Net Ionic Equations

 General Chemistry Index

Where are we going with this? This page will give the ability to demonstrate an understanding of the law of conservation of mass through the use of particle diagrams and mathematical models.

Net Ionic Equations 

Balancing Net Ionic Equations is really a subset of balancing equations, in general. So, what's special? Why does this get a special level of consideration?

Good question!

In the net ionic equation, you are looking at some stuff in solution mixed with some more stuff in solution. When a compound is in solution, it can break apart into the constituent ions. So, NaCl in water results in there being Na⁺ and Cl^- ions hanging around the polar H2O molecules.  If we mix other things in, it's possible to get other positive and negative ions all mixed up in the water. 

Then, suppose the positive ion from one compound joins with the negative ion from the other to form an insoluble solid? That solid will precipitate out of the solution leaving all the other ions floating around… just watching. 

So, you end up starting with a reaction like :this (Wait? Can you end up starting? Does that even make sense?)  :

NaCl + AgNO3 → NaNO3 + AgCl

Dealing with net ionic reactions, the idea is that the reactants are dissolved in water, so the parts come… apart. The equation is then written with even more subscripts!

aq means it is "aqueous" which means the ions are just mixed up in the water.

s means it is a not-dissolved solid. (This will usually be on the product side!)

l means liquid. 

So, we put the stuff into the liquid (which isn't really part of the reaction, but is the—if you will—container for the reaction). The ions come apart (we said that already) and so we rewrite the equations as…

Na⁺(aq) + Cl⁻(aq) + Ag⁺(aq) + NO3⁻³(aq )→

Now, on the product side where we had…

→ NaNO3 + AgCl

But, we now need to do that aq - s - l thing… meaning we need to know what will and what won't dissolve.  Skipping to the outcome, turns out that AgCl is insoluble. So…

Na⁺(aq) + Cl⁻(aq) + Ag⁺(aq) + NO3⁻(aq) → Na⁺(aq)  +  NO3⁻(aq)  + AgCl(s)

Comparing the reactant side to the product side, we can see some things that don't change… they were ions before and ended up ions after… They just "watched" the reaction. We call these spectator ions (really, no joking).

Spectator ions: ions in a chemical reaction that do not change during a chemical reaction.

Na⁺(aq) + Cl⁻(aq) + Ag⁺(aq) + NO3⁻(aq) → Na⁺(aq)  +  NO3⁻(aq)  + AgCl(s)

Now for the "net" part of this whole thing! In business, net profit is the money left over after the expenses are taken out. The part left over… 

So, we just cancel out the things that don't change. As if they aren't even there. As if they are just watching…

Cl⁻(aq) + Ag⁺(aq) →  + AgCl(s)

A net ionic equation shows only the ions found in the larger balanced chemical equation that are directly involved in the chemical reaction.

That's it.