MCAT® ExamChemical and Physical Foundations of Biological SystemsEasy
A student observes a reaction where solid silver chloride (AgCl) precipitates when solutions of silver nitrate (AgNO₃) and sodium chloride (NaCl) are mixed. The net ionic equation for this reaction is Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Which of the following statements BEST describes the role of the nitrate (NO₃⁻) and sodium (Na⁺) ions in this reaction?
- AThey are spectator ions and do not participate in the net reaction.
- BThey are reactants that form a soluble product.
- CThey are products that remain in solution.
- DThey are catalysts that speed up the precipitation.
Show answer & explanationAnswer & explanation
Correct answer: A. They are spectator ions and do not participate in the net reaction.
In a net ionic equation, ions that do not participate in the actual chemical change (i.e., they remain dissolved in solution before and after the reaction) are called spectator ions. Nitrate (NO₃⁻) and sodium (Na⁺) ions are present on both sides of the full ionic equation and do not form a precipitate or gas, nor do they act as reactants in the formation of AgCl(s).
Why the other options are wrong
- B. They are not reactants in the formation of the precipitate; they remain in solution.
- C. While they remain in solution, they are present initially and are not formed as new products of the precipitation.
- D. There is no indication that they affect the rate of the reaction; their role is simply to balance charges in the initial solutions.
Spectator Ions
Ions that are present in a solution during a chemical reaction but do not participate in the reaction itself, remaining unchanged in form before and after the reaction.
- They appear on both sides of the complete ionic equation.
- They are omitted from the net ionic equation.
- Their presence ensures charge neutrality in the initial solutions.
- Common spectator ions include alkali metal ions (Na⁺, K⁺) and nitrate ions (NO₃⁻).
Memory trick: Spectators just watch, they don't change their match!