Inorganic chemistry

Chemistry is a science of obtaining and exploring various properties, structures and chemical interactions of elements and their combinations. Essentially, chemistry is divided into organic chemistry, which explores carbon bonds due to their large number and specificity, and inorganic chemistry, which explores the bonds of all other elements and a small fraction of carbon bonds, with the exception of hydrocarbons and their derivatives. The term “chemical substance” is used for both inorganic compounds, i.e. those that do not contain carbon in a form other than cyanide, isocyanide, thiocyanide, cyanate, thiocyanate, carbonate, bicarbonate, carbon oxides and carbides, as well as all kinds of metal ores, minerals, metals and their alloys. Besides its links with physics, inorganic chemistry is also interrelated with other sciences, such as mineralogy, geology, geochemistry, cosmochemistry, and with many branches of applied science, e.g. inorganic chemical technology, metallurgy and ceramics. In terms of conducted research, inorganic chemistry can be divided into many different specialities, e.g. practical inorganic chemistry deals with obtaining new compounds, inorganic chemical technology focuses on the process of large-scale manufacturing of inorganic compounds and analytical inorganic chemistry aims at exploring structures of interest in terms of quality and quantity. Important questions in inorganic chemistry include also kinetics of chemical reactions, mineralogy and physical chemistry.

The nature of inorganic chemistry

Over the years, interest in chemistry has grown and become more targeted. Initially, in the 18th century, inorganic chemistry in practice boiled down to the exploration of combustion processes. In the 19th century, research began on readily available elements, such as hydrogen, oxygen and aluminium. Currently, constantly improved research techniques allow to even isolate and purify rare elements considered for applications in various fields of technology. This is due to more and more data obtained about such elements as gallium, niobium, tantalum, zirconium, beryllium, titanium and their compounds. Due to the discovery of radioactivity and radioactive transformations, it was possible to extend the scope of chemical research to elements with unstable nuclei. Other disciplines of inorganic chemistry, such as crystallography and solid state chemistry as a whole, are also gaining importance.

Determination of the structure of inorganic compounds

Physical and chemical tests are usually used to classify a substance in terms of its structure. These include:

• spectroscopic methods, which focus on all forms of radiation. As a result, it is possible to interpret the obtained spectra. This group of techniques includes, e.g. IR, NMR and UV-Vis spectroscopy,
• diffraction methods, used for determination of the full characteristics of a crystal, including its dimensions, shape and atoms arrangement, e.g. X-ray crystallography and electronography.

Classification of inorganic compounds

1. Oxides are compounds of oxygen with other elements, having the general formula of E_nO_m, in which oxygen is always present in the -2 oxidation state.
   • Due to their physical properties, oxides are divided into:
     • metal oxides, i.e. solids of high density and melting point, which conduct electricity when melted. These substances are often characterised by their colour, e.g. iron(III) oxide (Fe₂O₃) is brownish red, lead(IV) oxide (Pb₃O₄) is yellow, mercury(II) oxide (HgO) is yellow or red and chromium(III) oxide is green,
     • other oxides – oxides of metalloids and non-metals have different physical states, e.g. gaseous such as carbon(II) oxide (CO), carbon(IV) oxide (CO₂) and sulphur(IV) oxide (SO₂), solid including phosphorus(V) oxide (P₄O₁₀) and silicon(IV) oxide (SiO₂) and liquid, e.g. hydrogen oxide, i.e. water (H₂O).
   • Due to their chemical properties, oxides are divided into:
     • acidic compounds reacting with water to form acids and with bases to form salts. These are, e.g. carbon(IV) oxide (CO₂), phosphorus(V) oxide (P₄O₁₀), sulphur(VI) oxide (SO₃) and nitrogen(III) oxide (N₂O₃),
     • alkaline compounds reacting with water to form bases and with acids to form salts. These are mainly oxides with elements of groups 1 and 2 of the periodic table, apart from beryllium, i.e. sodium oxide (Na₂O), calcium oxide (CaO) and potassium oxide (K₂O),

• amphoteric compounds reacting with both acids and bases to form salts. However, they do not react with water. These include: zinc oxide (ZnO), aluminium oxide (Al₂O₃), beryllium oxide (BeO) and manganese(II) oxide (MnO).

1. Preparation: direct synthesis, thermal decomposition of salts, hydroxides and oxides, oxidation of oxides in lower oxidation states and reduction of oxides in higher oxidation states, combustion of organic compounds.

2. Hydrides are made of hydrogen combined with another element. Depending on the periodic table group in which the element is found, the general formula can be represented as EH_n – for groups 1–15, e.g. NH₃, and H_nE for groups 16 and 17, e.g. H₂
   • Due to the nature of the bond, hydrides are divided into:
     • metallic, which are formed by d-block elements, and which chemical composition cannot be noted using simple formulas, e.g. TiH_1.73,
     • saline, typical for elements of groups 1 and 2 (s-block) and lanthanides, where hydrogen is always in the -1 oxidation state, except for beryllium and magnesium hydride,

• covalent, which are formed by elements of groups 13 (B), 14-17, and in which hydrogen is present in the +1 oxidation state.

• Due to their chemical properties, hydrides can be divided into:
  • inert, which do not react with water, e.g. CH₄,
  • acidic, which react with water to form acids and with bases to form salts, e.g. HCl, HI,

• alkaline, which analogously form bases with water and salts with acids, e.g. NH₃.
• Preparation: direct synthesis, displacement reactions.

3. Acids are inorganic chemicals that consist of hydrogen cations (hydronium ions) and an acid residue. Their general formula is: H_nR, where R is an acid residue.
   • According to the type of acid residue, acids can be divided into:
     • hydracids, in which the acid residue is formed by non-metal atoms. These are aqueous solutions of non-metal hydrides, e.g. hydrochloric acid (HCl), hydrosulphuric acid (H₂S) and hydrofluoric acid (HF),
     • Hydracids preparation: direct synthesis, dissolution of the gaseous product in water,

• oxyacids, in which the acid residue is formed by a group containing non-metal atoms and oxygen atoms. These acids are obtained by dissolving the oxides of the corresponding non-metals in water. Oxyacids include, e.g. nitric acid (HNO₃), sulphuric acid (H₂SO₄) and phosphoric acid (H₃PO₄),
• Oxyacids preparation: dissolution of the oxide (acid anhydride) in water
• Both types of acids can be prepared by treating the salt of the acid produced with another acid of greater strength.

4. Hydroxides are compounds that can act as proton acceptors or that are capable of releasing hydroxyl groups. The general formula of hydroxides is E(OH)_n, and the hydroxyl group has a valency of 1. Each hydroxide reacts with acids to form a salt in a neutralisation reaction.
   • Due to their chemical nature, hydroxides are divided into:
     • alkaline, which are formed by metals of groups 1 and 2 except beryllium and magnesium, and the d-block metals in their lowest possible oxidation states. In reaction with acids, they form salts, such as lithium hydroxide (Li(OH)) and calcium hydroxide (Ca(OH)₂),
     • amphoteric, which form salts in reactions with both acids and bases. These include, e.g. aluminium hydroxide (Al(OH)₃), copper(II) hydroxide (Cu(OH)₂), chromium(III) hydroxide (Cr(OH)₃) and zinc hydroxide (Zn(OH)₂),

• Preparation: dissolution of the oxide (base anhydride) in water, reaction of elements of groups 1 and 2 with water, reaction of hydrides with water, displacement reaction between a strong base and a salt of an element whose oxide is insoluble in water.

5. Salts are chemical compounds created through a neutralisation reaction of hydroxides with oxyacids and hydracids. Their general formula is E_nR_m, where nE^m+ is a base-forming cation and mR^n- is an acid residue.
   • Salts are divided into:
     • simple, including hydracid and oxyacid salts. These can be acid salts, which are products of incomplete displacement of hydrides in polyprotic acids, e.g. NH₄HS, KH₂PO₄. Basic salts, i.e. alkali salts formed as a result of incomplete neutralisation of hydroxyl groups of hydroxides, e.g. Al(OH)Cl₂, Mg(OH)Cl. There are also hydrated salts, such as CuSO₄·5H₂O.
     • complex, i.e. double and triple salts, which have two or three different cations, respectively, connected to an acid residue in their structure. These include K₂SO₄·Al₂(SO₄)₃·24H₂O and KAl(SO₄)₂·12H₂O,

• Preparation: reaction of a metal and non-metal, reaction of an acid anhydride with a base anhydride, treatment of a base anhydride with an acid and of an acid anhydride with a base, reaction of an active metal with an acid, neutralisation of a hydroxide with an acid.

Physical chemistry

It is a field of science allowing to visualise and understand the physical and chemical transformations of matter along with the associated energy flows. The fundamental method used during research is the creation of theoretical mathematical and physical models based on experimental observations. A model is a mechanism used to reflect, in the simplest possible way, the most important features of the considered object or phenomenon. Physical chemistry involves creation of hypotheses, theories and laws of nature in relation to its subject. The main issues of this chemistry branch are: thermodynamics, chemical equilibria, phase equilibria, thermodynamic characteristics of solutions, electrochemistry, surface phenomena and colloids, chemical kinetics and the basics of quantum chemistry.

Quantum chemistry

This is a very important field of theoretical chemistry. It allows to understand the interactions between atoms and molecules, as well as the chemical processes between them. Thanks to the use of quantum mechanics, many parameters can be determined, including the energy of chemical bonds, angles between atoms, magnetic moments and ionizing potentials. This field of chemistry was started in 1927 when three scientists, E.U. Condon, W. Heitler and F. London, began research to explain the bonds in a diatomic hydrogen molecule. In Poland, Włodzimierz Kołos, who worked on the same phenomenon, also contributed to the development of quantum chemistry. His calculations of the dissociation energy of a hydrogen molecule turned out to be more accurate than spectroscopic methods.

Chemical kinetics

This is a branch of physical chemistry exploring the rate of chemical reactions using experimental and theoretical analysis. In order to define the kinetic equation for a reaction, experimental data on the relation between the concentration of reactants and the rate of reaction is necessary. In addition, kinetics deals with the determination of the effect of various variables, such as catalysts or temperature change, on the rate of a chemical reaction. Having the necessary experimental data, researchers perform theoretical analysis, which allows to determine the stoichiometry and then to select the appropriate rate equation.

Analytical chemistry

It is a branch of chemistry that studies qualitative and quantitative composition of substances. For this purpose, a number of classic methods are used, such as gravimetric methods and classic titration using indicators, as well as constantly developing physical and chemical methods, also known as instrumental analyses. All these are tests that require the use of appropriate equipment, e.g. chromatographic techniques, spectral analysis or electrochemical methods such as voltammetry and potentiometry.

Application of inorganic chemistry

Depending on the substance, inorganic chemicals have a number of applications in almost every industry, as well as in everyday life. To give an example:

• nitric(IV) oxide (NO₂) is used as a nitrating agent, is an oxidant and a catalyst for many reactions, and is also an intermediate needed in the production of nitric acid,
• due to its colour, chromium(III) oxide (Cr₂O₃) is used as a component of green paints and for tinting glass and porcelain glaze,
• silicon oxide (SiO₂), i.e. common sand, is a component of many very important products, including cement, mortar, glass and ceramics,
• calcium hydride (CaH₂) has been used in the production of pure metals from their oxides, the removal of water from organic liquids, and also as a source of hydrogen to inflate balloons,
• lithium hydride (LiH) is a strong reducer and a commonly used desiccant,
• nitrogen hydride (NH₃), also known as ammonia, is used in the production of fertilisers and as a refrigerant,
• nitric acid (HNO₃) has a wide range of industrial applications, e.g. it is used for cleaning metal surfaces, for obtaining fertilisers, plastics and varnishes, as well as in the pharmaceutical industry,
• sulphuric acid (H₂SO₄) is a great bactericide, it is used in the production of phosphate fertilisers, artificial fibres and during the refining of oils and fats; it is also used as an electrolyte in batteries,
• sodium hydroxide is used in the production of soap, cellulose, washing powders, viscose rayon and glass,
• potassium hydroxide is a desiccant and bleaching product; it is a raw material for saponification, it is a gas, e.g. atmospheric CO₂, absorber.

Chemical reactions involving inorganic compounds:

1. Synthesis, during which one product is formed from two or more substances.
2. Decomposition, i.e. breakdown, during which at least two products are formed from one substrate.
3. Displacement, when components are exchanged between the reactants during the reaction.
4. Redox, i.e. reactions of oxidation and reduction, during which the oxidation states of the elements involved change.