Reactions in Chemistry

Ionic Equations

In an ionic equation, ionic compounds are shown as positive and negative ions. Charges must be balanced on both sides of an ionic equation.

Some ionic compounds, such as metallic salts, split into positive and negative ions in an aqueous solution. These ions can then participate in reactions. Ionic equations can be used to model such chemical reactions. In an ionic equation, ionic compounds are shown as positive and negative ions. An ionic equation is balanced like a molecular equation. Additionally, the charge must also be balanced on the reactants and products side.

The reaction of aqueous solutions of sodium phosphate and calcium chloride to form sodium chloride and calcium phosphate can be written as a molecular equation. The molecular reaction is:
2Na3PO4(aq)+3CaCl2(aq)6NaCl(aq)+Ca3(PO4)2(s){2{\rm{Na}}_3}{\rm{PO}}_4{{(aq})}+{3{\rm{CaCl}}_2({aq})}\rightarrow6{\rm{NaCl}}{({aq})}+{\rm{Ca}}_3({\rm{PO}_4)_2}{(s)}
Note that both reactants and one of the products are aqueous, meaning they are in solution.
The ionic equation for this chemical reaction is:
6Na+(aq)+2PO43(aq)+3Ca2+(aq)+6Cl(aq)6Na+(aq)+6Cl(aq)+Ca3(PO4)2(s){6{\rm{Na}^+}}{(aq)}+{2{\rm{PO}_4}^{3-}}{(aq)}+3{\rm{Ca}^{2+}}{(aq)}+6{\rm{Cl}^-}{(aq)}\rightarrow6{\rm{Na}^+}{(aq)}+6{\rm{Cl}^-}{(aq)}+{\rm{Ca}}_3({\rm{PO}}_4)_2{(s)}
Calcium phosphate (Ca3(PO4)2) is not written as ions because it is insoluble in water following the solubility rules, guidelines for the solubility of ionic compounds based on patterns in data. Note that sodium and chlorine ions are present intact on both sides of the equation. Sodium and chlorine ions do not actually participate in the reaction even though they were in the solution. An ion that does not participate in a reaction and is present in both the reactant side and product side is called a spectator ion. If an ionic equation has all the ions involved, including spectator ions, it is a complete ionic equation. It is also possible to write the equation with the spectator ions removed. In this case, only those ions participating in the reaction are written. This form of an ionic equation without spectator ions is called a net ionic equation. The net ionic equation for this example is:
2PO43(aq)+3Ca2+(aq)Ca3(PO4)2(s){2{\rm{PO}_4}^{3-}}{(aq)}+3{\rm{Ca}^{2+}}{(aq)}\rightarrow{\rm{Ca}}_3({\rm{PO}}_4){_2(s)}