Much important chemistry, including virtually all of the reactions that make life possible, occurs in aqueous solutions. We will now consider the types of equations used to represent reactions that occur in water. For example, as we saw earlier, when we mix aqueous potassium chromate with aqueous barium nitrate, a reaction occurs to form solid barium chromate and dissolved potassium nitrate. One way to represent this reaction is by the equation
This is called the molecular equation for the reaction; it shows the complete formulas of all reactants and products. However, although this equation shows the reactants and products of the reaction, it does not give a very clear picture of what actually occurs in solution. As we have seen, aqueous solutions of potassium chromate, barium nitrate, and potassium nitrate contain the individual ions, not molecules as is implied by the molecular equation. Thus the complete ionic equation,
better represents the actual forms of the reactants and products in solution. In a complete ionic equation, all substances that are strong electrolytes are represented as ions. Notice that BaCrO4 is not written as the separate ions, because it is present as a solid; it is not dissolved.
The complete ionic equation reveals that only some of the ions participate in the reaction. Notice that the K and NO3 ions are present in solution both before and after the reaction. Ions such as these, which do not participate directly in a reaction in solution, are called spectator ions. The ions that participate in this reaction are the Ba^2 and CrO4^2 ions, which combine to form solid BaCrO4:
The complete ionic equation reveals that only some of the ions participate in the reaction. Notice that the K and NO3 ions are present in solution both before and after the reaction. Ions such as these, which do not participate directly in a reaction in solution, are called spectator ions. The ions that participate in this reaction are the Ba^2 and CrO4^2 ions, which combine to form solid BaCrO4:
This equation, called the net ionic equation, includes only those components that are directly involved in the reaction. Chemists usually write the net ionic equation for a reaction in solution, because it gives the actual forms of the reactants and products and includes only the species that undergo a change.