WRITE-UP

You will analyze the data for only three of the four solution sets. You and your partner will be assigned Sets I, II, and IV or Sets I, III, and IV. For each solution combination (within your assigned sets) that formed a precipitate, write a complete ionic equation and a balanced net ionic equation. Write your equations on the corresponding data (grid) sheet. Your lab report will consist of your objective, procedure, observations/data (for all four solution sets) and equations for three solution sets and your conclusions.

Hints for Writing Net Ionic Equations

1. A molecular equation depicts a chemical reaction by showing the complete formulas for all species involved in the reaction. For example:

Na2S(aq) + Fe(NO3)2(aq) ----> FeS(s) + 2NaNO3(aq)

(Eq.1)

2. The molecular equations are useful in stoichiometry where complete formulas of compounds are required for calculations. However, molecular equations do not accurately describe the chemical reactions that take place in solutions.

3. Complete ionic equations depict all of the chemical species present in the solution. The complete ionic equation represents species as ions if they ionize in solution. The complete ionic equation for the reaction above should be written as equation (2):

2Na+(aq) + S2-(aq) + Fe2+(aq) + 2NO3-(aq) ---> FeS(s) + 2Na+(aq) + 2NO3-(aq)

(Eq.2)

4. Those species that ionize in aqueous solution are:

5. Those species that do not ionize in aqueous solution are:

6. The complete ionic equation (2) was derived from the molecular equation (1). The phase designations help to determine which species ionize and which do not. For example, Na₂S, Fe(NO₃)₂, and NaNO₃ are all soluble salts (indicated by the (aq) designation). The FeS was followed by a (s) phase designation indicating an insoluble salt or a solid that does not ionize. Be aware that not all species indicated as aqueous, (aq), will ionize. For example, H₃PO₄(aq) is a weak acid that, according to the guidelines above, should not be written as separate ions in an ionic equation.

7. Finally, the net ionic equation includes only those ions which show the essence of the reaction. Ions that appear unchanged (i.e. appear exactly the same on both sides of the equation) in the reaction are called spectator ions. All spectator ions are canceled from both sides of the equation to arrive at the net ionic equation. The net ionic equation for equation (1) is given as equation (3):

S2-(aq) + Fe2+(aq) ----> FeS(s)

(Eq.3)

8. Note that the phase indicator (aq) is optional on ionic species but is required on weak acids, weak bases and other soluble, non-dissociating molecules (i.e. sugar - C₆H₁₂O₆). These systematic steps may be followed to write any net ionic equation for any chemical reaction.