Since water is very common on Earth, it may not surprise you to know that many chemical reactions are aqueous (their components are dissolved in water). When ions are a part of these reactions, we have several options for how to correctly write the chemical equations, depending on what information we need.
Since ionic compounds usually dissociate (split into their constituent ions) when they dissolve in water, the way we represent these compounds changes the appearance of the chemical equation. It may be more realistic to represent an ionic compound labelled as aqueous (aq) by writing its dissociated ions, for example CaCl2 (aq):
CaCl2(aq) ⟶ Ca2+(aq) + 2Cl−(aq)
Molecular Equations
Consider a reaction between ionic compounds taking place in an aqueous solution. Mixing aqueous solutions of CaCl2 and AgNO3 reacts to produce aqueous Ca(NO3)2 and solid AgCl:
CaCl2 (aq) + 2AgNO3 (aq) ⟶ Ca(NO3)2 (aq) + 2AgCl (s)
This balanced equation is called a molecular equation because it doesn’t explicitly represent the ionic species that are present in the solution – for example CaCl2 is represented as the compound, not its ions.
Complete Ionic Equations
Explicitly representing all dissolved ions in the reaction creates the complete ionic equation. In this example, the formulas for the dissolved ionic compounds are replaced by formulas for their dissociated ions:
Ca2+(aq) + 2Cl−(aq) + 2Ag+(aq) + 2NO3−(aq) ⟶ Ca2+(aq) + 2NO3−(aq) + 2AgCl (s)
AgCl does not dissolve in water to a significant extent, as signified by its physical state notation, s. Showing dissociated ions is not correct for solid AgCl (s).
Spectator Ions and the Net Ionic Equation
Examining the reaction equation above shows that two chemical species are present in identical form on both sides of the arrow, Ca2+(aq) and NO3−(aq).
These spectator ions are neither chemically nor physically changed by the reaction. (They maintain charge neutrality in the solution, so they are still important to the reaction). Spectator ions can be eliminated from the equation:
Ca2+(aq) + 2Cl−(aq) + 2Ag+(aq) + 2NO3−(aq) ⟶ Ca2+(aq) + 2NO3−(aq) + 2AgCl(s)
yielding a more concise chemical equation, the net ionic equation:
2Cl−(aq) + 2Ag+(aq) ⟶ 2AgCl(s)
Cl−(aq) + Ag+(aq) ⟶ AgCl(s)
This net ionic equation indicates that combining dissolved chloride and silver(I) ions produces solid silver chloride, regardless of the source of these ions. (The spectator ions do not participate in the reaction, so their identity must not be important here.) The molecular and complete ionic equations provide additional information, namely, the ionic compounds used as sources of Cl− and Ag+.
Molecular and Ionic Equations
When carbon dioxide is dissolved in an aqueous solution of sodium hydroxide, the mixture reacts to yield aqueous sodium carbonate and liquid water. Write balanced molecular, complete ionic, and net ionic equations for this process.
Solution
Begin by identifying formulas for the reactants and products and arranging them properly in chemical equation form:
CO2(aq) + NaOH(aq) ⟶ Na2CO3(aq) + H2O(l) (unbalanced)
The reaction is balanced by changing the coefficient for NaOH to 2, resulting in the molecular equation for this reaction:
CO2(aq) + 2NaOH(aq) ⟶ Na2CO3(aq) + H2O(l)
The two dissolved ionic compounds, NaOH and Na2CO3, can be represented as dissociated ions to yield the complete ionic equation:
CO2(aq) + 2Na+(aq) + 2OH−(aq) ⟶ 2Na+(aq) + CO32−(aq) + H2O(l)
Finally, identify the spectator ion(s), in this case Na+(aq), and remove it from each side of the equation to generate the net ionic equation:
CO2(aq) + 2Na+(aq) + 2OH−(aq) ⟶ 2Na+(aq) + CO32−(aq) + H2O(l)
CO2(aq) + 2OH−(aq) ⟶ CO32−(aq) + H2O(l)