Properties And Reactions Of Elements And Compounds of P Block Elements - Session 1

Thermal decomposition is a process in which a substance breaks-down into simpler substances when it is heated to a high temperature. Thermal decomposition occurs when the heat energy supplied to a substance exceeds the energy required to break the bonds holding its molecules together. The energy absorbed by the substance causes its molecules to vibrate more vigorously, eventually leading to bond breaking and the release of simpler substances. For example, Ammonium salts decompose on heating into other compounds.

The thermal decomposition of ammonium salts involves the breakdown of the NH₄⁺ and the anion in the salt. The decomposition occurs under high temperatures. The reaction products depend on the specific ammonium salt and the conditions of the reaction.For example, NH₄Cl can undergo thermal decomposition when heated to temperatures above 338 °C. It forms NH₃ and HCl as products.

Similarly, (NH4)2SO4 can undergo thermal decomposition when heated, forming NH₃, SO₂ and water vapour. Ammonium Carbonate decomposes on heating to give NH₃, CO₂ and water vapour. Products of decomposition of Ammonium Nitrate are Nitrogen Gas and water vapour.

Decomposition of Ammonium Nitrite gives Nitrous Oxide gas and water vapour. What can be the possible products of decomposition of Ammonium Chromate?.Ammonium Chromate decomposes on heating to give Nitrogen gas, Chromium oxide and water vapour.

Allotropy is the property of some chemical elements and compounds to exist in multiple forms. These multiple forms are called allotropes. Allotropes have different physical and chemical properties despite being composed of the same atoms or molecules. For example, diamond and graphite both are allotropes of carbon.

Graphite is purely made up of carbon atoms. Have you ever thought what is a diamond made of? Diamond is also purely made up of carbon atoms. But then, if diamond and graphite are both are purely made up of carbon atoms, why are their properties are different from each other. Diamond is transparent and hard. It is also costly. Meanwhile Graphite is cheaper and its color is dark gray to black. Answer to this question is very simple. Diamond and graphite both are allotropes of carbon. Arrangement of carbon atoms is different in both of them.

Let us have a look at arrangement of atoms in graphite and diamond. In graphite carbon atoms are in the form of hexagonal rings. These rings are arranged in the form of layers. While in diamond carbon atoms are in tetrahedral arrangement. Due to this reason their properties are different from each other. Graphite is soft and diamond is hard. Graphite is a good conductor of electricity. Diamond is a bad conductor of electricity.

Now we shall discuss some more examples of allotropes. Oxygen has two main allotropes. These are Oxygen gas and Ozone. Oxygen is a diatomic molecule composed of two oxygen atoms. It is represented by the chemical formula O2. It is a colorless, odourless gas that is essential for life and makes up about 21% of the Earth's atmosphere.

Ozone, on the other hand, is a triatomic molecule composed of three oxygen atoms. It is represented by the chemical formula O3 . It is a pale blue gas with a pungent odour. It is formed naturally in the Earth's atmosphere when ultraviolet light acts upon oxygen molecules. Ozone is an unstable and reactive molecule, while oxygen is a stable and relatively unreactive molecule.

Allotropes of sulfur are categorized into two forms. One is Crystalline and another is Amorphous. Crystalline allotropes of sulfur are Rhombic Sulfur and Monoclinic Sulfur. Rhombic Sulfur is a yellow crystalline solid. It has octahedral shape. It is the most stable form of sulfur at room temperature and pressure. It consists of S8 molecules arranged in a rhombic crystal lattice. Monoclinic Sulfur is a crystalline form of sulfur. In its monoclinic form, sulfur forms long, needle-like crystals that are usually yellow. It is not very stable and tends to change into the more stable rhombic form of sulfur over time.

Amorphous solids are those in which atoms are not arranged in definite pattern. Amorphous Allotropes of sulfur include Plastic Sulfur and Colloidal Sulfur. Plastic Sulfur is a unique allotrope because it can be moulded and shaped like plastic when heated to a particular temperature. It is created by melting sulfur and then rapidly cooling it. This causes the Sulfur atoms to arrange themselves in a long-chain polymer.

Colloidal Sulfur is a type of sulfur that is dispersed in a liquid medium to form a colloidal suspension. A colloidal suspension is a mixture of particles that are small enough to remain suspended in the liquid and not settle to the bottom. In the case of colloidal sulfur, small particles of sulfur are dispersed in a liquid such as water or oil. Colloidal sulfur has several applications in medicine and skincare.

Oxoacids, also known as Oxyacids, are a class of acids that contain oxygen, hydrogen, and one or more other elements. The general formula for an oxoacid shown here. n represents number of hydrogen atoms. m represents number of oxygen atoms. X represents any non-metallic or polyatomic ion.

An example of Oxoacid is H₂SO₄. Sulfuric acid is the oxoacid of sulfur. In same way, H₂SO₃ and H₂S₂O₃ are also oxoacids of sulfur.

Oxoacids of nitrogen include HNO3 and HNO2. There are four Oxoacids of chlorine. These are HOCl, HOClO, HOClO2 , and HOClO3 . The structures of these acids are illustrated here. Can you name any Oxoacid of bromine?.

When Aluminium reacts with Hydrochloric Acid, it forms Aluminium Chloride. Aluminium Chloride is a salt. It is a group 3A halide. Hydrogen gas is also formed in this reaction. In the same way, Aluminium can react with Bromine to form Aluminium Bromide. Compounds in which the central atom has incomplete octet are called electron deficient compounds. Group 3A halides are mostly electron deficient. To understand this electron deficiency let’s discuss the formation of dimer by Aluminium Chloride.

Aluminium chloride can exist as dimer. It is aDimer because two Aluminium chloride molecules are attached by a coordinate covalent bond. Coordinate covalent bond is the bond formed when an atom shares its pair of electrons with electron deficient atom. The molecular formula of dimer of Aluminium Chloride is Al₂Cl₆.But how does the Aluminium chloride exist as a dimer? To answer this question we shall first discuss structure of Aluminium Chloride.

As we can see in the Lewis Dot Structure of Aluminium Chloride, Aluminium is surrounded by 3 chlorine atoms. The Aluminium atom has three valence electrons. Each valence electron is shared with each of the three chlorine atoms to make a covalent bond. This shows that the octet of chlorine atoms is complete. But Aluminium atom is surrounded by only six electrons. Octet of Aluminium atom is not complete. It requires a pair of electrons to complete its octet. We can also say that Aluminium atom in Aluminium chloride is electron deficient.

In same way other group 3A halides like Aluminium Bromide and Boron Trifluoride are also electron deficient due to incomplete octet on central atom. In Boron Trifluoride, Boron is surrounded by six electrons. It has incomplete octet. It is electron deficient.

As we know that the Aluminium atom in Aluminium Chloride is electron deficient. To complete the octet of the Aluminium atom in Aluminium Chloride, the chlorine atom of one molecule of Aluminium Chloride shares a pair of electrons with Aluminium atom of another Aluminium Chloride molecule. This results in the formation of coordinate covalent bond between Aluminium atom of one molecule and chlorine atom of another molecule. Overall, there are two coordinate covalent bonds in one dimer of Aluminium Chloride.

Group 3A halides have acidic nature. For example, Aluminium Chloride acts as a Lewis acid. A Lewis acid is a chemical specie that can accept a lone pair of electrons. We know that the Aluminium atom in Aluminium Chloride has an incomplete octet. It is deficient of a pair of electrons. It can accept a lone pair of electrons from Lewis base. Lewis base is a chemical specie that can donate a lone pair of electrons. Aluminium chloride reacts with Ammonia to form an adduct as illustrated. An Adduct is a product that contains all the atoms of reactants.