Download 7.4 Acids and bases

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- + OHHCO3- + 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