Q.Discuss the consequences of high enthalpy of H-H bond in terms of chemical reactivity of dihydrogen.

The H–H bond in dihydrogen has a very high bond dissociation enthalpy of 435.88 kJ mol⁻¹, one of the strongest single covalent bonds known.

Consequence for chemical reactivity: Even though many reactions of H2 (with O2, N2, halogens, metal oxides, etc.) are thermodynamically highly favourable (large negative ΔG), the high bond enthalpy means a large amount of energy must first be supplied to break the H–H bond before new bonds can form. This gives these reactions a high activation energy, so:

• At room temperature, dihydrogen is comparatively unreactive/inert (kinetically stable) despite being thermodynamically capable of reacting.
• Reactions require an external energy input to proceed at a useful rate: high temperature (e.g., $H_2 + N_2 \xrightarrow{catalyst, high\ T, high\ P} 2NH_3$, Haber process), a catalyst (e.g., hydrogenation of oils using Ni/Pd/Pt), light (photochemical reaction with Cl2), or an electric spark/arc (combustion with O2, or dissociation in the atomic hydrogen torch). …

State the numerical H–H bond enthalpy, distinguish thermodynamic favourability from kinetic feasibility, and list the typical energy inputs (heat, ca …

• Confusing "thermodynamically favourable" with "happens readily/fast" — a strong bond can make an exothermic reaction very slow at room temperature. …