Give reasons for the following :

(a) Red phosphorus is less reactive than white phosphorus.

(b) Electron gain enthalpies of halogens are largely negative.

(c) N₂O₅ is more acidic than N₂O₃.

(a) White Phosphorus exists as P₄ units. The four phosphorus atoms lie at the corners of a regular tetrahedron with ∠PPP = 60°.

Since PPP angle is much smaller (60°) than the normal tetrahedral angle (109° -28°), therefore, P₄ molecules are highly strained. Due to this angle strain , white phosphorus is very reactive. Like white phosphorus, red phosphorus also exists as P₄ tetrahedral but these are joined together through covalent bonds to give a polymeric structure.

Because of polymeric structure, it is less reactive than white phosphorus.

(b) Electron gain enthalpies of halogens are largely negative are due to the following reasons:

As we know that electron gain enthalpy means tendency to accept an additional electron. Halogens have one electron less than the nearest noble gas configuration. Therefore, they have a strong tendency to accept an additional electron and hence halogens have large negative electron gain enthalpies.

Note: Further as we move down the group, the electron gain enthalpies become less and less negative as the size of the halogen increases. However, electron gain enthalpy of fluorine is less negative than that of chlorine. This is due to small size of fluorine atom. As a result of which strong electron-electron repulsions are present. Thus, amongst halogens, chlorine has the most negative electron gain enthalpy.

(c) N₂O₅ is more acidic than N₂O₃.

As we know that higher the oxidation state of the central atom in the given, higher will be the acidic character.

In N₂O₅, let the oxidation state of central atom N is x

∴ 2x + 5× (-2) = 0

⇒ 2x – 10 = 0

⇒ x = 5

Thus, the oxidation state of nitrogen in N₂O₅ is 5

On the other hand, in N₂O₃, let the oxidation state of central atom N is y

∴ 2y + 3× (-2) = 0

⇒ 2y – 6 = 0

⇒ y = 3

Thus, the oxidation state of nitrogen in N₂O₃ is 3

Hence, the oxidation state of nitrogen in N₂O₅ is higher than the oxidation state of nitrogen in N₂O₃. As a result, N₂O₅ is more acidic.