# 16: Acid–Base Equilibria

Acids and bases have been defined differently by three sets of theories. One is the Arrhenius definition, which revolves around the idea that acids are substances that ionize (break off) in an aqueous solution to produce hydrogen (H+) ions while bases produce hydroxide (OH-) ions in solution. On the other hand, the Bronsted-Lowry definition defines acids as substances that donate protons (H+) whereas bases are substances that accept protons. Also, the Lewis theory of acids and bases states that acids are electron pair acceptors while bases are electron pair donors. Acids and bases can be defined by their physical and chemical observations.

• 16.1: Acids and Bases - A Brief Review
In chemistry, acids and bases have been defined differently by three sets of theories: One is the Arrhenius definition defined above, which revolves around the idea that acids are substances that ionize (break off) in an aqueous solution to produce hydrogen (H+) ions while bases produce hydroxide (OH-) ions in solution. The other two definitions are discussed in detail alter in the chapter and include the Brønsted-Lowry definition and the Lewis theory.
• 16.2: Brønsted–Lowry Acids and Bases
A compound that can donate a proton (a hydrogen ion) to another compound is called a Brønsted-Lowry acid. The compound that accepts the proton is called a Brønsted-Lowry base. The species remaining after a Brønsted-Lowry acid has lost a proton is the conjugate base of the acid. The species formed when a Brønsted-Lowry base gains a proton is the conjugate acid of the base. Thus, an acid-base reaction occurs when a proton is transferred from an acid to a base.
• 16.3: The Autoionization of Water
Water is amphiprotic: it can act as an acid by donating a proton to a base to form the hydroxide ion, or as a base by accepting a proton from an acid to form the hydronium ion ($$H_3O^+$$). The autoionization of liquid water produces $$OH^−$$ and $$H_3O^+$$ ions. The equilibrium constant for this reaction is called the ion-product constant of liquid water (Kw) and is defined as $$K_w = [H_3O^+][OH^−]$$. At 25°C, $$K_w$$ is $$1.01 \times 10^{−14}$$; hence $$pH + pOH = pK_w = 14.00$$.
• 16.4: The pH Scale
The concentration of hydronium ion in a solution of an acid in water is greater than $$1.0 \times 10^{-7}\; M$$ at 25 °C. The concentration of hydroxide ion in a solution of a base in water is greater than $$1.0 \times 10^{-7}\; M$$ at 25 °C. The concentration of H3O+ in a solution can be expressed as the pH of the solution; $$\ce{pH} = -\log \ce{H3O+}$$. The concentration of OH− can be expressed as the pOH of the solution: $$\ce{pOH} = -\log[\ce{OH-}]$$. In pure water, pH = 7 and pOH = 7.
• 16.5: Strong Acids and Bases
Acid–base reactions always contain two conjugate acid–base pairs. Each acid and each base has an associated ionization constant that corresponds to its acid or base strength. Two species that differ by only a proton constitute a conjugate acid–base pair. The magnitude of the equilibrium constant for an ionization reaction can be used to determine the relative strengths of acids and bases.
• 16.6: Weak Acids
The strengths of Brønsted-Lowry acids and bases in aqueous solutions can be determined by their acid or base ionization constants. Stronger acids form weaker conjugate bases, and weaker acids form stronger conjugate bases. Thus strong acids are completely ionized in aqueous solution because their conjugate bases are weaker bases than water. Weak acids are only partially ionized because their conjugate bases are strong enough to compete successfully with water for possession of protons.
• 16.7: Weak Bases
The pH of a solution of a weak base can be calculated in a way which is very similar to that used for a weak acid. Instead of an acid constant Ka, a base constant Kb must be used.
• 16.8: Relationship Between Ka and Kb
• 16.9: Acid-Base Properties of Salt Solutions
A salt can dissolve in water to produce a neutral, a basic, or an acidic solution, depending on whether it contains the conjugate base of a weak acid as the anion ( A−A− ), the conjugate acid of a weak base as the cation ( BH+ ), or both. Salts that contain small, highly charged metal ions produce acidic solutions in water. The reaction of a salt with water to produce an acidic or a basic solution is called a hydrolysis reaction.
• 16.10: Acid-Base Behavior and Chemical Structure
Inductive effects and charge delocalization significantly influence the acidity or basicity of a compound. The acid–base strength of a molecule depends strongly on its structure. The weaker the A–H or B–H+ bond, the more likely it is to dissociate to form an $$H^+$$ ion. In addition, any factor that stabilizes the lone pair on the conjugate base favors the dissociation of $$H^+$$, making the conjugate acid a stronger acid.
• 16.11: Lewis Acids and Bases
Lewis proposed that the electron pair is the dominant actor in acid-base chemistry. An Lewis acid is a substance that accepts a pair of electrons, and in doing so, forms a covalent bond with the entity that supplies the electrons. A Lewis base is a substance that donates an unshared pair of electrons to a recipient species with which the electrons can be shared. Lewis acis/base theory is a powerful tool for describing many chemical reactions used in organic and inorganic chemistry.
• 16.E: Acid–Base Equilibria (Exercises)
These are homework exercises to accompany the Textmap created for "Chemistry: The Central Science" by Brown et al.
• 16.S: Acid–Base Equilibria (Summary)