Question

What are the products for the following reaction: `Na_3PO_4 + CaCl_2 -> ?`

Answer 1

The products are soluble sodium chloride, `"NaCl"`, and the insoluble precipitate calcium phosphate, `"Ca"_3"(PO"_4)_2`.

Explanation:

Balanced equation

`2color(blue)("Na"_3)color(red)("PO"_4)("aq")+3color(orange)("Ca")color(green)("Cl"_2)("aq")` `rarr` `color(orange)("Ca"_3)color(red)(("PO"_4))_color(red)(2)("s")+6color(blue)("Na")color(green)("Cl")("aq")+`

The products of this reaction can be predicted because this is a double replacement reaction, also called a double displacement, and if a precipitate forms, a metathesis. These reactions can also form an insoluble gas that bubbles out of solution, or water in a neutralization reaction.

The cations in the above reaction are the sodium and calcium ions, `"Na"^+"` and `"Ca"^(2+)`. The anions are the phosphate ion, `"PO"_4"^(3-)`, and the nitrate ion, `"NO"_3"^(-)`.

One of the two products is a precipitate because it is insoluble in water. You can determine which is the precipitate by using a list or table of solubility rules, such as the one below.

Notice that you first look at the anions, and then the exceptions involving cations. The anions in the products are `"Cl"^(-)` and `"PO"_4"^(3-)`. If you look at the Soluble Ionic Compounds , most `"Cl"^(-)` compounds, with a few exceptions, are soluble in water. Therefore, `"NaCl"` is not one of the exceptions, so it is soluble in water, and is not the precipitate.

The other anion is `"PO"_4"^(3-)`. It's listed under Insoluble Ionic Compounds . Most `"PO"_4"^(3-)` compounds are insoluble in water, with exceptions being the alkali metals (group 1 on the periodic table), and the ammonium cation, `"NH"_4"`. Since `"Ca"` is not an alkali metal, the compound `"Ca"_3"(PO"_4)_2"` is insoluble in water and will precipitate out of solution as a solid.