Q.Which of the following compounds will not evolve H₂ gas on reaction with alkali metals ?

Alkali metals are chemically very reactive, and reactivity increases from Li to Cs as ionisation energy falls down the group; they react especially readily with electronegative elements such as oxygen and the halogens.

With oxygen: Li forms only the normal oxide (4Li + O₂ → 2Li₂O); Na forms mainly the peroxide (2Na + O₂ → Na₂O₂); K, Rb, Cs go further and form the superoxide (M + O₂ → MO₂). All these oxides are basic.

With hydrogen (~673 K, Li needs 1073 K): all form ionic hydrides, 2M + H₂ → 2M⁺H⁻. Reactivity toward H₂ decreases Li→Cs even as the hydride's ionic character increases and its stability falls; the hydrides are strong, increasingly powerful reducing agents down the group.

With halogens: 2M + X₂ → 2MX, all ionic except LiI (which shows covalent character, since the small, highly polarising Li⁺ distorts the large, easily-polarised I⁻); reactivity increases down the group as ionisation enthalpy falls.

With liquid ammonia: alkali metals dissolve to give deep blue, electrically conducting solutions -- M + (x+y)NH₃ → [M(NH₃)ₓ]⁺ + [e(NH₃)ᵧ]⁻ -- the blue colour coming from the "ammoniated electron"; such solutions are paramagnetic and slowly liberate H₂ on standing (forming an amide), while concentrated solutions turn bronze and diamagnetic.

With water/alcohols/alkynes (any active-hydrogen compound): 2M + 2H₂O → 2MOH + H₂; similarly with alcohols (2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂) and terminal alkynes (HC≡CH + Na → HC≡C-Na, → Na-C≡C-Na with excess metal).

Reducing activity: M(s) → M⁺(g) + e⁻ -- readily lost valence electrons make alkali metals good reducing agents generally.

With carbon: only Li reacts directly (2Li + 2C → Li₂C₂); the rest need a carbon compound like acetylene instead.