Net ionic equations are equations that show only the soluble, strong electrolytes reacting (these are represented as ions) and omit the
spectator ions,which go through the reaction unchanged. When you encounter net ionicequations on the
SAT II Chemistry test, you’ll need to remember thefollowing solubility rules, so memorize them! Also keep in mind thatnet ionic equations, which are the bare bones of the chemical reaction,usually take place in aqueous environments. Here are those solubilityrules:
- Most alkali metal compounds and
compounds are soluble. - Cl-, Br-, I- compounds are soluble, except when they contain Ag+,
, or Pb2+. - F- compounds are soluble, except when they contain group 2A metals.
, 
, 
, and CH3COO- compounds are soluble.
compounds are soluble, except when they include Ca2+, Sr2+, Ba2+, Ag+, Pb2+, or 
.
, 
, 
, 
, S2-, OH-, and O2- compounds are insoluble.- Group 2A metal oxides are classified as strong bases even though they are not very soluble.
The two solubility rules that you will usethe most are numbers 1 and 4. You must memorize that all group 1A metaland ammonium compounds are soluble. As soon as you see a compound
,Li, Na, K, Rb, Cs, or Fr, you should know that it’s soluble. Also, allnitrates are soluble—look at the end of the compound. If it ends in
, you know that it’s soluble.
What’s the big deal with solubility? Well,if the ion is soluble, it won’t form a precipitate, and this means itdoesn’t react and should be left out of the net ionic equation. The keyis first to write the compound’s chemical formula and then determine ifit’s soluble. If it is soluble, then ionize it—if it isn’t, don’tionize it; leave it as a molecule.
Here are some additional rules about common reaction types that you should be familiar with for the exam:
- If an insoluble precipitate or gas can be formed in a reaction, it probably will be.
- Oxides (except group 1A) are insoluble, and when reacted withwater, they form either acids (nonmetal oxides) or bases (metaloxides).
- There are six strong acids that completely ionize: HCl, HBr, HI, HNO3, H2SO4, HClO4. All other acids are weak and are written together, as molecules.
- The strong bases that ionize are oxides and hydroxides ofgroup 1A and 2A metals. All other oxides and hydroxides are consideredweak and written together, as molecules.
Now try writing some net ionic equations, using the rules above.
Example
Write the net ionic equation for a mixture of solutions of silver nitrate and lithium bromide.
Explanation
Ag+ + 
+ Li+ + Br- 
This is a double replacement reaction. Bothcompounds are soluble, so everything ionizes. If anything is formed, itwill come from recombining the “inside” two ions with the “outside” twoions to make LiNO3 and AgBr. If either of them is
insoluble,a precipitate will be formed, and the ions that react to form it willbe in our net ionic equation; the other ions are spectators and shouldbe omitted! As we said, the two possible products are lithium nitrateand silver bromide. Since halides are soluble
except those containing silver, mercury, or lead, we have a precipitate of silver bromide, and our net ionic equation looks like this:
Ag+ + Br-
AgBr
Example
Hydrochloric acid and sodium hydroxide are mixed. Write the net ionic equation.
Explanation
This is a mixture of a strong acid and a strong base, so each ionizes completely.
H+ + Cl- + Na+ + OH- 
The two possible compounds formed are sodiumchloride, which is soluble, and water, which is molecular; thus wateris the only product in our net ionic equation.
H+ + OH-
H2O
Example
Chlorine gas is bubbled into a solution of potassium iodide; write the net ionic equation.
Explanation
This one is a single replacement, so you need toconsider the activity series. Since halogens are involved, you candetermine their activity by using the periodic table: Cl is more activethan I.
Cl2 + K+ + I- 
Remember that halogen is diatomic and that all potassium compounds are soluble. The resulting compound is also soluble, so K+ is a spectator and is left out of the final equation.
Cl2 + I-
I2 + Cl
