Q1(p). Why 511 nm light dissociates Br₂ but not H₂

Answer in brief

A 511 nm photon carries 234 kJ mol⁻¹: more than D₀(Br₂) = 190 but far less than D₀(H₂) = 432 kJ mol⁻¹; and H₂ does not absorb visible light at all.

Key points

• Photon energy E = hc/λ = 3.89 × 10⁻¹⁹ J = 2.43 eV; per mole E = 234 kJ mol⁻¹.
• Br₂: D₀ = 190 kJ mol⁻¹ < 234 ⇒ dissociation possible; Br₂ absorbs in the visible.
• H₂: D₀ = 432 kJ mol⁻¹ > 234 ⇒ one photon is too weak; it would need λ < 277 nm.
• H₂ is colourless: its first electronic absorption lies in the vacuum UV; by the Grotthuss–Draper law only absorbed light can act.
• So H₂ enters the chain only thermally: Br + H₂ → HBr + H.