A spectator ion is an
ion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
that exists both as a
reactant and a
product in a
chemical equation
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the Product (chemistry), product entities are on the right-hand side ...
of an
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...
.
For example, in the reaction of aqueous solutions of
sodium carbonate
Sodium carbonate (also known as washing soda, soda ash, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...
and
copper(II) sulfate
Copper(II) sulfate is an inorganic compound with the chemical formula . It forms hydrates , where ''n'' can range from 1 to 7. The pentahydrate (''n'' = 5), a bright blue crystal, is the most commonly encountered hydrate of copper(II) sulfate, whi ...
:
:2
( aq) +
(aq) +
(aq) +
(aq) → 2
(aq) +
(aq) +
( s)
The and ions are spectator ions since they remain unchanged on both sides of the equation. They simply "watch" the other ions react and does not participate in any reaction, hence the name.
They are present in total ionic equations to balance the charges of the ions. Whereas the and ions combine to form a
precipitate
In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...
of solid . In
reaction stoichiometry, spectator ions are removed from a complete ionic equation to form a net ionic equation. For the above example this yields:
So:
2 (aq) +
(aq) +
(aq) +
(aq) →
2 (aq) +
(aq) +
(s) (where
x = spectator ion)
:⇒
(aq) +
(aq) →
(s)
Spectator ion concentration only affects the
Debye length
In plasmas and electrolytes, the Debye length \lambda_\text (Debye radius or Debye–Hückel screening length), is a measure of a charge carrier's net electrostatic effect in a solution and how far its electrostatic effect persists. With each D ...
. In contrast,
potential determining ions, whose concentrations affect surface potential (by surface chemical reactions) as well the Debye length.
Net ionic equation
A net ionic equation ignores the spectator ions that were part of the original equation.
So, the net ionic equation only shows the ions which reacted to produce a precipitate.
Therefore, the total ionic reaction is different from the net reaction.
See also
*
Catalysis
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
References
Acid–base chemistry
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