The oxidation state of P in PCl₅ is +5 and that in PCl₃ is +3.  Since elements in the +5 oxidation state have less tendency to lose electrons than in the +3 oxidation state, therefore,  P in the +5 oxidation state has more tendency to share its electrons with C1 than in the +3 oxidation state.  Thus, PCl₅ is more covalent than PC1₃.

Since oxidation state of N in HNO₂. + 3, therefore, it can be increased to its maximum O.N. of + 5 and hence it acts as a reducing it.  For example, it reduces KMnO₄ to manganese salts.
\(2\ KMnO_4\ +\ 5\ HNO_2\ +\ 3\ H_2SO_4\ \longrightarrow K_2SO_4\ +\ 2\ MnSO_4\ +\ 5\ HNO_3\ +\ 3\ H_2O\)
For more such reactions, refer to 'Supplement Your Knowledge' page 7/29.

Due to presence of a triple bond between the two N-atoms, the bond dissociation enthalpy (941.4 kJ mol^-1) is very high.  Hence, N₂ is least reactive.

Sites different from active site of enzymes where a molecule can bind and affect the active site is called allosteric site.

shape-selective catalysis: The catalysis which depends upon the pore structure of the catalyst and molecular size of reactant and product molecules is called shape-selective catalysis, e.g. Zeolites are shape-selective catalysis due to their honey- comb structure. ZSM-5 (Zeolite Sieve of Molecular porosity is used to convert methanol into gasoline (petrol).