Net ionic equations

1. Writing Net Ionic Equations
Learning to write the molecular
equations as net ionic equations

2. Rules for Writing Net Ionic Equations
 Write weak electrolytes such as slightly soluble
salts, weak acids, weak bases, gases, organic
compounds, water, and complex ions as molecules.
 Write strong electrolytes such as soluble salts,
strong acids and strong bases as ions.
 Cancel spectator ions (ions that are the same as
reactants and products) from the equation.

3. Identifying the Weak Electrolytes
 If a reactant or product is a salt (product of acid +
base reaction), check the solubility rules. Remember
that all sodium salts, potassium salts, ammonium
salts and nitrate salts are soluble.
 If a reactant or product is an acid (forms H+
in water)
 If a reactant or product is a base (forms OH−
in
water)

4. Other Weak Electrolytes or Nonelectrolytes
 Common gases are usually oxides such as CO2, H2O, SO2, SO3, NO, NO2,
etc.
 Other common gases are the diatomic molecules such as H2, N2, O2, etc.
 Organic compounds (compounds that contain carbon) are usually
nonelectrolytes.
 Water is the most common weak electrolyte.
 Complex ions are also weak electrolytes.

5. Other Information
 Remember that metals in their solid state are not ions and do not
have charges.
 Most oxides are written as molecules.
 Write a (s) behind insoluble salts and write a (g) behind gases.
 Reactions that do not form a weak electrolyte or nonelectrolyte will
be all spectator ions since there is no driving force for the reaction.

6. First Example
Pb(NO3)2 + Na2S →
Write the balanced equation with the predicted products.
Pb(NO3)2 + Na2S → PbS + 2NaNO3 Look up the solubilities
of each of the salts in the equation.
Pb(NO3)2(aq)+ Na2S(aq) → PbS(s) + 2NaNO3(aq)
Remember that all sodium and all nitrate salts are soluble. Put a (s) for slightly
soluble salts. Now write the equation ionically.
Pb2+
+ 2NO3
−
+ 2Na+
+ S2-
→ PbS(s) + 2Na+
+ 2NO3
−
Note that subscripts
(except those in polyatomic ions) are brought out front when the compound is
written ionically. Write the net ionic equation.
Pb2+
+ S2-
→ PbS(s)
Remove all ions that are the same on the reactants and products side.

7. Second Example
NaC2H3O2 + HCl →
Write the balanced equation with the predicted products.
NaC2H3O2 + HCl → NaCl + HC2H3O2 Look up the
solubilities of each of the salts and the strengths of each of the acids.
NaC2H3O2(aq) + HCl(aq) → NaCl (aq) + HC2H3O2
Remember that all sodium salts are soluble. Don’t put an (aq) for weak acids.
Now write the equation ionically.
Na+
+ C2H3O2
−
+ H+
+ Cl−
→ Na+
+ Cl−
+ HC2H3O2 Note that subscripts of
polyatomic ions are not brought out front when the compound is written
ionically. Write the net ionic equation.
H+
+ C2H3O2
−
→ HC2H3O2 Remove all
ions that are the same on the reactants and products side.

8. Third Example
K + H2O →
Write the balanced equation with the predicted products.
2K + 2H2O → 2ΚOH + H2 Look up
the solubility or strength of the base.
2K(s) + 2H2O(l ) → 2KOH(aq) + H2(g)
Remember that K is metal not an ion, KOH is a strong base, and water is a weak
electrolyte. Now write the equation ionically.
2K(s) + 2H2O(l ) → 2K+
+ 2OH−
+ H2(g) Write the net ionic equation.
2K(s) + 2H2O(l ) → 2K+
+ 2OH−
+ H2(g)
There are no ions the same on both sides of the equation.

9. Fourth Example
Zn + FeCl2 →
Write the balanced equation with the predicted products.
Zn + FeCl2 → ZnCl2 + Fe Look up
the solubility of the two salts.
Zn(s) + FeCl2(aq) → ZnCl2(aq) + Fe(s)
Remember that Zn and Fe are metals not ions. The only insoluble chlorides are
silver, lead(II), and Hg(I). Now write the equation ionically.
Zn(s) + Fe2+
+ 2Cl−
→ Zn2+
+ 2Cl−
+ Fe(s) Write the net ionic equation.
Zn(s) + Fe2+
→ Zn2+
+ Fe(s)
Remove all ions that are the same on the reactants and products side.

10. Fifth Example
H2O + SO3 →
Write the balanced equation with the predicted products.
H2O + SO3 → H2SO4 Look up
the strength of the acid.
H2O(l ) + SO3(g) → H2SO4(aq)
Remember that water is a weak electrolyte, sulfur trioxide is a gas, and sulfuric
acid is a strong acid. Now write the equation ionically.
H2O(l ) + SO3(g) → Η+
+ HSO4
−
In this case the first H ion is strong, but the
second H ion is weak. Therefore, the first H ion is ionized and the second one is
not. Write the net ionic equation.
H2O(l ) + SO3(g) → Η+
+ HSO4
−
There are no ions the same on both sides of the equation.

11. Sixth Example
C2H4 + O2 →
Write the balanced equation with the predicted products.
2C2H4 + 6O2 → 4CO2 + 4H2O
What can be written as ions?
2C2H4 + 6O2(g) → 4CO2(g) + 4H2O (l )
Remember that water is a weak electrolyte, carbon dioxide and oxygen are gases,
and ethane (C2H4) is an organic compound. Now write the equation ionically.
2C2H4 + 6O2(g) → 4CO2(g) + 4H2O (l ) In this case none of compounds can
be ionized; therefore the molecular, ionic and net ionic equations are all the same.

12. Summary
 Write weak electrolytes such as slightly soluble salts,
weak acids, weak bases, gases, organic compounds, water,
oxides, and metals as molecules or atoms.
 Break strong electrolytes such as soluble salts, strong
acids and strong bases into their respective ions.
 Cancel spectator ions (ions that are the same as reactants
and products) from the equation.