In this question, you have to identify the ions that form the precipitate and the net ionic equation for the formation of the precipitate. The three sections below step you through this process.
Determining the Precipitate
Precipitation reactions occur because a cation from one aqueous solution can combine with an anion from the other aqueous solution in order to form an insoluble solid. Since the cation and the anion come from the two separate solutions, there are two possible cation-anion sets to consider. For sodium carbonate mixed wih iron(III) nitrate, those two sets include sodium nitrate and iron(III) carbonate. So when trying to determine the precipitate, it is important to determine which one of these two ionic compounds is insoluble in water. One of them will be insoluble; the one that is insoluble is the precipitate. To determine which one is insoluble, use the list of Solubility Rules and the same pattern of thinking that was used in the first activity - Soluble or Insoluble?.
Determining the Chemical Formula of the Precipitate
Once you determine the two ions that precipitate, you will have to identify the chemical formula of the precipitate. To do so, you will need to know the ion charges for the cation and the anion. The charge for elemental ions can be determined from its location in the periodic table ... unless it is a transition metal, lead, or tin. For transition metals, lead and tin, the charge is indicated in the name by a Roman numeral. For polyatomic ions, the charge (and symbols) can be determined using the Polyatomic Ion List (a button is provided in the Concept Builder for this).
Once you have determined ion charges, you will have to use the rule of electrical neutrality. The rule of electrical neutrality states that since the overall charge of an ionic compound is zero (neutral), the total amount of positive charge is equal to the total amount of negative charge. This rule, when combined with knowledge of the ion charges, helps one to determine the numbers of each of the ions. These numbers are included as the subscripts in the formula. The table below shows several examples:
| Cation | Anion | Formula of Compound |
|---|
| Ca2+ | Cl- | CaCl2 |
| Al3+ | Cl- | AlCl3 |
| Ca2+ | SO42- | CaSO4 |
| Al3+ | SO42- | Al2(SO4)3 |
| Ca2+ | PO43- | Ca3(PO4)2 |
Writing the Net Ionic Equation
A net ionic equation shows the reactants and products of the precipitation reaction. The reactants are the ions that combine to form the precipitate; identifying these ions was discussed above in the Determining the Precipitate section. The formula for the precipitate was discussed in the section (above) titled Determining the Chemical Formula of the Precipitate. So once you have determined the ions that are combining (along with their charges) and have determined the chemical formula of the precipitate, you are ready to identify the net ionic equation. Scan the list of five equations and identify the equation with the identified ions (and charges) and precipitate. The final consideration is the coefficients in the net ionic equation. The coefficients are used to balance charge and to balance atoms (mass). For instance, if the precipitate is Ca3(PO4)2, then there must be three ions of Ca2+ and two ions of PO43- on the reactant side of the equation.