Net Ionic Equations

Rules for writing net ionic equations:

  1. Write the molecular equation. Make sure all formulas are written correctly.

  2. Example: Sodium chromate reacts with lead (II) nitrate.

    Na2CrO4 + Pb(NO3)2 --> NaNO3 + PbCrO4

  3. Determine which substances are not soluble and leave them as is. This will include anything on the insoluble list.

  4. Check the solubility rules. You see that the only substance that is considered insoluble is the PbCrO4. All chromates are insoluble. This substance will not form ions in solution. It will be a solid.

  5. The other substances should all be soluble - which means they will be aqueous solutions.

  6. You will find that all alkali metals (group 1 elements) are soluble and they are soluble all the time. That makes the Na2CrO4 and NaNO3 soluble. You also see that all nitrates are soluble. That makes the Pb(NO3)2 soluble as well.

  7. Once you have determined which substances are soluble, break those substances into their respective ions.

  8. Now you will take the substances that have been identified as soluble and write their ions:

    Na2CrO4 will break down into Na+ ions and CrO42- ions
    NaNO3will break down into Na+ ions and NO3- ions
    Pb(NO3)will break down into 2 Pb2+ ions and NO3- ions

    Equation with ions: Na+ + CrO42- + Pb2+ + NO3- --> Na+ + NO3- + PbCrO4

  9. Cancel identical ions on reactant and product side. They must be identical. These are the spectator ions. (Cl- is not identical to Cl2, therefore, you would not cancel them out)

  10. You will notice that there are two identical spectator ions in the equation. They are the Na+ on both sides and the NO3- on both sides. These are spectator ions. Cancel them out. You are left with the final net ionic equation of:

    Pb2+ + CrO42- --> PbCrO4

You have just written your first (and certainly not your last) net ionic equation!

Example 1: CaCO3(s) + HCl --> ?

Molecular Equation: CaCO3(s) + HCl(aq) --> CaCl2(aq) + H2O(l) + CO2(g)

Total Ionic Equation: CaCO3(s) + H+(aq) + Cl- --> Ca2+(aq) + Cl-(aq) + H2O(l) + CO2(g)

Net Ionic Equation: CaCO3(s) + H+ --> Ca2+(aq) + H2O(l) + CO2(g)

Notes: The calcium carbonate is a solid which means it will not form ions, even as a reactant. HCl is a strong acid and will fully ionize, or form ions. Calcium chloride falls under the chloride rule - all chlorides are soluble except silver, mercury, and lead. Carbon dioxide and water are a gas and a liquid, plus they are molecules (nonmetal-nonmetal compounds) and all molecules are insoluble. The only spectator ion present is the chloride ion - it will be the only one canceled out.

Example 2: Fe(s) + AgNO3 --> ?

Molecular Equation: Fe(s) + AgNO3(aq) --> Ag(s) + Fe(NO3)2(aq)

Total Ionic Equation: Fe(s) + Ag+(aq) + NO3-(aq) --> Ag(s) + Fe2+(aq) + NO3-(aq)

Net Ionic Equation: Fe(s) + Ag+(aq) --> Ag(s) + Fe2+(aq)

Notes: Any single element by itself will not form ions. A single element is always a neutral atom of that element. The iron as a reactant is a solid as well which verifies that the Fe atom will not form ions. All nitrates are soluble which means the silver nitrate will form ions. On the product side, again the Ag formed is a neutral atom and will be a solid. This means that no ions will form. All nitrates are soluble and the iron (II) nitrate that is formed will form ions. As you can see from the equation, the only spectator ion is the nitrate ion. Cancel the nitrate ion out and you are left with the net ionic equation.