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A spectator ion is any electrically charged chemical species that appears between the reactants and again in the products of the reaction when the reaction is written in the form of an ionic equation. In other words, they are ions that do not undergo any type of transformation during a chemical reaction, but are present during it.
It is common for chemical reactions involving ionic compounds to directly intervene only some of the ions present in the solution. Those that are not directly involved in the chemical reaction are the spectator ions.
How to recognize a spectator ion in a chemical reaction
When we represent chemical reactions between ionic compounds in the form of molecular equations it can be difficult to quickly recognize spectator ions. In fact, it can even be difficult to recognize which ions are involved in the reaction.
A molecular equation is a chemical equation in which all species are represented by their empirical or neutral molecular formulas as if they were all molecular compounds (although ionic compounds are not, of course, molecular compounds). . Molecular equations have the benefit of making stoichiometric calculations easier, however they misrepresent how reactions between ions actually occur. For this, there are two other types of chemical equations that are ionic equations and net ionic equations.
The Total Ionic Equation and the Net Ionic Equation
Identifying spectator ions is very easy when a reaction is written in the form of its ionic equation and its net ionic equation. This is because spectator ions are simply those that appear in the total ionic equation but not in the net ionic equation.
Steps to write the net ionic equation
To show how to write ionic equations and recognize spectator ions in them, consider as an example the reaction between lead(II) nitrate (Pb(NO 3 ) 2 ) and potassium iodide (KI) to form lead iodide. (II) (PbI 2 ) that precipitates in solid form and potassium nitrate (KNO 3 ) that remains in solution.
Below are the steps for obtaining the net ionic equation for this and any other reaction, and, in the process, recognizing the spectator ions involved.
- Step 1: Write and balance the molecular equation
In the case of the example we are using, the reaction is:
- Step 2: Separate all ionic compounds that are in solution into their constituent ions by writing them in brackets. This is not done with ionic compounds that are in the solid state.
In our example, this involves ionizing lead(II) nitrate, potassium nitrate, and potassium iodide, since they are all in solution (hence the subscript ac) but not lead(II) iodide), since it is in solid state.
- Step 3: Multiply all the ions by the stoichiometric coefficients to obtain the total ionic equation.
The purpose of this step is to remove the brackets and separate all the ions to write them independently, as they are actually in solution:
This is the overall ionic equation of the reaction. Shows all the ions that are present during the chemical reaction. Note that the ions highlighted in blue (potassium and nitrate ions) appear in both reactants and unchanged products.
- Step 4: Cancel or remove all repeating ions in reactants and products.
In this case, it is the ions highlighted in blue in the total ionic equation, that is, the potassium and nitrate ions.
This represents the net ionic equation of the reaction, and therefore shows the chemical reaction actually taking place, i.e., the precipitation of lead(II) and iodide ions as solid lead(II) iodide.
Observing the difference between the net ionic equation and the total ionic equation makes the concept of a spectator ion much clearer. Potassium and nitrate ions do not participate in the chemical reaction, which involves only lead and iodide.
However, it is not possible to have a solution that contains only lead(II) ions and another that only contains iodide ions and mix them for the reaction presented in the net ionic equation to occur. In order to have lead(II) ions in solution, there must necessarily be a counterion that maintains the neutrality of the solution. In this case, nitrate ions. The same happens with the iodide ion and potassium ions.
Examples of spectator ions
Below are some additional examples of chemical reactions between ionic compounds in which spectator ions are identified by the total ionic equation.
Example 1: Reaction between barium chloride and sodium sulfate
The balanced molecular equation is:
The total ionic equation is:
The net ionic equation is:
The spectator ions in this case are the sodium cation (Na + ) and the chloride anion (Cl – ).
Example 2: Reaction between calcium nitrate and potassium phosphate
The balanced molecular equation is:
The total ionic equation is:
The net ionic equation is:
The spectator ions in this case are the potassium cation (K + ) and the nitrate anion (NO 3 – ).
Example 3: Oxide-reduction reaction between zinc and copper (II) sulfate
The balanced molecular equation is:
The total ionic equation is:
The net ionic equation is:
In this case the only spectator ion is the sulfate anion (SO 4 – ).
Example 4: Dissolution of metallic magnesium with hydrochloric acid
The balanced molecular equation is:
The total ionic equation is:
The net ionic equation is:
In this case the only spectator ion is the chloride anion (Cl – ).
References
- BBC Bytesize (nd). Ionic equations and spectator ions . Retrieved from https://www.bbc.co.uk/bitesize/guides/zsmgpbk/revision/5
- Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
- Writing and balancing chemical equations . (2020, October 30). Retrieved from https://espanol.libretexts.org/@go/page/1818
- Khan Academy (nd). Net ionic and complete ionic equations . Retrieved from https://es.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:chemical-reactions/x2eef969c74e0d802:net-ionic-equations/a/complete-ionic-and-net-ionic-equations
