In chemistry, spectator ions are ions that occur as both reactants and products in a chemical equation, but do not affect the equilibrium of the reaction. In other words, they “spectate” or “watch” the other ions react in an aqueous solution (when the solvent is water). Because spectator ions occur on both sides of the reaction arrow, they are “cancelled out” and do not appear in the net ionic equation.
Spectator Ions and the Net Ionic Equation
For example, the chemical equation for the reaction between silver nitrate (AgNO3) and sodium chloride (NaCl) in water shows aqueous sodium nitrate (NaNO3) and solid silver chloride (AgCl) as products:
AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s)
Writing the total ionic equation reveals the spectator ions:
Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) → Na+(aq) + NO3–(aq) + AgCl(s)
The sodium ion (Na+) and nitrate ion (NO3-) appears on both sides of the reaction, so you cancel them out or eliminate them:
Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) → Na+(aq) + NO3–(aq) + AgCl(s)
This leaves the net ionic equation:
Ag+(aq) + Cl–(aq) → AgCl(s)
Note the net ionic equation only shows the chemical species directly participating in the reaction. By convention, write the cation (in this case, Ag+) first, followed by the anion (in this case, Cl–). The net ionic equation is a balanced chemical equation. The number and type of atoms on both sides of the reaction arrow is the same. The net charge on both sides of the reaction arrow is the same. In this case, the “+” and “-” on the left side of the arrow neutralize each other, so the net charge on both sides of the arrow is 0.
How to Find Spectator Ions
Usually, you’re looking for a double replacement (double displacement) reaction in aqueous solution where one of the products precipitates as a solid. This type of reaction has the following general form:
AB(aq) + CD(aq) → AD(aq) + CB(s) or AB(aq) + CD(aq) → AD(s) + CB(aq)
Often, this type of reaction occurs between two salts or between acids and bases as a neutralization reaction. If you don’t know whether a precipitate forms, consult a solubility chart or recall solubility rules.
Some species commonly occur as spectator ions:
| Common Spectator Cations | Common Spectator Anions |
|---|---|
| Li+ (lithium ion) | Cl– (chloride ion) |
| Na+ (sodium ion) | Br– (bromide ion) |
| K+ (potassium ion) | I– (iodide ion) |
| Rb+ (rubidium ion) | NO3– (nitrate ion) |
| Sr2+ (strontium ion) | ClO4– (perchlorate ion) |
| Ba2+ (barium ion) | SO42- (sulfate ion) |
There are exceptions. For example, calcium sulfate (CaSO4) is insoluble and forms a precipitate.
Spectator Ion Example Problem
For example, identify the spectator ions and write the net ionic equation for the reaction between sodium chloride (NaCl) and copper sulfate (CuSO4) in water.
The first step is predicting the products of the reaction. From the solubility rules, you know both sodium chloride and copper sulfate dissociate into their ions in water. So, assuming the ions exchange partners in the reaction (a double replacement reaction), the products are sodium sulfate and copper chloride. Balancing the ions for charge, their formulas are Na2SO4 and CuCl2. Again from the solubility rules, you know sodium sulfate is aqueous, but copper chloride forms a precipitate.
NaCl(aq) + CuSO4(aq) → Na2SO4(aq) + CuCl2(s)
Balancing the equation requires introducing coefficients:
2NaCl(a) + CuSO4(aq) → Na2SO4(aq) + CuCl2(aq)
The next step is writing the total ionic equation:
2Na+(aq) + 2Cl–(aq) + Cu2+(aq) + SO42-(aq) → 2Na+(aq) + SO42-(aq) + CuCl2(s)
Identify the spectator ions by looking for ions occurring on both sides of the reaction arrow. They are Na+ and SO42-.
2Na+(aq) + 2Cl–(aq) + Cu2+(aq) + SO42-(aq) → 2Na+(aq) + SO42-(aq) + CuCl2(s)
Eliminate the spectator ions:
2Na+(aq) + 2Cl–(aq) + Cu2+(aq) + SO42-(aq) → 2Na+(aq) + SO42-(aq) + CuCl2(s)
This leaves the net ionic equation:
2Cl–(aq) + Cu2+(aq) → CuCl2(s)
Rearrange the equation so the cation appears before the anion in the reactants:
Cu2+(aq) + 2Cl–(aq) → CuCl2(s)
Importance of Spectator Ions
Although they don’t participate in the net reaction or affect equilibrium, spectator ions are important. Their presence affects the Debye length or Debye radius of the charge carriers in a solution. What this means is these ions acts as an electrical screen, either attracting oppositely-charged ions or repelling like-charged ions. In liquids, the Debye length affects electrolyte and colloid conductivity.
References
- Atkins P.; de Paula, J. (2006). Physical Chemistry (8th ed.). W.H. Freeman. ISBN 978-0-7167-8759-4.
- Laidler, K. J. (1978). Physical Chemistry with Biological Applications. Benjamin/Cummings. ISBN 978-0-8053-5680-9.
- Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry: Principles and Modern Applications (8th ed.). Upper Saddle River, N.J: Prentice Hall. ISBN 978-0-13-014329-7.
- Zumdahl, Steven S. (1997). Chemistry (4th ed.). Boston, MA: Houghton Mifflin Company. ISBN 9780669417944.