Question
Explain the reactivity of halogens towards hydrogen.

Answer

  • All halogens react with hydrogen and form hydrogen halides HX where X = F, Cl, Br, I.
  • The affinity and reactivity decrease down the group from F to I.
    (a) $H _2+ F _2 \xrightarrow{\text { dark }} 2 HF$
    (b) $H _2+ Cl _2 \xrightarrow[\text { sunlight }]{\text { diffused }} 2 HCl$
    (c) $H _2+ Br _2 \xrightarrow{\text { heat }} 2 HBr$
    (d) $H _2+ I _2 \stackrel{\text { heat }}{\rightleftharpoons} 2 HI$

    Property$H F$$HCl$$HBr$HI
    Boiling point $( K )$293189206238
    Melting point $( K )$190159185222
    Bond length $( H - X ) pm$$91.7$$127.4$$141.4$$160.9$
    Bond dissociation enthalpy $\left( kJ mol ^{-1}\right)$574432136295

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