How would you account for the following:

1. NF₃ is an exothermic compound but NCl₃ is not.

2. The acidic strength of compounds increase in the order:

PH₃<H₂S<HCl

3. SF₆ is kinetically inert.

1. An exothermic compound is more stable than an endothermic compound. Exothermic compound means a compound formed from its elements by releasing heat energy. Since energy is released, it means that the compound formed is more stable.

We know, Cl lies below F in the periodic table due to which Cl is a larger atom, than F. Now, as the difference in size between N and F is small, N-F bond is quite strong. So NF₃ is an exothermic compound whereas NCl₃ is an endothermic compound (as N-Cl bond is weak in nature).

2. The acidity of any compound depends on the stability of the conjugate base, after deprotonation:

We know in the same period, electronegativity increases from left to right, so their electronegativity is in the increasing order P < S < Cl.

After the formation of the respective conjugate bases, Cl being more electronegative, the Cl^- formed is more stable as because it can hold the negative charge better ( most electronegative amongst P, S, Cl). So HCl is the best acid followed by H₂S and PH₃.

So, the acidity order follows as:

PH₃<H₂S<HCl

3. If we look at the structure of SF₆, we can see that in SF₆ , the S atom is surrounded by or sterically protected by six F atoms from the attack by the reagents to such an extent that even reactions like hydrolysis (which are thermodynamically favourable) do not occur. As F does not have vacant d-orbitals to accept the incoming electrons donated by H₂O molecules, it makes SF₆ inert.