In many ionic reactions not all the chemical species undergo a change. Those that do not change are called spectator ions. When chemists remove the spectator ions from a chemical reaction they produce what is termed a net ionic reaction. Net ionic equations indicate only the substance that undergo a change and are very important in our study of chemistry.
In order to write net ionic reactions we need to do the following steps:
  1. balance the equation for atoms.
  2. write out the dissolved chemical species as they exist in solution. Precipitates, liquids and gases are not written as ions. This is called a total ionic equation.
  3. remove common aqueous ions, ( ions that are in both the reactant and product) . The resulting equation is called the net ionic equation.
Step #2 ; Writing the dissolving of ionic solids. (Arrhenius model )
To write the dissolving of an ionic solid we:
  1. place the formula for the ionic solid in the reactants.
  2. place the formulas for the ions that compose the solid in the products (taken from a table of ions)
  3. balance for the charge and atoms by changing the coefficients of the products.
Examples;
1. Dissolving of Calcium Chloride
 
Reactants
 
Products
Write formulas of solid and ions
CaCl2(s)
__Ca 2+(aq)
+
__Cl-(aq)
Balance for charge and atoms
CaCl2(s)
__Ca2+(aq)
+
2 Cl -(aq)
2. Dissolving of Barium Phosphate
 
Reactant
Product
Formulas
Ba3(PO4)2(s)
___Ba 2+ (aq)
+
___PO43- (aq)
Balance
Ba3(PO4)2(s)
_3_Ba 2+ (aq)
+
_2_PO43- (aq)
Notice that in each case the numbers of atoms is balanced and so to is the charge in the products.
  1. There is + 2 , -2 for Calcium Chloride
  2. There is +6 and -6 for the Barium Phosphate.
The ratio of the ions to the solid always remains constant . If there were 2 CaCl2 (s) then we would have 2 Ca 2+ (aq) and 4 Cl - (aq) .
How many Ba 2+ ions would be present if we had 3 Ba3(PO4)2 (s)? Answer 9.
Writing Net Ionic Equations (all three steps )
A solution of Barium chloride combines with a solution of Sodium Carbonate to form a precipitate of Barium Carbonate and a solution of Sodium Chloride
1. Write the formula equations and balance
___BaCl2 (aq) + ___Na2CO3 (aq) ___BaCO3 (s) + _2__NaCl (aq)
2. Write the aqueous substances as separate ions. Leave solids, liquids and gases unchanged.
_Ba2+(aq) + 2Cl - (aq) + 2Na +(aq) + CO3 2-(aq) ___BaCO3 (s) + 2Na +(aq) + 2Cl- (aq)
3. Cancel common aqueous ions. (Net Ionic Equations )
Ba 2+ (aq)
+
CO32- (aq)
BaCO3(s)
Assignment #3: Writing Net Ionic Equations
Balance the following reactions
Write the total and net ionic equations for the following using the procedure above.
  1. ___Zn (s) +___ HCl (aq) ----->___ ZnCl2 (aq) + ___H2 (g)
  2. ___FeCl3 (aq) +___ AgNO3 (aq) ------>___ AgCl (s) +___ Fe(NO3)3 (aq)
  3. ___KOH (aq) + ___H3PO4(aq) ------>___ K3PO4(aq) +___ H2O (l)
  4. ___HCl (aq) + ___ Na2CO3 (aq) -----> ___ H2O(l) + ___CO2(g) + ___NaCl (aq)
  5. ___Ba(OH)2 (aq) + ___Fe2(SO4)3 (aq) -----> ___ Fe(OH)3 (s) + ___ BaSO4 (aq)
  6. ___Mg (s) + ___AgNO3 (aq) -----> ___Ag (s) + ___ Mg(NO3)2 (aq)
  7. ___NaOH (aq) + ___CO2 (g) -----> ___Na2CO3 (aq) + ___H2O (l)