Writing Precipitation Equations

This page shows the procedure for predicting whether mixing two aqueous solution of ionic compounds will lead to a precipitation reaction and shows you how to write complete and net ionic equations for the reactions that take place. The following is a typical problem. 

Predict whether a precipitate will form when water solutions of silver nitrate, AgNO3(aq), and sodium sulfide, Na2S(aq), are mixed.  If there is a precipitation reaction, write the complete and net ionic equation that describes the reaction.

 

Study Sheet

Tip-off – When you are asked to predict whether a precipitation reaction takes place when two aqueous solutions of ionic compounds are mixed and to write complete and net ionic equations for the reaction, if it takes place. 

 

General Steps  

Step 1:   Determine the formulas for the possible products using the general double displacement equation. (Remember to consider ion charges when writing your formulas.)

AB  +  CD        AD  +  CB  

Step 2:    Predict whether either of the possible products is water insoluble. If either possible product is insoluble, a precipitation reaction takes place, and you will continue with step 3. If neither is insoluble, write “No reaction”.

Step 3:    Follow these steps to write the complete equation.

Write the formulas for the reactants separated by a “+”.

Separate the formulas for the reactants and products with a single arrow.

Write the formulas for the products separated by a “+”.

Write the physical state for each formula.

1) The insoluble product will be followed by (s).

2) Water-soluble ionic compounds will be followed by (aq).

Balance the equation.

Step 4:    Follow these steps to write the net ionic equation.

Write the complete ionic equation by describing water-soluble ionic compounds as separate ions and insoluble ionic compounds with a complete formula.

Eliminate the formulas for the ions that are unchanged in the reaction (the spectator ions).

Rewrite what is left after the spectator ions are removed.

Balance the equation.

EXAMPLE 1 – Predicting Precipitation Reactions:  Predict whether a precipitate will form when water solutions of silver nitrate, AgNO3(aq), and sodium sulfide, Na2S(aq), are mixed.  If there is a precipitation reaction, write the complete and net ionic equation that describes the reaction.

Solution:

Step 1:  Determine the possible products using the general double displacement equation.  

AB  +  CD        AD  +  CB

In AgNO3, Ag+ is A, and NO3 is B. In Na2S, Na+ is C, and S2− is D. The possible products from the mixture of AgNO3(aq) and Na2S(aq) are Ag2S and NaNO3. (Remember to consider charge when you determine the formulas for the possible products.)

AgNO3(aq) + Na2S(aq)  to  Ag2S and NaNO3

Step 2:    Predict whether either of the possible products is water insoluble.

According to our solubility guidelines, most sulfides are insoluble, and compounds with Ag+ are not listed as an exception. Therefore, Ag2S would be insoluble. Because compounds containing Na+ and NO3 are soluble, NaNO3 is soluble.

Step 3:    Write the complete equation. (Don’t forget to balance the equation.)

2AgNO3(aq) + Na2S(aq)    Ag2S(s) + 2NaNO3(aq)

Step 4:    Write the net ionic equation.

Write the complete ionic equation, describing the aqueous ionic compounds, AgNO3(aq), Na2S(aq) and NaNO3(aq), as ions. Describe the solid with a complete formula.

2Ag+(aq)  +  2NO3(aq)  +  2Na+(aq)  +  S2−(aq)   
                    →    Ag2S(s)  +  2Na+(aq)  +  2NO3(aq)

The nitrate and sodium ions have the same form on each side of the equation, so they are eliminated as spectator ions.

2Ag+(aq)  +  S2−(aq)    Ag2S(s)

EXAMPLE 2 – Predicting Precipitation Reactions:  Predict whether a precipitate will form when water solutions of barium chloride, BaCl2(aq), and sodium sulfate, Na2SO4(aq), are mixed.  If there is a precipitation reaction, write the complete and net ionic equation that describes the reaction.

Solution:

Step 1:  Determine the possible products using the general double displacement equation.  

AB  +  CD       AD  +  CB

In BaCl2, A is Ba2+, and B is Cl. In Na2SO4, C is Na+, and D is SO42−. The possible products from the reaction of BaCl2(aq) and Na2SO4(aq) are BaSO4 and NaCl. (Remember to consider charge when you determine the formulas for the possible products.)

BaCl2(aq) + Na2SO4(aq)  to  BaSO4 and NaCl

Step 2:    Predict whether either of the possible products is water insoluble.

According to our solubility guidelines, most sulfates are soluble, but BaSO4 is an exception. It is insoluble and would precipitate from the mixture. Because compounds containing Na+ and Cl are soluble, NaCl is soluble. 

Step 3:    Write the complete equation. (Don’t forget to balance the equation.)

BaCl2(aq)  +  Na2SO4(aq) 

                            →   BaSO4(s)  +  2NaCl(aq)

Step 4:    Write the complete ionic equation, describing the aqueous ionic compounds as ions. Describe the solid as a complete formula.

Ba2+(aq)  +  2Cl(aq)  +  2Na+(aq)  +  SO42−(aq)   
                  BaSO4 (s)  +  2Na+(aq)  +  2Cl(aq)

The chloride and sodium ions have the same form on each side of the equation, so they are eliminated as spectator ions.

Ba2+(aq)  +  SO42−(aq)        BaSO4(s)

This is the reaction used in industry to form barium sulfate, which is used in paint preparations and in x-ray photography.