Net Ionic Equation Definition

How to Write the Net Ionic Equation

The net ionic equation lists only species active in the reaction.
The net ionic equation lists only species active in the reaction. Steve McAlister, Getty Images

Net Ionic Equation Definition

The net ionic equation is a chemical equation for a reaction which lists only those species participating in the reaction. The net ionic equation is commonly used in acid-base neutralization reactions, double displacement reactions, and redox reactions. In other words, the net ionic equation applies to reactions that are strong electrolytes in water.

Net Ionic Equation Example

The net ionic equation for the reaction that results from mixing 1 M HCl and 1 M NaOH is:

H+(aq) + OH-(aq) → H2O(l)

The Cl- and Na+ions do not react and are not listed in the net ionic equation.

How to Write a Net Ionic Equation

There are three steps to writing a net ionic equation:

  1. Balance the chemical equation.
  2. Write the equation in terms of all of the ions in the solution. In other words, break all of the strong electrolytes into the ions they form in aqueous solution. Make sure to indicate the formula and charge of each ion, use coefficients (numbers in front of a species) to indicate the quantity of each ion, and write (aq) after each ion to indicate it's in aqueous solution.
  3. In the net ionic equation, all species with (s), (l), and (g) will be unchanged. Any (aq) that remain on both sides of the equation (reactants and products) can be cancelled out. These are called "spectator ions" and they don't participate in the reaction.

Tips for Writing the Net Ionic Equation

The key to knowing which species dissociate into ions and which form solids (precipitates) is to be able to recognize molecular and ionic compounds, know the strong acids and bases, and predict the solubility of compounds.

Molecular compounds, like sucrose or sugar, don't dissociate in water. Ionic compounds, like sodium chloride, dissociate according to solubility rules. Strong acids and bases completely dissociate into ions, while weak acids and bases only partially dissociate.

For the ionic compounds, it helps to consult the solubility rules.

Follow the rules in order:

  • All alkali metal salts are soluble. (e.g., salts of Li, Na, K, etc. - consult a periodic table if you're unsure)
  • All NH4+ salts are soluble.
  • All NO3-, C2H3O2-, ClO3-, and ClO4- salts are soluble.
  • All Ag+, Pb2+, and Hg22+ salts are insoluble.
  • All Cl-, Br-, and I- salts are soluble.
  • All CO32-, O2-, S2-, OH-, PO43-, CrO42-, Cr2O72-, and SO32- salts are insoluble (with  exceptions).
  • All SO42- salts are soluble (with exceptions).

For example, following these rules you know sodium sulfate is soluble, while iron sulfate is not.

The six strong acids that completely dissociate are HCl, HBr, HI, HNO3, H2SO4, HClO4. The oxides and hydroxides of alkali (group 1A) and alkaline earth (group 2A) metals are strong bases that completely dissociate.

Net Ionic Equation Example Problem

For example, consider the reaction between sodium chloride and silver nitrate in water. Let's write the net ionic equation.

First, you need to know the formulas for these compounds. It's a good idea to memorize common ions, but if you don't know them, this is the reaction, written with (aq) following the species to indicate they are in water:

NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)

How do you know silver nitrate and silver chloride form and that silver chloride is a solid?

Use the solubility rules to determine both reactants dissociate in water. In order for a reaction to occur, they must exchange ions. Again using the solubility rules, you know sodium nitrate is soluble (remains aqueous) because all alkali metal salts are soluble. Chloride salts are insoluble, so you know AgCl precipitates.

Knowing this, you can rewrite the equation to show all the ions (the complete ionic equation):

Na+(aq) + Cl​​​(aq) + Ag​+(aq) + NO​3​​(aq) → Na​+​​(aq) + NO​3​​(aq) + AgCl(s)

The sodium and nitrate ions are present on both sides of the reaction and are not changed by the reaction, so you can cancel them from both sides of the reaction. This leaves you with the net ionic equation:

Cl-(aq) + Ag+(aq) → AgCl(s)

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Helmenstine, Anne Marie, Ph.D. "Net Ionic Equation Definition." ThoughtCo, Nov. 23, 2016, thoughtco.com/net-ionic-equation-in-chemistry-604575. Helmenstine, Anne Marie, Ph.D. (2016, November 23). Net Ionic Equation Definition. Retrieved from https://www.thoughtco.com/net-ionic-equation-in-chemistry-604575 Helmenstine, Anne Marie, Ph.D. "Net Ionic Equation Definition." ThoughtCo. https://www.thoughtco.com/net-ionic-equation-in-chemistry-604575 (accessed November 18, 2017).