Topic

Concentrated nitric acid is a strong oxidizing agent. It is used for oxidizing most metals. The products of oxidation depend on the concentration of the acid, temperature, and also on the material undergoing oxidation.
3Cu + 8 HNO_3(dil) → 3 Cu(NO₃)₂ + 2 NO + 4H₂O
Cu + 4 HNO_3(conc) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

Almost all halogens are coloured. This is because halogens absorb radiations in the visible region. This results in the excitation of valence electrons to a higher energy region. Since the amount of energy required for excitation differs for each halogen, each halogen displays a different colour.

Hydride NH₃ PH₃ AsH₃ SbH₃
H-M-H angle 107^o 92^o 91^o 90^o
The above trend in the H?M?H bond angle can be explained on the basis of the electronegativity of the central atom. Since nitrogen is highly electronegative, there is high electron density around nitrogen. This causes greater repulsion between the electron pairs around nitrogen, resulting in maximum bond angle. We know that electronegativity decreases on moving down a group. Consequently, the repulsive interactions between the electron pairs decrease, thereby decreasing the H-M-H bond angle.

N (unlike P) lacks the d-orbital. This restricts nitrogen to expand its coordination number beyond four. Hence, R₃N=O does not exist.

(i) 4 NaCl + MnO₂ + 4 H₂SO₄ → MnCl₂ + 4NaHSO₄ + 2H₂O + Cl₂
(ii) Cl₂ + NaI → 2 NaCl + I₂

Catenation is much more common in phosphorous compounds than in nitrogen compounds. This is because of the relative weakness of the N?N single bond as compared to the P?P single bond. Since nitrogen atom is smaller, there is greater repulsion of electron density of two nitrogen atoms, thereby weakening the N?N single bond.

On heating, orthophosphorus acid (H₃PO₃) disproportionates to give orthophosphoric acid (H₃PO₄) and phosphine (PH₃). The oxidation states of P in various species involved in the reaction are mentioned below.
4H₃PO₃ → 3H₃PO₄ + PH₃.

The elements of group 16 are collectively called chalcogens.
(i) Elements of group 16 have six valence electrons each. The general electronic configuration of these elements is ns² np⁴, where n varies from 2 to 6.
(ii) Oxidation state: As these elements have six valence electrons (ns² np⁴), they should display an oxidation state of ?2. However, only oxygen predominantly shows the oxidation state of ?2 owing to its high electronegativity. It also exhibits the oxidation state of ?1 (H2O2), zero (O2), and +2 (OF2). However, the stability of the ?2 oxidation state decreases on moving down a group due to a decrease in the electronegativity of the elements. The heavier elements of the group show an oxidation state of +2, +4, and +6 due to the availability of d-orbitals.
(iii) Formation of hydrides: These elements form hydrides of formula H₂E, where E = O, S, Se, Te, PO. Oxygen and sulphur also form hydrides of type H₂E₂. These hydrides are quite volatile in nature.

Neil Bartlett initially carried out a reaction between oxygen and PtF₆ . This resulted in the formation of a red compound, O⁺₂ [PtF₆ ]^-.
Later, he realized that the first ionization energy of oxygen (1175 kJ/mol) and Xe (1170 kJ/mol) is almost the same. Thus, he tried to prepare a compound with Xe and PtF₆ . He was successful and a red-coloured compound, Xe⁺[PtF₆]^- was formed.

(i) Cl2 can be prepared from HCl by Deacons process.
4 HCl + O₂ → 2Cl₂ + 2H₂ O
(ii) HCl can be prepared from Cl2 on treating it with water.
Cl₂ + H₂ O → HCl + HOCl

Freons or chlorofluorocarbons (CFCs) are aerosols that accelerate the depletion of ozone. In the presence of ultraviolet radiations, molecules of CFCs break down to form chlorine-free radicals that combine with ozone to form oxygen.

Sulphur dioxide causes harm to the environment in many ways:
1. It combines with water vapour present in the atmosphere to form sulphuric acid. This causes acid rain. Acid rain damages soil, plants, and buildings, especially those made of marble.
2. Even in very low concentrations, SO₂ causes irritation in the respiratory tract. It causes throat and eye irritation and can also affect the larynx to cause breathlessness.
3. It is extremely harmful to plants. Plants exposed to sulphur dioxide for a long time lose colour from their leaves. This condition is known as chlorosis. This happens because the formation of chlorophyll is affected by the presence of sulphur dioxide.

(i) Cl₂ + H₂ O → HCl + HOCl
(ii) 2 F₂ + 2 H₂ O→ 4H ⁺ + 4 F ^- + O₂ + 4 HF

Fluorine forms only one oxoacid i.e., HOF because of its high electronegativity and small size.

Both chlorine and oxygen have almost the same electronegativity values, but chlorine rarely forms hydrogen bonding. This is because in comparison to chlorine, oxygen has a smaller size and as a result, a higher electron density per unit volume.