Spectator ions

What are Spectator ions?

The ions which do not participate in chemical reactions and present the same on both sides of the reactions are called Spectator ions. In the net chemical reaction, the spectator ions are cancelled from both sides of the equation. In other words, “spectate” means “watch” the other ions react in an aqueous solution.

The net ionic equation provides the information of ions that exist in an aqueous medium. Salts get dissolved in polar solvents like water and are present as cations and anions in the dissolved state. The net ionic equation indicates the chemical species that undergo a chemical change. The ions that appear on both sides of the equation remain constant and therefore considered to be spectator ions

Table of Contents

• Role of spectators ion
• Net ionic equation and spectators ion
• Problem practice
• Frequently Asked Questions-FAQs

Role of spectators ion :

• NaCl in aqueous solution produces Na⁺ and Cl^– ions. AgNO₃ in aqueous solution produces Ag ⁺ and NO₃^– ions. When both react with each other they produce AgCl precipitate and the other ions Na⁺ and NO₃^– ions are present in ionic form in the solution. These ions are known as spectator ions.
  Na⁺ + Cl^– + Ag⁺ + NO₃^– → AgCl (s) + Na⁺ + NO₃^–

• The spectator ions do not participate in chemical reactions.
• They present in the same form throughout the reaction.
• Their role is only to spector the chemical reaction.
• While writing net ionic equations the spectator ions are omitted from the reactant and product side.

Net ionic equation and spectators ion

• The ionic equation that includes only the particles that participate in the reaction is called net ionic equation.
• In this equation, the spectator ions are removed from the reactant and the product side.

Steps for writing net ionic equations are:

Let us write the ionic equation of AgNO₃ + NaCl → NaNO₃ + AgCl

Step -1 : Write the equation in the ionic form

Na⁺ + Cl^– + Ag⁺ + NO₃^– → AgCl (s) + Na⁺ + NO₃^–

Step -2 :Eliminate the spectator ions from the equation.

Na⁺ (aq) + Cl^– (aq) + Ag⁺ (aq) + NO₃^– (aq) → AgCl (s) + Na⁺ (aq) + NO₃^– (aq)

Step -3 : The net ionic equation can now be written

Ag⁺ + Cl^– → AgCl

Problem practice:

1. Write the net ionic equation when aqueous solutions of copper(II) chloride and potassium phosphate are mixed, a precipitate of copper(II) phosphate is formed.

Solution:

The balance chemical equation of the above reaction is

3CuCl₂ (aq) + 2K₃PO₄(aq) → Cu₃(PO₄)₂(s) + 6KCl(aq)

The ionic equation of the above reaction is

3Cu²⁺ + 6Cl^– + 6K⁺ + PO₄^3- → Cu₃(PO₄)₂ + 6K⁺ + 6Cl^–

Eliminate the spectator ions 6Cl^– and 6K⁺ from the equation.

The net ionic equation is:

3Cu²⁺ + PO₄^3- → Cu₃(PO₄)₂

2. Write the net ionic equation that represents the reaction of an aqueous solution of sodium chromate and lead(ll) nitrate which react to form a yellow precipitate of lead(ll) chromate and an aqueous solution of sodium nitrate

Solution

The balance chemical equation the above reaction is

Na₂CrO₄(aq) + Pb(NO₃)₂(aq) → PbCrO₄(s) + 2NaNO₃(aq)

The ionic equation of the above reaction is

2Na⁺(aq) + CrO₄^2-(aq) + Pb²⁺(aq) + 2NO₃^–(aq) → PbCrO₄(s) + 2Na⁺(aq) + 2NO₃^–(aq)

Eliminate the spectator ions 2Na⁺ and 2NO₃^– from the equation.

The net ionic equation is

CrO₄^2-(aq) + Pb²⁺(aq) → PbCrO₄(s)