CHAPTER

p-BLOCK ELEMENTS

Syllabus

Group 16 Elements: General introduction, electronic configuration, oxidation states, occurrence, trends in physical and chemical properties, dioxygen: Preparation, Properties and uses, classification of Oxides, Ozone, Sulphur-allotropic forms; compounds of Sulphur: Preparation, Properties and uses of Sulphur- dioxide; Sulphuric Acid: industrial process of manufacture, properties and uses; Oxoacids of Sulphur (Structures only).

Group 17 Elements: General introduction, electronic configuration, oxidation states, occurrence, trends in physical and chemical properties; compounds of halogens, Preparation, properties and uses of Chlorine and Hydrochloric acid, interhalogen compounds, Oxoacids of halogens (structures only).

Group 18 Elements: General introduction, electronic configuration, occurrence, trends in physical and chemical properties, uses.

Chapter Analysis

201 2017 2018

List of Topics D D OD OD D/OD

Draw the structure 1Q 1Q 1Q

(2 marks) (2 marks)

What happens when 1Q 1Q (2 marks)* (2 marks)* Give reason 1Q 1Q 1Q 1Q

(1 mark) (1 mark) (1 mark) (2 marks)

Complete the reactions 1Q (2 marks)*

• * One question of 2 marks with two choices was asked.

On the basis of above analysis, it can be said that from exam point of view, Draw the Structure of Compounds, What Happens When and Give Reason are the most important type of questions asked from different topics of the chapter.

TOPIC-1

Compounds Group-16 Elements, Properties and Some Important

Revision Notes

<math>\rightharpoonup</math> Oxygen family : <math>{}_{8}\text{O}</math>, <math>{}_{16}\text{S}</math>, <math>{}_{34}\text{Se}</math>, <math>{}_{52}\text{Te}</math>, <math>{}_{84}\text{Po}</math>

> Physical properties:

1. Atomic and ionic radii: Smaller than the corresponding elements of group 15 due to increase in nuclear charge. Down the group they increase due to addition of a new shell.

1. Ionisation enthalpy: I.E.₁ is lower than those of corresponding elements of group 15 due to increase in nuclear charge. I.E.2 is higher than those of group 15 due to smaller size of the ions and greater effective nuclear charge. Down the group I.E. decreases.

1. Electron gain enthalpy: Oxygen has less electron gain enthalpy than sulphur. From sulphur, the value again become less negative upto polonium.

116 Oswaal CBSE Chapterwise & Topicwise Question Bank, CHEMISTRY, Class – XII

1. Electronegativity: More electronegative than group 15 elements. It decreases down the group due to increase in atomic size.
2. Oxygen and sulphur are non-metals, selenium and tellurium are metalloids and polonium is a metal.

.... P. 115

1. Melting and boiling points: Increases regularly from O to Te due to increase in size and hence greater van der Waals force. Po has lesser melting and boiling point than Te due to maximum inert pair effect.

Group-16 Elements, Properties and

Some Important Compounds

TOPIC - 1

1. Density: Increases down the group.

.... P. 121

TOPIC - 3

1. Oxidation state: Oxygen shows an oxidation state of –2 only (except <math>OF_2</math> and <math>H_2O_2</math>). All other elements show +2, +4 and +6 oxidation states.

TOPIC - 2

Group-17 Elements, Properties and

Some Important Compounds

Chemical properties :

1. With hydrogen: Form hydroxide of type <math>H_2E</math> (E = O, S, Se, Te and Po).

Group-18 Elements, Properties and

Some Important Compounds

.... P. 128

Properties of hydrides:

1. Thermal stability: Decreases down the group.
2. Acidic character: Increases down the group.
3. Reducing nature : All are reducing agents except H₂O.
4. Boiling point: From <math>H_2O</math> to <math>H_2S</math> there is a sudden drop which increases from <math>H_2S</math> to <math>H_2Te</math>.
5. With halogens: Form halides of type <math>EX_2</math>, <math>EX_4</math> and <math>EX_6</math> where E is an element of group and X is a halogen.

Properties of halides:

1. Only hexafluorides are the only stable halides.
2. All elements except selenium form dichlorides and dibromides. These dihalides are <math>sp^3</math> hybridised and have tetrahedral structure.

1. Preparation of SF₄ and SF₆:

<math display="block">2Se_{2}Cl_{2} \rightarrow SeCl_{4} + 3Se</math>

<math display="block">3SCl_{2} + 4NaF \rightarrow SF_{4} + 6_{2}Cl_{2} + 4NaCl</math>

<math display="block">S + 3F_{2} \xrightarrow{Burn} SF_{6}</math>

(b) Properties of SF₄ : SF₄ is readily hydrolysed.

<math>SF_4 + 2H_2O \rightarrow 4HF + SO_2</math>

1. SF₆ is <math>sp^3d^2</math> hybridised and octahedral whereas SF₄ is <math>sp^3d</math> hybridised and is trigonal bipyramidal with one position occupied by a lone pair of electrons.
2. With oxygen: Form oxides of EO₂ and EO₃.

➤ Dioxygen (O₂)

Preparation:

In laboratory,

<math>2KClO_3 \xrightarrow{MnO_2} 2KCl + 3O_2</math>

Properties:

1. Slightly soluble in water and appreciably soluble in alkaline pyrogallol solution.
2. Paramagnetic.
3. Forms oxides with metals and non-metals.

<math>2Mg(s) + O_2(g) \rightarrow 2MgO(s)</math>

<math>C(s) + O_2(g) \rightarrow CO_2(g)</math>

<math>4Al + 3O_2 \rightarrow 2Al_2O_3</math>

<math>P_4 + 5O_2 \rightarrow P_4O_{10}</math>

<math>2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2</math>

<math>CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O</math>

<math>4HCl + O_2 \rightarrow 2Cl_2 + 2H_2O</math>

➣ Classification of oxides : On the basis of chemical nature.

1. Basic: Na₂O, CaO, etc. metallic oxides.
2. Acidic : <math>CO_2</math>, <math>SO_2</math>, etc. non-metallic oxides.
3. Amphoteric : SnO₂, Al₂O₃, etc. are amphoteric oxides.
4. Neutral: H₂O, CO, etc. are neutral oxides.
5. Poly-oxide: Oxides having oxygen more than required oxygen such as peroxide (<math>Na_2O_2</math>), super oxide (<math>Na_2O_2</math>), super oxide (<math>Na_2O_2</math>), super oxide (<math>Na_2O_2</math>), super oxide (<math>Na_2O_2</math>), super oxide (<math>Na_2O_2</math>), sup dioxide (PbO₂), higher oxide (<math>Mn_2O_7</math>).
6. Sub-oxide: Oxides having oxygen less than required, e.g., C₃O₂.