Chapter 4: Group VA and Group VIA Elements – Chemistry 2nd Year Notes

This chapter covers Group VA and Group VIA Elements from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Group VA is the nitrogen family (N, P, As, Sb, Bi) with the outer configuration ns2 np3, and Group VIA is the oxygen family or chalcogens (O, S, Se, Te, Po) with ns2 np4. Both begin with a typical non-metal at the top and become metallic towards the bottom.

Two of the most important industrial processes in chemistry belong to this chapter: the Haber process for making ammonia and the Contact process for making sulphuric acid, the king of chemicals.

Learning Objectives

• List the members and configurations of Groups VA and VIA.
• Describe how metallic character changes down each group.
• Explain the manufacture of ammonia by the Haber process and its uses.
• Describe the oxides and oxoacids of nitrogen and sulphur.
• Explain the manufacture of sulphuric acid by the Contact process.
• Describe the allotropy of sulphur.

Key Concepts

The Two Families

In Group VA, nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids and bismuth is a metal. In Group VIA, oxygen, sulphur and selenium are non-metals, tellurium is a metalloid and polonium is a radioactive metal. In both groups metallic character increases down the group while the acidic strength of the oxides decreases.

Nitrogen and the Haber Process

Nitrogen makes up about 78 percent of air but is very unreactive because of the strong triple bond in the N2 molecule. It is fixed mainly as ammonia. Ammonia (NH3) is manufactured by the Haber process, in which nitrogen and hydrogen combine over an iron catalyst: N2 + 3H2 gives 2NH3 (reversible and exothermic), using about 200 atm pressure and 450 degrees Celsius. High pressure and a moderate temperature give the best yield, and unreacted gases are recycled. Ammonia is used to make nitric acid, urea and ammonium fertilizers.

Oxides and Oxoacids of Nitrogen

Nitrogen forms several oxides showing oxidation states from +1 to +5, for example nitrous oxide (N2O, laughing gas), nitric oxide (NO) and nitrogen dioxide (NO2, a brown gas). Its most important oxoacid is nitric acid (HNO3), a strong acid and powerful oxidising agent made industrially by the Ostwald process.

The Oxygen Family and Allotropy of Sulphur

Oxygen is the most abundant element in the earth crust and is essential for respiration and combustion. Sulphur is a yellow non-metal that shows allotropy: rhombic sulphur is stable at room temperature and monoclinic sulphur is stable above 96 degrees Celsius, both made of S8 rings.

Sulphuric Acid and the Contact Process

Sulphuric acid (H2SO4) is manufactured by the Contact process in three stages: burning sulphur to sulphur dioxide (S + O2 gives SO2), oxidising SO2 to SO3 over a vanadium(V) oxide catalyst (2SO2 + O2 gives 2SO3), and absorbing SO3 in concentrated sulphuric acid to form oleum, which is then diluted. Concentrated sulphuric acid is a strong acid, a dehydrating agent and an oxidising agent, and is used to make fertilizers, detergents, paints and batteries.

Important Definitions

Formulas & Rules

Item	Fact
Group VA / VIA	ns2 np3 (N,P,As,Sb,Bi) / ns2 np4 (O,S,Se,Te,Po)
Ammonia (Haber)	N2 + 3H2 gives 2NH3; Fe, 200 atm, 450 C
Sulphuric acid (Contact)	catalyst V2O5
Sulphur allotropes	rhombic (below 96 C) and monoclinic (above 96 C)
Conc. H2SO4	strong acid + dehydrating + oxidising agent

Diagrams & Illustrations

Position of Groups VA and VIA: a periodic-table schematic of the nitrogen family (ns2 np3) and the oxygen family or chalcogens (ns2 np4), noting that non-metal changes to metal down each group.

Haber process: a flow diagram showing nitrogen and hydrogen compressed to about 200 atm, heated to 450 degrees Celsius over an iron catalyst to give ammonia, which is cooled and removed while unreacted gases are recycled.

Contact process: a four-step flow chart: sulphur to SO2, SO2 to SO3 over vanadium(V) oxide, SO3 absorbed as oleum, and oleum diluted to sulphuric acid.

Solved Examples & Numericals

Example 1: Le Chatelier in the Haber process

Why is high pressure used in the Haber process? The forward reaction reduces the number of gas molecules (4 to 2), so by Le Chatelier principle high pressure shifts the equilibrium toward ammonia, increasing the yield.

Example 2: Role of concentrated sulphuric acid

What happens when concentrated sulphuric acid is added to sugar? It acts as a dehydrating agent, removing hydrogen and oxygen as water and leaving a black mass of carbon.

Example 3: Identifying a gas

A brown gas is evolved when copper reacts with concentrated nitric acid. Name it. Nitrogen dioxide (NO2), because concentrated nitric acid is a strong oxidising agent.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the manufacture of ammonia by the Haber process.

Ammonia is manufactured by combining nitrogen from the fractional distillation of liquid air with hydrogen obtained from natural gas. The purified gases are mixed in a one to three ratio, compressed to about 200 atmospheres, and passed over a finely divided iron catalyst at about 450 degrees Celsius, where they combine according to the reversible, exothermic reaction N2 + 3H2 gives 2NH3. Because the forward reaction reduces the number of gas molecules and gives out heat, a high pressure and a moderate temperature give the best practical yield, and a catalyst speeds up the reaction. The mixture leaving the converter is cooled so that ammonia liquefies and is removed, while the unreacted nitrogen and hydrogen are recycled.

Q2: Describe the manufacture of sulphuric acid by the Contact process.

In the Contact process, sulphur is first burned in air to give sulphur dioxide (S + O2 gives SO2). The purified, dried sulphur dioxide is mixed with more air and passed over a vanadium(V) oxide catalyst at about 450 degrees Celsius, where it is oxidised to sulphur trioxide (2SO2 + O2 gives 2SO3). Because the reaction gives out heat and reduces the number of gas molecules, a moderate temperature and ordinary pressure are used. The sulphur trioxide is not dissolved directly in water (which would form a mist) but is absorbed in concentrated sulphuric acid to form oleum (SO3 + H2SO4 gives H2S2O7), which is then carefully diluted with water to give sulphuric acid of the required concentration.

Q3: Discuss the properties and uses of concentrated sulphuric acid.

Concentrated sulphuric acid shows three important kinds of behaviour. As a strong acid it reacts with metals, bases and carbonates to form sulphates. As a powerful dehydrating agent it removes the elements of water from other substances, for example charring sugar to carbon and drying gases. As an oxidising agent, especially when hot and concentrated, it oxidises metals such as copper, itself being reduced to sulphur dioxide. Because of this versatility it is one of the most widely used industrial chemicals, employed in making fertilizers, detergents, dyes, paints and explosives, and in car batteries and metal pickling.

MCQs with Answers

Quick Revision Summary

• VA = ns2 np3 (N,P,As,Sb,Bi); VIA = ns2 np4 (O,S,Se,Te,Po); metallic character increases down.
• Ammonia by Haber process: N2 + 3H2 gives 2NH3, Fe catalyst, 200 atm, 450 C.
• Sulphuric acid by Contact process: V2O5 catalyst; SO3 absorbed as oleum then diluted.
• Sulphur allotropes: rhombic (below 96 C) and monoclinic (above 96 C), both S8.
• Concentrated H2SO4 is a strong acid, dehydrating and oxidising agent.

Exam Tips

• Learn the Haber and Contact process conditions and equations perfectly; they appear almost every year.
• Always justify the chosen conditions with Le Chatelier principle.
• Remember the catalysts: Fe for Haber, V2O5 for Contact.
• For concentrated H2SO4 name its three roles: acid, dehydrating and oxidising, with an example each.
• Note that SO3 is absorbed in H2SO4, not water, to avoid an acid mist.