Chemical Reactions in Aqueous Solutions

Many chemical reactions occur in aqueous solutions. These reactions fall into 3 main categories: precipitation, acid–base, and oxidation–reduction.

In a typical precipitation reaction, dissolved cations and anions come together to form an insoluble ionic compound that precipitates from the solution. For example, the reaction between aqueous potassium chloride and aqueous silver nitrate leads to the precipitation of insoluble silver chloride.

In an acid–base reaction, the acid and base neutralize each other, such as when a proton (H⁺ ion) from an acid and a OH⁻ ion from a base react to form water. The counterions typically react to form a salt. For example, hydrogen bromide reacts with potassium hydroxide to give water and potassium bromide.

In an oxidation–reduction, or redox, reaction, electrons are transferred from one reactant to another. For example, an atom of potassium loses one electron to an atom of chlorine when they combine to form potassium chloride.

When a chemical reaction occurs in an aqueous solution, water-soluble molecular solids dissolve as intact molecules, whereas water-soluble ionic solids exist as separated ions. Ionic solids that are water-insoluble remain undissolved.

While writing equations for reactions that occur in aqueous solution, the nature of the dissolved substance must be indicated.

Consider the precipitation reaction between aqueous lead(II) nitrate and aqueous sodium iodide to form solid lead(II) iodide and aqueous sodium nitrate.

The balanced equation is called the molecular equation because the complete neutral formulas of the compounds are written as if they existed as molecules or whole units in solution.

Water-soluble ionic compounds such as lead(II) nitrate, sodium iodide, and sodium nitrate exist as ions in solution, whereas water-insoluble lead(II) iodide exists as an ionic solid.

The molecular equation can, therefore, be rewritten as a complete ionic equation, which shows soluble ionic species as free ions in solution.

The ions that appear in identical forms on both sides of the complete ionic equation are called spectator ions. Canceling out spectator ions leaves the net ionic equation, which includes only the ions and molecules that actually participate in the reaction.