Sulphide

Sulphide, also sometimes called sulfide, is sulphur’s inorganic anion. Sulphide is basically a compound that contains one or more S²⁻ ions. The chemical formula of sulphide is also S²⁻. Many metallic elements, including sulphides, are usually found to occur naturally as minerals. For example, pyrite, which is a sulphide of iron with the formula FeS₂, is one of the most abundant sulphur minerals. In addition, mercury, copper, silver, zinc, cadmium and many other elements tend to occur as sulphides in nature. We will learn more about this compound below.

Sulphide

Classes of Sulphides

In general, there are three classes of sulphides which include organic sulphides (also known as thioethers), inorganic sulphides and phosphine sulphides.

Inorganic Sulphides

Inorganic sulphides are usually ionic compounds. They contain negatively charged sulphide ions, S²⁻. In other words, we can say that these compounds are basically salts of a very weak acid – hydrogen sulphide.

Organic Sulphides

In organic sulphides, the sulphur atom forms a covalent bond with two organic groups. These sulphides are also sometimes called thioethers.

Phosphine Sulphides

Lastly, phosphine sulphides are sulphides that are formed when organic phosphines react with sulphur. Here, the sulphur atom, which is bonded to phosphorus, has both ionic and covalent properties.

Properties and Chemical Reaction of Sulphide

S²⁻ ion does not exist in aqueous alkaline solutions of Na₂S. Sulphide is converted to hydrosulphide.

– S²⁻ + H₂O → SH⁻ + OH⁻

When treated with an acid, sulphide salts convert to hydrogen sulphide.

– S²⁻ + H⁺ → SH⁻

– SH⁻ + H⁺ → H₂S

Usually, carrying out the oxidation of sulphide is a complex process. It largely depends on the conditions at the time of reaction. As such, in different conditions, oxidation can result in elemental sulphur, polythionates, sulphite, polysulphides or sulphate.

Metal sulphides react with halogens, forming sulphur and metal salts.

– 8 MgS + 8 I₂ → S₈ + 8 MgI₂

As stated above, just to recap, when sulphide is treated with a standard acid, it is converted into hydrogen sulphide (H₂S), along with the salt of a metal. Sulphur or sulphate is formed once sulphide is oxidised. Sulphur metals or salts of metals are formed once non-metals like bromine, iodine and chlorine react with metal sulphides, viz.

8 MgS + 8 I2 -> S8 + 8 MgI

Also, using a relevant oxidiser and sulphide, one could prepare sulphur, viz.

16 HNO₃ + 24 H₂S -> 16 NO + 3 S₈ + 32 H₂O

Sulphides of alkali metals and alkaline-earth metals are slightly soluble in water, and they appear to be primarily ionic. However, sulphides of copper and zinc are the least-soluble compounds.

Sulphides are not particularly toxic. Only a few metal sulphides, on exposure to a strong mineral acid or gastric acids, produce toxic hydrogen sulphide.

Organic sulphides tend to be highly flammable. On burning sulphide, we get sulphur dioxide (SO₂) gas.

Almost all organic sulphides have a strong and very unpleasant smell. Hydrogen sulphide and some of its salts can be taken as examples.

Dissolved free sulphides or sulphides present in aqueous solutions are very aggressive species for the corrosion of many metals. They are responsible for different forms of corrosion.

Preparation of Sulphide

Sulphides can be prepared in the following ways:

When elements combine directly.

Fe_(s) + S_(s) → FeS_(s)
By the reduction of sulphate.

MgSO_4(s) + 4C_(s) → MgS_(s) + 4CO_(g)
An insoluble sulphide undergoes precipitation.

M²⁺ + H₂S_(g) → MS_(s) + 2H⁺_(aq)

Structure of Sulphides

Sulphides that are formed from semimetals (metalloids) or some nonmetallic elements are molecular. They have sulphide bridges in a polymeric structure. Sulphide (organic) is an angular functional group, the C–S–C angle approaching 90°. The C–S bonds are about 180 pm.

Metal Derivatives

Solid sulphides get precipitated once sulphide sources (NaHS, Na₂S, and H₂S) react with transition metal cations in an aqueous solution. Inorganic sulphides have relatively low water solubility. A popular example of this is cadmium yellow (CdS). Ag₂S, on the other hand, is a black-coloured stain that is found in sterling silver. These are sometimes just called salts.

The bonding in transition metal sulphides is highly covalent. Due to this, they have semiconductor properties, and it is also related to the colours. As for the applications, they are often used as pigments, found in solar cells, and they also act as catalysts for some chemical reactions.

Many important metal ores are sulphides. Some of them are pyrite (iron disulphide), galena (lead sulphide), argentite (silver sulphide), cinnabar (mercury sulphide), realgar (arsenic sulphide), pentlandite (nickel sulphide), sphalerite (zinc sulphide) and chalcopyrite (iron-copper sulphide).

Frequently Asked Questions – FAQs

What is the chemical formula of sulphide?

S^2- is the chemical formula of sulphide. It is an inorganic anion.

What are sulphides used for?

Sulphides are used in thermoelectric devices as they are semiconductors. Tungsten and molybdenum sulphides are used as commercial lubricants.

What is pyrite?

Pyrite is a sulphide of the metal iron. It has the chemical formula FeS₂.

Give an example of organic sulphide.

Methyl phenyl sulphide is an example of organic sulphide. Its formula is C₆H₅-S-CH₃. It is also called thioanisole because of its similarity in structure to anisole C₆H₅-O-CH₃.

Name some common sulphides.

Cinnabar – mercury sulphide, Galena – lead sulphide, pentlandite – nickel sulphide, sphalerite – zinc sulphide, Chalcopyrite – iron – copper sulphide, Argentite – silver sulphide, Realgar (Arsenic sulphide) are some common sulphides.