8.4 Ionic Equations

The chemical equations we have discussed so far have shown the identities and the number of formula units of the reactants and the products, but they told us very little about what was occurring in solution. In contrast, equations that show only the hydrated species focus our attention on the chemistry that is taking place and allow us to see similarities between reactions that might not otherwise be apparent.

Let’s consider the reaction of silver nitrate with potassium dichromate. When aqueous solutions of silver nitrate and potassium dichromate are mixed, silver dichromate forms as a red solid. The overall chemical equationA chemical equation that shows all the reactants and products as undissociated, electrically neutral compounds. for the reaction shows each reactant and product as undissociated, electrically neutral compounds:

Although Equation 8.4(eq1) gives the identity of the reactants and the products, it does not show the identities of the actual species in solution. Because ionic substances such as AgNO₃ and K₂Cr₂O₇ are strong electrolytes, they dissociate completely in aqueous solution to form ions. In contrast, because Ag₂Cr₂O₇ is not very soluble, it separates from the solution as a solid. To find out what is actually occurring in solution, it is more informative to write the reaction as a complete ionic equationA chemical equation that shows which ions and molecules are hydrated and which are present in other forms and phases., showing which ions and molecules are hydrated and which are present in other forms and phases:

Note that K⁺(aq) and NO₃⁻(aq) ions are present on both sides of the equation, and their coefficients are the same on both sides. These ions are called spectator ionsIons that do not participate in the actual reaction. because they do not participate in the actual reaction. Canceling the spectator ions gives the net ionic equationA chemical equation that shows only those species that participate in the chemical reaction., which shows only those species that participate in the chemical reaction:

Both mass and charge must be conserved in chemical reactions because the numbers of electrons and protons do not change. For charge to be conserved, the sum of the charges of the ions multiplied by their coefficients must be the same on both sides of the equation. In Equation 8.4(eq3), the charge on the left side is 2(+1) + 1(−2) = 0, which is the same as the charge of a neutral Ag₂Cr₂O₇ formula unit.

By eliminating the spectator ions, we can focus on the chemistry that takes place in a solution. For example, the overall chemical equation for the reaction between silver fluoride and ammonium dichromate is as follows:

The complete ionic equation for this reaction is as follows:

Because two NH₄⁺(aq) and two F⁻(aq) ions appear on both sides of Equation 8.4(eq5), they are spectator ions. They can therefore be canceled to give the net ionic equation (Equation 8.4(eq6)), which is identical to Equation 8.4(eq3):

If we look at net ionic equations, it becomes apparent that many different combinations of reactants can result in the same net chemical reaction. For example, we can predict that silver fluoride could be replaced by silver nitrate in the preceding reaction without affecting the outcome of the reaction.

If we write the complete ionic equation for the reaction in Equation 8.4(eq7), we see that Na⁺(aq) and Br⁻(aq) are spectator ions and are not involved in the reaction:

The overall reaction is therefore simply the combination of H⁺(aq) and OH⁻(aq) to produce H₂O, as shown in the net ionic equation:

The net ionic equation for the reaction of any strong acid with any strong base is identical to Equation 8.4(eq8).

In the case of the reaction of a weak acid with a strong base, the ionic equation needs to reflect the fact that the weak acid stays mostly undissociated. For this reason, the formula of the weak acid is left whole and not written as ions as was the case for the strong acid reactions. Consider the reaction of nitrous acid with sodium hydroxide shown in Equation 8.4(eq9)