Complete-Formula Equations

Let's work with these combinations of ions from exercise 9 in your workbook.

Ba²⁺
NO₃^-

Cu²⁺
SO₄^2-

The compound that contains both barium and nitrate ions is called barium nitrate and has the formula Ba(NO₃)₂. The compound that contains both copper(II) ions and sulfate ions is called copper(II) sulfate (or cupric sulfate) and has the formula CuSO₄.

_{barium nitrate}	_{copper(II) sulfate}
Ba(NO₃)₂	CuSO₄

When these compounds react with one another we get a double displacement reaction and the products are barium sulfate and copper(II) nitrate. The formulas are BaSO₄ and Cu(NO₃)₂.

Ba(NO₃)₂ + CuSO₄ BaSO₄ + Cu(NO₃)₂

To balance this equation we have to make sure that we have the same number of bariums, nitrates, coppers, and sulfates on each side of the equation. We do, so the equation is balanced.

Please note that it is a coincidence that each formula contains two nitrate ions. This is because both barium and copper ions have a charge of +2.

Precipitation reactions are often represented using complete-formula equations. Their advantage is that they show the compounds that are involved. Their disadvantage is that they do not show the forms of the chemicals that are actually involved in the reaction. (Sometimes these equations are called "molecular" equations, even though the compounds are ionic rather than molecular.)