7.4 Acids and bases

Acids and bases are renowned for being corrosive and reactive. This can be true however, acids and bases are common substances which have important roles in industry, research, biology and the environment. For example our bodies are largely constructed from proteins which are chains of amino acids. Many foods contain acids and bases including citrus, which contains citric acid, and caffeine, which is base. A mixture of nitric and hydrochloric acid, called Aqua regia, was historically used to extract pure gold from ores and alloys. Substances which are classified as acids or bases will have certain properties. Some of these properties include

Acids (Properties) Bases (Properties)

What is an acid and what is a base?

Arrhenius acid and bases.

The definition of an acid and a base has changed throughout history because more has become known and understood about acids and bases. The first useful description of an acid and a base was produced by Svante Arrhenius. An Arrhenius acid is a substance which produces hydrogen ions (H+) in a water solution. An Arrhenius base is a substance which a substance which produces hydroxide ions (OH-) in a water solution.

Strong and weak acids and bases

It is important to note that acids are non-metal compounds which contain covalent bonding. Therefore they must ionise to produce hydrogen ions. Some acids will ionise completely and 100% of their hydrogens become ions. These are called strong acids. In a solution of strong acid there is no acid molecules only ions. Weak acids do not ionise completely. As a result acid molecules as well as ions will be in a weak acid solution.

Strong bases include metal hydroxides and metal oxides. Metal hyroxides such as NaOH dissociate completely into hydroxide ions and a metal cation in solutions. The metal oxides first react with water to produce a metal hydroxide and then completely dissociate providing 100% hydroxide ions. An example of this process if described below for MgO

MgO + H2O Mg(OH)2

Mg(OH)2 Mg2+ + 2(OH-)

Weak bases include a range of substances including metal carbonates and metal hydrogen carbonates like Na(CO3)2 and NaHCO3. These don’t directly dissociate into hydroxide ions but instead produce a small amount of hydroxide ions through a reaction with water in solutions.

CO32+ +H2O ⇄HCO3- + OH-

HCO3- + H2O ⇄ OH- + H2CO3

Reactions of acids

Acids undergo several predictable reactions with different reactants. It is important for chemists to know these reactions as they can be involved in the chemical synthesis and also used to identify known substances. For example you may recall producing carbon dioxide and hydrogen gas using acid reactions in year 9.

Neutralisation reactions

When acids and bases react they form salts and water. Pure water is neither acidic nor basic it is neutral. For this reason a reaction between and acid and a base is called a neutralisation. In a neutralisation reaction, if just the right amount of acid is mixed to the right amount of acid. There will be no excess reagent and the resulting salt solution will also be neutral.

Neutralisation reactions involve acids reacting with strong bases, so either Metal hydroxides or metal oxides. They follow these general word equations:

Acid + Metal Hydroxide Salt + Water

E.g. HCl + NaOH NaCl + H2O

Or

Acid + Oxide Salt + Water

E.g. 2HCl + MgO MgCl + H2O

Acid + metal carbonate reactions

Acids react with metal carbonates, like CaCO3, and metal hydrogen carbonates, like NaHCO3 according to the following general equation. Metal carbonates are examples of weak bases.

Acid + Metal Carbonate Carbon dioxide + Water + Salt

An example of this reaction is that between vinegar and bicarb soda:

CH3COOH + NaHCO3 CO2 + H2O + NaCH3COO

Acid and metal reactions

Acids react with metals to produce hydrogen gas and a salt. An example of this is seen between HCl and K which produces H2 gas KCl and a lot of energy which is seen as light and heat. In this specific reaction the energy from this reaction ignites the hydrogen.

Metal + Acid Hydrogen + Salt