Example #1

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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. Silver sulfide is used to make ceramics. Because compounds containing Na+ and NO3- are soluble, NaNO3 is soluble.

Step #3:    Write the complete equation. (Dont 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)

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