Summary and Main Ideas

Summary

• The Arrhenius definition of acids and bases defines an acid as a substance that increases the concentration of hydronium ions \((\text{H}_{3}\text{O}^{+})\) in a solution. A base is defined as a substance that increases the concentration of hydroxide ions \((\text{OH}^{-}\)) in a solution. However, this definition only applies to aqueous solutions (in water).

• The Brønsted-Lowry definition is much broader. An acid is a substance that donates protons (\(\text{H}^{+}\)) and a base is a substance that accepts protons.

• In different reactions, certain substances can act as both an acid and a base. These substances are amphoteric substances. Amphiprotic substances are amphoteric substances that are Brønsted-Lowry acids and bases. Water is both amphoteric and amphiprotic.

• A conjugate acid-base pair refers to two compounds (one reactant and one product) that differ only by a hydrogen ion (\(\text{H}^{+}\)) and a charge of +1.

• A large percentage of molecules in a strong acid or base dissociate or ionise to form ions in solution.

• Only a small percentage of molecules in a weak acid or base dissociate or ionise to form ions in solution.

• In a concentrated solution there is a high ratio of dissolved substance to solvent.

• In a dilute solution there is a low ratio of dissolved substance to solvent.

• \(\text{K}_{\text{a}}\) and \(\text{K}_{\text{b}}\) are the equilibrium constants for the reaction of an acid or a base with water. A large \(\text{K}_{\text{a}}\) or \(\text{K}_{\text{b}}\) means that the acid or base is strong. A small \(\text{K}_{\text{a}}\) or \(\text{K}_{\text{b}}\) means that the acid or base is weak.

• When an acid and a base react, they form a salt and water. The salt is made up of a cation from the base and an anion from the acid. An example of a salt is sodium chloride \((\text{NaCl})\), which is the product of the reaction between sodium hydroxide \((\text{NaOH})\) and hydrochloric acid \((\text{HCl})\).

• The reaction between an acid and a base is a neutralisation reaction.

• In the reaction between an acid and a metal the products are a salt and hydrogen.

• In the reaction between an acid and a metal hydroxide or metal oxide the products are a salt and water.

• In the reaction between an acid and a metal carbontae or metal hydrogen carbonate the products are a salt, water and carbon dioxide.

• The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from \(\text{0}\) to \(\text{14}\). Values greater than \(\text{7}\) indicate a base, while those less than \(\text{7}\) indicate an acid.

• Water ionises to a small extent. \(\textbf{K}_{\textbf{w}}\) is a measure of this auto-ionisation. \(\text{K}_{\text{w}}\) is \(\text{1} \times \text{10}^{-\text{14}}\) at \(\text{25}\) \(\text{℃}\).

• An indicator is a compound that is a different colour in a basic solution, an acidic solution, and at the end-point of a reaction. They are used to determine the end-point during a neutralisation reaction.

• Titrations are the method used to determine the concentration of a known substance using another, standard, solution. Acid-base titrations are an example.

• Two notable applications of acids and bases are in the chloralkali industry, and in hair products including permanent waving applications, hair relaxers, and hair dyes.