Ionic equation

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An ionic equation is a chemical equation in which electrolytes are written as dissociated ions. Ionic equations are used for single and double displacement reactions which occur in aqueous solutions. For example in the following precipitation reaction:

CaCl_2(aq) + 2AgNO_3(aq) → Ca(NO₃)_2(aq) + 2AgCl_(solid)

the full ionic equation would be:

Ca²⁺ + 2Cl^- + 2Ag⁺ + 2NO₃^- → Ca²⁺ + 2NO₃^- + 2AgCl_(solid)

and the net ionic equation would be:

2Cl^- + 2Ag⁺ → 2AgCl_(solid)

or in reduced balanced form

Ag⁺ + Cl^- → AgCl_(solid).

In this aqueous reaction the Ca²⁺ and the NO₃^- ions remain in solution and are not part of the reaction. They are termed spectator ions and do not participate directly in the reaction, as they exist with the same oxidation state on both the reactant and product side of the chemical equation. They are only needed for charge balance of the original reagents.

In a neutralization or acid/base reaction, the net ionic equation will always be:

H⁺ + OH^- → H₂O

There are a few acid/base reactions that produce a precipitate in addition to the water molecule shown above. An example would be the reaction of barium hydroxide with phosphoric acid because the insoluble salt barium phosphate is produced in addition to water.

Double displacement reactions that feature a carbonate reacting with an acid have the net ionic equation:

2 H⁺ + CO₃^2- → H₂O + CO₂