16.4. Net Ionic Equations http://www.ck12.org
Net ionic equation:
3Cu^2 +(aq)+2PO^34 −(aq)→Cu 3 (PO 4 ) 2 (s)Step 3: Think about your result.
For a precipitation reaction, the net ionic equation always shows the two ions that come together to form the
precipitate. The equation is balanced by mass and charge.
Practice Problem- Write the net ionic equation for the reaction of calcium nitrate with lithium hydroxide. The products are
aqueous lithium nitrate and a calcium hydroxide precipitate.
Some other double-replacement reactions do not produce a precipitate as one of the products. The production of a
gas and/or a molecular compound such as water may also drive the reaction. For example, consider the reaction of
a solution of sodium carbonate with a solution of hydrochloric acid (HCl). The products of the reaction are aqueous
sodium chloride, carbon dioxide, and water. The balanced molecular equation is:
Na 2 CO 3 (aq)+2HCl(aq)→2NaCl(aq)+CO 2 (g)+H 2 O(l)The ionic equation is:
2Na+(aq)+CO^23 −(aq)+2H+(aq)+2Cl−(aq)→2Na+(aq)+2Cl−(aq)+CO 2 (g)+H 2 O(l)
The sodium and chloride ions are spectator ions, making the final net ionic equation:
2H+(aq)+CO^23 −(aq)→CO 2 (g)+H 2 O(l)You will obtain the correct net ionic equation for any reaction as long as you follow the steps in the examples.
A single-replacement reaction is one in which a pure, neutral element replaces another element in a compound. A
neutral element would not carry a charge, so it will not be a spectator ion. The example below shows the reaction of
solid magnesium metal with aqueous silver nitrate to form aqueous magnesium nitrate and silver metal.
Balanced molecular equation:
Mg(s)+2AgNO 3 (aq)→Mg(NO 3 ) 2 (aq)+2Ag(s)Ionic equation:
Mg(s)+2Ag+(aq)+2NO− 3 (aq)→Mg^2 +(aq)+2NO− 3 (aq)+2Ag(s)The only spectator ion is the nitrate ion, so the net ionic equation is:
Mg(s)+2Ag+(aq)→Mg^2 +(aq)+2Ag(s)Notice that the overall charge on both sides of the equation is now +2, instead of zero, like it was in the previous
examples. This is typical for a single-replacement reaction. Because both sides of the reaction carry the same
total charge, it is still balanced. This type of single-replacement reaction is called a metal replacement. Other
common categories of single-replacement reactions are hydrogen replacement and halogen replacement. These
were discussed in the chapter,Chemical Reactions.