Net Ionic Equations
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 ionic equations on
the SAT II Chemistry test, you’ll need to remember the following
solubility rules, so memorize them! Also keep in mind that net ionic
equations, which are the bare bones of the chemical reaction, usually
take place in aqueous environments. Here are those solubility rules:
Most alkali metal compounds and compounds are soluble.
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
- compounds are soluble, except
when they include Ca2+, Sr2+,
Ba2+, Ag+, Pb2+,
- , , , , S2-,
OH-, and O2- compounds
2A metal oxides are classified as strong bases even though they
are not very soluble.
The two solubility rules that you will use the most are
numbers 1 and 4. You must memorize that all group 1A metal and 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, all nitrates 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 it doesn’t
react and should be left out of the net ionic equation. The key is
first to write the compound’s chemical formula and then determine
if it’s soluble. If it is soluble, then ionize it—if it isn’t, don’t
ionize 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
with water, they form either acids (nonmetal oxides) or bases (metal
- 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
of group 1A and 2A metals. All other oxides and hydroxides are considered
weak and written together, as molecules.
Now try writing some net ionic equations, using the rules
Write the net ionic equation for a mixture of solutions
of silver nitrate and lithium bromide.
Ag+ + + Li+ + Br-
This is a double replacement reaction. Both compounds
are soluble, so everything ionizes. If anything is formed, it will
come from recombining the “inside” two ions with the “outside” two
ions to make LiNO3 and AgBr. If either of
them is insoluble, a precipitate will be formed,
and the ions that react to form it will be in our net ionic equation;
the other ions are spectators and should be omitted! As we said,
the two possible products are lithium nitrate and 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
Hydrochloric acid and sodium hydroxide are mixed. Write
the net ionic equation.
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 sodium chloride,
which is soluble, and water, which is molecular; thus water is the
only product in our net ionic equation.
H+ + OH-H2O
Chlorine gas is bubbled into a solution of potassium iodide;
write the net ionic equation.
This one is a single replacement, so you need to consider
the activity series. Since halogens are involved, you can determine
their activity by using the periodic table: Cl is more active than
Cl2 + K+ +
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-