Properties And Reactions Of Elements And Compounds of P Block Elements - Session 3

Peroxides are a class of chemical compounds that contain an oxygen- oxygen single bond. Peroxides have functional group R-O-O-R . R represents any element. Oxidation state of oxygen in peroxides is negative one.Some examples of peroxides include hydrogen peroxide H2O2 and metal peroxides such as sodium peroxide Na2O2 and magnesium peroxide Mg2O2 .

Hydrogen peroxide acts as reducing agent. When hydrogen peroxide reacts with silver oxide, it reduces the silver oxide into silver. Oxidation state of silver changes from positive one in silver oxide to zero in silver. Silver oxide undergoes reduction.Hydrogen peroxide also acts as oxidizing agent. When hydrogen peroxide reacts with lead sulfide, it oxidizes lead sulfide to lead sulfate. Oxidation state of sulfur changes from negative two in lead sulfide to positive six in lead sulfate. Hydrogen peroxide itself undergoes reduction to water.

Some of the P Block elements exhibit variable oxidation states. For example carbon atom has an oxidation state of negative four in some compounds such as methane CH4 . This is because carbon is more electronegative than hydrogen atom. Carbon also exhibits positive four oxidation state in carbon dioxide CO2 . This is because oxygen is more electronegative than carbon. Carbon in carbon formic acid has an oxidation state of positive two. Carbon in formaldehyde has an oxidation state of zero. Carbon in methanol has an oxidation state of negative two.

Nitrogen can exhibit a range of oxidation states from negative three in compounds like ammonia NH3 to positive five in compounds like nitric acid HNO3 .Oxygen generally has an oxidation state of negative two. For example in water H2O oxygen has negative two oxidation state. Oxygen can also exhibit positive oxidation states in some compounds. For example in hydrogen peroxide H2O2 oxidation state of oxygen is negative one.

Halogens such as chlorine generally has an oxidation state of negative one. For example in hydrogen chloride HCl oxidation state of chlorine is negative one. Chlorine can also exhibit positive oxidation state. For example, in oxoacids of chlorine such as perchloric acid HClO4 , oxidation state of chlorine is positive seven. This is because oxygen is more electronegative than chlorine.

Phosphorus can exhibit oxidation states ranging from negative three in compounds like phosphine PH3 to positive five in compounds like phosphoric acid H3PO4 .Sulfur (S) - Sulfur can exhibit oxidation states ranging from negative two in compounds like hydrogen sulfide H2S to positive six in compounds like sulfuric acid H2SO4 .

The hydrogen halides are a group of binary acids consisting of hydrogen and a halogen atom. Fluorine, chlorine, bromine and iodine are called halogens. These hydrogen halides include hydrofluoric acid HF , hydrochloric acid HCl , hydrobromic acid HBr and hydroiodic acid HI . Hydrofluoric acid HF is the weakest acid among hydrogen halides. Hydroiodic acid HI is the strongest acid among hydrogen halides.

The acidic strength of these hydrogen halides rises as we move downward in the periodic table from Hydrogen fluoride HF to Hydrogen iodide HI. As we move downward in the group, the size of the halogen atom grows. This results in a decrease in the bond strength between hydrogen and the halogen atom. Therefore, it becomes easier to dissociate the bond between hydrogen atom and halogen atom. As a result hydrogen ion H+ ion is easily released resulting in a rise in acidic strength.

Another factor responsible for rise in acidic strength from hydrogen fluoride to hydrogen iodide is electronegativity. The electronegativity of the halogen atoms decreases as we move downward in the group. This means that the halogen atoms become less effective at attracting electrons towards themselves. Therefore, the shared pair of electrons in the hydrogen halogen bond is less polarized towards the halogen atom. This makes it easier to break the bond and release hydrogen ions.

Stability of the halide ion also determines the acidic strength of hydrogen halide. When a hydrogen halide is dissolved in water, it is dissociated into hydrogen ions and halide ion. If halide ion of a hydrogen halide is stable, then that hydrogen halide is highly acidic. For example, iodide ion I- ion is more stable than chloride ion Cl- ion due to large side of iodide ion. This makes the hydrogen iodide more acidic than hydrogen chloride.

Hydrolysis is a chemical reaction in which water is used to break-down a compound into smaller components. Compounds of p-block elements such as boron, carbon, nitrogen, oxygen, fluorine, and silicon can undergo hydrolysis reactions. The hydrolysis of p-block compounds can result in the formation of acids or bases, depending on the nature of the compound.Hydrolysis of Boron trichloride BCl3 results in formation of boric acid and hydrochloric acid. In same way Hydrolysis of Silicon results in the formation of silicic acid and hydrochloric acid.

Chlorides of the group fifteen elements also undergo hydrolysis. For example, hydrolysis of Ammonium Chloride NH4Cl results in formation of ammonia and hydrochloric acid.Phosphorous trichloride on hydrolysis forms phosphorous acid and hydrochloric acid. Hydrolysis reactions can be used to synthesize new compounds and to study the chemical properties of different elements.

Elements of the group eighteen are called noble gases. These elements include helium, neon, argon, krypton, xenon and radon. Noble gases are generally unreactive and do not readily form chemical bonds with other elements. Their valence shell is completely filled with electrons. However, under extreme conditions, they can form compounds with some other elements. Among noble gases, Xenon forms stable compounds.

Xenon reacts with fluorine in the presence of a catalyst to form xenon tetrafluoride. This reaction is highly exothermic, and the product is a yellow crystalline solid that is highly reactive.When xenon reacts with fluorine gas in the presence of a strong oxidizing agent, such as cobalt fluoride, xenon hexafluoride is formed. This reaction is also highly exothermic, and the product is a colorless crystalline solid. Xenon hexafluoride can react with water to form xenon oxide and hydrogen fluoride.

Amphoteric nature refers to the ability of a chemical species to act as both an acid and a base, depending on the reaction conditions. Many p-block elements exhibit amphoteric behavior. They can react with both acids and bases to form a variety of products. For example, Aluminum can react with acid such as hydrochloric acid to form Aluminum chloride and hydrogen gas. Aluminum can also react with base such as sodium hydroxide to form sodium aluminate and hydrogen gas.

In same way sulfur can also react with both acids and bases. Sulfur reacts with sulfuric acid to form sulfur dioxide gas and water. When sulfur reacts with a base such as sodium hydroxide, sodium sulfide, sodium thiosulfide and water is formed.