About Aqueous Reactions

An aqueous solution is any solution where water is the dissolving medium. Any time two or more elements are dissolved in water to form new substances, you have an aqueous reaction.

There are three basic types of aqueous reactions, as indicated by the properties of the reactions involved — precipitation reactions, acid-base reactions, and oxidation-reduction reactions (or redox reactions).

In a precipitation reaction, negatively-charged ions (called anions) and positively-charged ions (called cations) present in the solution combine to form a solid precipitate.

For example, combining silver nitrate (AgNo₃) and ordinary table salt (NaCl) in water will form an aqueous solution that encourages both compounds to dissolve. The resulting anions (Cl^-) and cations (Ag⁺) will tend to precipitate together forming silver chloride (AgCl).

Ag⁺(aq) + Cl^-(aq) –> AgCl(s)

Note that the 'aq' indicates an aqueous solution, while 's' indicates a solid.

Acids are substances that tend to lose a proton (H⁺ cation), while bases tend to accept protons. (Alternately, bases tend to lose a OH^- anion.) When both are present in an aqueous solution, the anions and cations combine to form water, thus reducing the overall acidic/basic nature of the solution.

For example, consider the addition of an acid — hydrochloric acid (HCl) — and a base — sodium hydroxide (NaOH) — to water. The resulting aqueous solution will tend to dissolve equal parts of both substances in an acid-base reaction:

H⁺ + OH^- –> H₂O

FYI, the remainders of the original acid and base will combine to form table salt — NaCl.

Also known as redox reactions, these reactions involve the oxidation of one substance and the reduction of the other. (Oxidation is another term for burning, or the loss of electrons; reduction is the gaining of electrons.) Simply put, in an oxidation-reduction reaction one reactant loses electrons to another reactant.

For example, combining hydrochloric acid (HCl) and zinc (Zn) in an aqueous solution causes the zinc atoms to oxidize, or lose two electrons:

Zn(s) –> Zn²⁺ + 2e^-

The hydrochloric acid is then reduced, as hydrogen gains electrons:

2H⁺(aq) + 2e^- –> H₂(g)

The resulting ionic equation is:

Zn(s) + 2H⁺(aq) –> Zn²⁺ + H₂(g)

Two things should be noted when examining reactions in aqueous solutions.

• Spectator Ions — The net ionic equations that describe these reactions only consider the elements involved in the reaction. Thus, the redox reaction above only includes the zinc and hydrogen but excludes the chlorine. Ions that do not participate directly in the reaction (in this case, chlorine) are called spectator ions. (A complete ionic equation would include all of the reactants, including the chlorine.)
• Balanced Equation — The total charge represented on both sides of the ionic equation must equal each other, yielding a balanced equation. Check the examples above — in the precipitation reaction, both sides have a zero charge; both sides in the acid-base reaction also have a zero charge, and both sides of the oxidation-reduction reaction have a 2⁺ charge. As you can see, the charges can be zero or non-zero values, so long as they are equal on both sides.

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Click one of these links to get more help from another Cramlet in this same chapter:

• Diffusion
• Chemical Properties of Water
• About Aqueous Reactions