction.
As it is unchanged during the reaction, the spectator ion is the sulfate ion, .
Example 3: Writing a Net Ionic Equation from the Molecular Equation for the reaction between Ammonium Hydroxide and Hydrochloric Acid
Consider the following equation:
What is the net ionic equation for the neutralization between ammonium hydroxide and hydrochloric acid?
Answer
In this question, we are given the molecular equation for a neutralization reaction and asked to identify the corresponding net ionic equation. A molecular equation lists aqueous solutions as compounds, while a net ionic equation lists the individual ions that take part in the reaction, omitting spectator ions.
To solve this problem, we can firstly write the ionic equation. From there, we can identify and remove the spectator ions to determine the net ionic equation.
The ionic equation for this reaction is
In this ionic equation, we have written the aqueous compounds as individual ions, but, as a liquid, water is not broken up into its individual ions.
We can now determine the spectator ions in this reaction. and appear on both sides of the equation and are unchanged, so, they do not take part in the reaction. The other two ions, and , combine to form water.
Leaving out the spectator ions gives us the net ionic equation and the correct answer, option A:
Example 4: Balancing a Net Ionic Equation
Consider the following equation:
What are the correct substitutions for and to complete and balance the partial ionic equation?
Answer
This question is asking us to find the value of and identify product that will result in a properly balanced net ionic equation. To get this result, we need to ensure that the total charge of each side of the equation is the same and that the number of each atom on each side of the equation is the same.
First, let’s balance the charge. The right side of the equation is neutral, so the left side must be neutral as well. The iron ion has a charge that needs to be balanced out by ions, each with a charge. For the left side of the equation to be neutral, must equal 3.
Next, the identity of the product, . In the chemical equation, we have a iron ion reacting with three hydroxide ions. As is the only product formed, then this substance must contain one iron atom, three hydrogen atoms, and three oxygen atoms to ensure the equation is balanced.
From the answer choices, only D and E have the correct chemical formula for , iron(III) hydroxide. However, as we know that must equal 3, then choice D is the correct answer.
Key Points
- In a chemical equation, solid, liquid, and gas substances will always be indicated the same way, but we can indicate compounds in aqueous solution in several different ways.
- Molecular equations list aqueous solutions as compounds.
- Ionic equations list aqueous solutions as individual ions.
- Net ionic equations list the individual ions that participate in the reaction.
- A spectator ion is an ion that does not participate in the reaction.
- Spectator ions appear as individual ions on the left and right sides of an ionic equation but are omitted from net ionic equations.
- When writing an ionic equation, we must ensure that the number of each type of atom and the overall charge are balanced on both sides.
