S1, Ep 16: Net Ionic Equations are different from any equations you have ever written!

Caroline Christian, Ph.D.

Net ionic equations are usually taught in the context of precipitation reactions, but they can also be taught in terms of neutralization reactions. Since they are usually taught as a precipitation reaction, we will cover these. We just discussed solubility rules, and there are going to be lots of overlaps between these two posts.  Here is a cool picture of two clear liquids coming together to form a yellow precipitate!

The first thing they usually teach you how to write is the molecular equation; this is just the ionic compounds reacting to form the precipitate.  For example, NaCl(aq) + AgNO3(aq) --> AgCl(s) + NaNO3(aq).  Another thing I should cover before going any farther is if they ever ask you to predict the products of a double displacement reaction (which is what the previous reaction is) just switch ion partners.  For my example the NaCl got split up and the Cl- went with the Ag+ from the AgNO3 and the Na+ went with the NO3- from the other reactant.  So why didn't the Na+ go with the Ag+?  Because these are both cations - see the blog post "Differences between metals and nonmetals" if you don't remember what this means - and they will repel each other.  Positive attracts negative, hence why the Na+ will be attracted to the NO3-.

The next thing usually taught is how to write the total ionic equation, which is all the aqueous substances broken down into ions, and the solid, liquid, and gaseous substances left together. (The reason the solid, liquid, and gaseous substances are left together is because these substances will not be broken down into ions in solution.) For our example: NaCl(aq) + AgNO3(aq) --> AgCl(s) + NaNO3(aq), this is its total ionic equation: Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) --> AgCl(s) + Na+(aq) + NO3-(aq).  All I did is break down the aqueous substances NaCl, AgNO3, and NaNO3 into ions, and I left the solid AgCl alone.

Next is the net ionic equation; cross out all spectator ions (ions that are the same on both sides of the arrow).  So for our example, the spectator ions would include Na+(aq) and NO3-(aq) ions. And our net ionic equation is: Cl-(aq) + Ag+(aq) --> AgCl(s). Our picture at the top has the molecular equation of 2KI(aq) + Pb(NO3)2(aq) --> 2KNO3(aq) + PbI2(s), what is its net ionic equation?