Inorganic chemistry

(CC) Photo: Axel Lankhorst
A chemistry laboratory.

Inorganic chemistry is a subdiscipline of chemistry involving the scientific study of the properties and reactions of all chemical elements and chemical compounds other than the vast number of organic compounds (compounds containing at least one carbon-hydrogen covalent bond).

There are a number of subdivisions of inorganic chemistry such as the four subdivisions of the American Chemical Society's Division of Inorganic Chemistry, namely organometallic chemistry, bioinorganic chemistry, solid-state chemistry and nanoscience.

Inorganic chemistry is closely related to other disciplines such as materials science, earth science, mineralogy, geology and crystallography.

Distinctions between inorganic and organic chemistry

The distinction or boundary between inorganic chemistry and organic chemistry is not very well defined. In general, the above definition of inorganic chemistry seemingly excludes carbon compounds but it does not exclude elemental carbon itself. Hence, carbon oxides, carbon sulfides, cyanides and cyanates, metallic carbides and carbonates are included as inorganic compounds.

As another example of the ill-defined distinction between inorganic and organic chemistry, oxalic acid (H₂C₂0₄) is commonly considered to be an organic compound even though it does not contain a carbon-hydrogen bond.

Classification of inorganic compounds

Inorganic chemistry encompasses a very complicated variety of substances which the distinguished American chemist, F. Albert Cotton (1930 − 2007), grouped into these four classes:

The chemical elements: These have a variety of structure and properties and include:

• Atomic gases such argon (Ar) and krypton (Kr) , as well as molecular gases such as hydrogen (H₂) and oxygen (O₂).
• Molecular solids such as the phosphorus allotrope (P₄), the sulphur allotrope (S₈), and the carbon allotrope (C₆₀).
• Network solids such as diamond and graphite.
• Metals, either solid such as copper (Cu) and tungsten (W) or liquid such as mercury (Hg) and gallium (Ga).

Ionic compounds: These are always solids at reference conditions of 0 °C temperature and 101.325 kPa absolute pressure and include:

• Simple ionic compounds such as sodium chloride (NaCl), which are soluble in water or other polar solvents.
• Ionic oxides that are insoluble in water, such as zirconium oxide (ZrO₂) and mixed oxides such as the mineral "spinel" (MgAl₂O₄), the mineral "diopside" (CaMg(SiO₃)₂) and various silicates.
• Other binary halides, carbides, arsenides, nitrides and similar materials. A few examples are silver chloride (AgCl), silicon carbide (SiC), gallium arsenide (GaAs), and boron nitride (BN), some of which could also be considered to be network solids.
• Compounds containing polyatomic ions (also called "complex ions") such as the silicon hexafluoride anion [SiF₆] ²⁻, the cobalt hexammine complex cation [Co(NH₃) ₆] ³⁺ and the ferricyanide anion [Fe(CN) ₆] ³⁻.

Molecular compounds: These may be solids, liquids or gases and include:

• Simple binary compounds such as phosphorus trifluoride (PF₃), sulfur dioxide (SO₂) and osmium tetroxide (OsO₄).
• Organometallic compounds that characteristically have metal−to−carbon bonds such as nickel carbonyl (Ni(CO)₄) and tetra-benzyl-zirconium (Zr(CH₂C₆H₅)₄).
• Complex metal-containing compounds.

Inorganic polymers and superconductors: These include numerous and varied inorganic polymers and superconductors. One example of an inorganic polymer has the chemical formula of YBa₂Cu₃O₇.

Typical inorganic chemical reactions

There is no universally accepted list of the typical, important inorganic reactions. Although there are numerous available sources (books, journal and Internet websites) that include such lists, they all differ to some extent from each other. The inorganic reaction types listed and explained below were drawn from many of the available sources:

Synthesis reaction: (also referred to as combination or composition reaction)

This is a reaction in which two or more reactants combine to form a single product, where each reactant is a chemical element or compound and the reaction product consist of the two reactants. Examples include:

• sodium + chlorine → sodium chloride

2Na + Cl₂ → 2NaCl

• carbon dioxide + water → carbonic acid

CO₂ + H₂O → H₂CO₃

• hydrogen + sulphur → hydrogen sulphide

H₂ + S → H₂S

Decomposition reaction: (may be thermal, electrolytic or catalytic decomposition reaction)

This is a reaction in which a chemical compound is separated into elements or simpler compounds. It is often defined as being the opposite of a synthesis reaction. Examples include:

• hydrogen peroxide → water + oxygen     (Hydrogen peroxide spontaneously decomposes into water and gaseous oxygen.)

2H₂O₂ → 2H₂O + O₂

• calcium carbonate + heat → calcium oxide + carbon dioxide     (When heated, calcium carbonate decomposes into calcium oxide and gaseous carbon dioxide.)

CaCO₃ + heat → CaO + CO₂

Single displacement reaction: (also referred to as substitution or single replacement reaction)

This is a reaction characterized by one element being displaced from a compound by another element. Examples include:

• copper + hydrochloric acid → cupric chloride + hydrogen

Cu + 2HCl → CuCl₂ + H₂

• zinc + cupric sulfate → copper + zinc sulfate

Zn + CuSO₄ → Cu + ZnSO₄

Metathesis reaction: (also referred to as exchange or double displacement or double replacement reaction)

This is a reaction in which two compounds exchange bonds or ions to form new, different compounds. Examples include:

• sodium sulfate + barium chloride → barium sulfate + sodium chloride

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

• silver nitrate + hydrochloric acid → nitric acid + silver chloride

AgNO₃ + HCl → HNO₃ + AgCl

Precipitation reaction: (a specific type of metathesis referred to as aqueous metathesis)

This is a reaction that occurs when two inorganic salt solutions, as in the example below, react to form a solution containing a soluble product and another product that is insoluble and precipitates out of the solution:

• calcium chloride + silver nitrate → calcium nitrate + silver chloride     (Insoluble silver chloride precipitates out of the aqueous solution.)

CaCl₂ (aq) + 2AgNO₃ (aq) → Ca(NO₃)₂ (aq) + 2AgCl (s)

Neutralization reaction: (another specific type of metathesis that is sometimes referred to as an acid-base reaction)

This is a reaction in which an acid and a base react to form a salt. Water is also produced in neutralizations with Arrhenius acids, which dissociate in aqueous solution to form hydrogen ions (H ⁺), and Arrhenius bases, which form hydroxide ions (OH⁻). However, water is not produced in all neutralizations as is the case with ammonia. Examples include:

• nitric acid + sodium hydroxide → sodium nitrate + water

HNO₃ + NaOH → NaNO₃ + H₂O

• hydrochloric acid + ammonia → ammonium chloride

HCl + NH₃ → NH₄Cl

Redox reaction: (also referred to as oxidation-reduction reaction)

This is a reaction in which the oxidation numbers of atoms are changed. Examples include:

• hydrogen + fluorine → hydrogen fluoride

H₂ + F₂ → 2HF

    Hydrogen is oxidized by its oxidation number increasing from zero to +1. Fluorine is reduced by its oxidation number decreasing from zero to -1.

• iron + cupric sulfate → ferrous sulfate + copper

Fe + CuSO₄ → FeSO₄ + Cu

    Iron is oxidized by its oxidation number increasing from zero to +2. Copper is reduced by its oxidation number decreasing from +2 to zero.

Analysis and characterization of inorganic compounds

The number of known chemical elements that occur naturally on Earth is 94 and the number of diverse inorganic chemical compounds derived by combinations of those elements is virtually innumerable. The characterization of those compounds includes the measurement of chemical and physical properties such as boiling points, melting points, density, solubility, refractive index and the pH and electrical conductivity of solutions.

The techniques of qualitative and quantitative analytical chemistry can provide the composition of a chemical compound in terms of its constituent chemical elements and can thus determine the chemical formula of a compound.

Modern laboratory equipment and techniques can provide many more details for characterizing chemical compounds. Some of the more commonly used modern techniques are:

• Chromatography: A process for separating mixtures into their component constituents.
• X-ray diffraction or X-ray crystallography: A technique that determines three-dimensional arrangement of atoms within a molecule.
• Spectrometry or qualitative Spectroscopy: A technique for the identification of substances through the electromagnetic spectrum emitted from or absorbed by them.
• Voltammetry: An electrochemical method for studying a chemical substance by measuring the electrical potential and/or electric current in an electrochemical cell containing the substance.