The P-Block Elements - II

Reactivity towards hydrogen:

• All group 15 elements from trihydrides, MH₃. 
• It belongs to sp₃ hybridisation.
• The stability of hydrides decreases down the group due to decrease in bond dissociation energy down the group.
• NH3 > PH₃ > AsH₃ > SbH₃> BiH3

Boiling point:

• PH₃< AsH₃ < NH₃ < SbH₃ < BiH3
• Boiling point increases with increase in size due to increase in van der Waals forces.
• Boiling point of NH₃ is more because of hydrogen bonding.

Bond angle:

• NH₃ (107.8°) > PH₃ (99.5°) > AsH₃ (91.8°) ≈ SbH₃ (91.3°) > BiH₃ (90°)
• Electronegativity of N is highest. Therefore, the lone pairs will be towards nitrogen and hence more repulsion between bond pairs. Therefore bond angle is the highest. After nitrogen, the electronegativity decreases down the group.
• Basicity decreases as NH₃ > PH₃> AsH₃ > SbH₃ < BiH₃. This is because the lone pair of electrons are concentrated more on nitrogen and hence the basicity will be maximum in the case of NH₃. It will decrease down the group as the electronegativity decreases down the group. The reducing power of hydrides increases down the group due to decrease in bond dissociation energy down the group.

Reactivity towards oxygen:

• All group 15 elements from trioxides (M₂O₃) and pentoxides (M₂O₅).
• Acidic character of oxides decreases and basicity increases down the group. This is because the size of nitrogen is very small.
• It has a strong positive field in a very small area. Therefore, it attracts the electrons of water O-H bond to itself and release H+ ions easily.
• As we move down the group, the atomic size increases and so, the acidic character of oxide decreases and basicity increases down the group.

Reactivity towards halogen:

Group 15 elements form trihalides and pentahalides.

Trihalides

These are covalent compounds and become ionic down the group with sp₃ hybridisation, pyramidal shape.

Pentahalides

• They are lewis acids because of the presence of vacant d – orbitals.
• They possess sp₃d hybridisation and hence possess trigonal birpyamidal shape.
• PCl₅ + Cl ^- ® [ PCl₆ ]-
• PCl₅ is ionic in solid state and exist as[ PCl₄ ]⁺ [ PCl₆ ]^- .
• In PCl₅, there are three equatorial bonds and two axial bonds. The axial bonds are longer than equatorial bonds because of greater repulsion from equatorial bonds.
• Nitrogen does not form pentahalides due to absence of d- orbitals.

Reactivity towards metals:
All elements react with metals to form binary compounds in –3 oxidation state.

Anomalous behaviour of nitrogen:

• The behaviour of nitrogen differs from rest of the elements. Reasons:
• It has a small size.
• It does not have d – orbitals
• It has high electronegativity
• It has high ionization enthalpy

Physical Properties:

It is a colourless, odourless, tasteless and non – toxic gas.
It is chemically un-reactive at ordinary temperature due to triple bond in N ≡ N which has high bond dissociation energy.

Ammonia:

Ammonia molecule is trigonal pyramidal with nitrogen atom at the apex.

It has 3 bond pairs and 1 lone pair.

N is sp₃ hybridised.

Haber’s process:

N ₂ (g ) + 3H ₂ (g ) -> 2NH 3 (g )    ∆_f H⁰  =-46.1KJ mol^-1

Pressure 200 x 10 Pa Temperature 773 K Catalyst is FeO with small amounts of K₂O and Al₂O3

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