The net ionic form is useful whenever we're talking about reactions of ions that occur in solutions, usually water.
Only those ions that are actually participating in the reaction will be listed in the chemical equation then.
I'll try to explain how to write them (and where you have to be careful!) using the following example:
In an aqueous solution (water), barium chloride reacts with sodium sulfate to form barium sulfate and sodium chloride.
1.) The molecular equation
Write down the 'normal' overall balanced chemical reaction (the molecular equation).
2.) Think about solubility
We know that BaCl2, Na2SO4 and NaCl are easily soluble, while BaSO4 isn't.
Latter can therefore be seen as a white precipitate after the reaction occured.
If you don't know whether a substance is soluble or not, just look it up. Often it will also be indicated in the equation by symbols like aq (meaning the substance is dissolved) or an arrow pointing downward (meaning the substance is a precipitate in a solution):
(Better check with your teacher before you write symbols like the arrow because different people use different symbols but it always helped me to put those arrows.)
If an ionic compound is soluble in a solvent, it will dissociate - it will separate into free moving ions.
BaCl2, Na2SO4 and NaCl will therefore exist as individual ions in the solution, not as a whole compound.
3.) Break apart the soluble compounds into ions
4.) Write out the whole chemical equation with the seperated ions (the total ionic equation)
Note that barium sulfate does not dissolve and therefore does not separate into ions!
5.) Simplify
Cancel those things out that appear on both sides of the equation. Those ions do not actually take part in the reaction. They are also called spectator ions because they are only watching but not participating ;)
I used circles to indicate which parts are still left over now.
6.) The net ionic equation (finally!)
Only write down the parts/ions/compounds that are still left over.
You're done!
Don't hesitate to ask if you still have problems or questions!
