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In this lesson we will review three ways to write equations for reactions in aqueous solutions: molecular equations, ionic equations, and net ionic equations.
When equations are written to show all the reactants and products, including their physical state, the equations are called molecular equations; that is, they show all of the elements and compounds that are involved in the reaction. The reactions in the previous sections have been shown using molecular equations, such as:
2AgNO3(aq) + Cu(s) → 2Ag(s) + Cu(NO3)2(aq)
Because many reactions occur in solution which is what the aqueous (aq) symbol represents, chemists also use two other ways to show reactions when only specific parts of the compounds are taking part in the reaction.
An ionic equation can be written whenever soluble ionic compounds are used in a reaction. When solid AgNO3 is dissolved in water, the water molecules separate the cations and anions in the silver nitrate. When AgNO3(aq) is written, the chemist knows that the ions have been separated and can be thought of as Ag+(aq) and NO3− (aq). The solid copper is not soluble in
water and does not form ions when placed in water so its representation remains the same. As the reaction proceeds, the copper metal forms copper(II) ion and the silver ion forms insoluble elemental silver. The soluble product of the reaction is the Cu(NO3)2 which is written as Cu2+(aq) and 2NO3− (aq).
Written in ionic form the reaction looks like this:
2Ag+(aq) + 2NO3−(aq) + Cu(s) → 2Ag(s) + Cu2+(aq) + 2NO3− (aq)
The third form of equation that can be written is the net ionic equation. Just as the net weight on a grocery product tells the consumer the weight of the contents, or your net pay tells you how much money you have in your pocket, a net ionic equation tells the chemist exactly what ions and species reacted, omitting from the reaction those ions which can be found in exactly the same form in products and reactants. They are rather like “beaker bums”—observing the reaction, but not taking part in it. For that reason, these ions are called spectator ions.
Removing the spectator ions from the previous example shows a net ionic equation of
2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+(aq) .
The nitrate ions’ only purpose was as an anion to provide a source of silver ions.
In double replacement reactions, if one of the products is not soluble or is a weak electrolyte that product can not be written as aqueous ions and must be included in its molecular form in the equation.
If aqueous calcium chloride reacts with aqueous lead(II) acetate, solid lead(II) chloride will form, with the calcium acetate remaining in solution. The net ionic equation for the reaction would simply be:
Pb2+(aq) + 2Cl− (aq) → PbCl2(s)
The calcium ions and acetate ions would remain in solution. Only upon evaporation of the water in the solution would solid calcium acetate be found.