Equations for Precipitation Reactions
Home Up Predicting Precipitation Reactions Equations for Precipitation Reactions

 

Answers to Exercise 6
Answers to Exercise 7

Writing Equations for Precipitation Reactions

Precipitation reactions can be represented using several types of chemical equations: complete-formula equations (also known as "molecular" equations), complete ionic equations, and net ionic equations.  Each provides a different perspective on the chemicals involved in the reaction.

An example of how to write each type of equation is given below using one of the combinations from exercise 5 in your workbook.

Complete-Formula Equations

Let's work with these combinations of ions from exercise 5 in your workbook.
Ba2+
NO3-		 Cu2+
SO42-		    
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 rtarrow.gif (850 bytes) 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.)

Now please do exercise 6 in your workbook and check your answers before continuing with the lesson.

 

Complete Ionic Equations

Complete ionic equations represent soluble ionic compounds as separated ions.

Ba(NO3)2 + CuSO4 rtarrow.gif (850 bytes) BaSO4(ppt) + Cu(NO3)2
Ba2+ + 2 NO3- + Cu2+ + SO42- rtarrow.gif (850 bytes) BaSO4(ppt) + Cu2+ + 2 NO3-

In the example shown above, barium nitrate exists in solution as separate barium ions and nitrate ions.  Copper(II) sulfate exists in solution as separate copper(II) ions and sulfate ions. After the barium and sulfate ions combine to form the solid barium sulfate precipitate, the copper(II) ions and the nitrate ions remain in solution. This representation is called the complete ionic equation. The barium and sulfate ions actually react to form the barium sulfate product. The copper(II) ions and the nitrate ions are not actually involved in the reaction. They are called spectator ions.

 

Net Ionic Equations

If we eliminate the spectator ions from the complete ionic equation we have what is called the net ionic equation. It shows only the ions that are actually involved in the reaction.

Ba2+ + 2 NO3- + Cu2+ + SO42-  rtarrow.gif (850 bytes) BaSO4(ppt) + Cu2+ + 2 NO3-
Ba2+ + SO42- rtarrow.gif (850 bytes) BaSO4(ppt)

Take some time now to do exercise 7 and write the net ionic equations for the reactions you have been working with. Check your answers before continuing with the lesson.

 

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