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BCECE Engineering BCECE Engineering Solved Paper-2015

  • question_answer
    The reactivity order of halogenations of alkianes is                                                                 \[{{F}_{2}}>C{{l}_{2}}>B{{r}_{2}}>{{I}_{2}}.\] (i) Lower the activation energy for the chain initiation step, more reactive is the halogen. (ii) Lower the activation energy for the first chain o- propagation step, more reactive is the halogen. (iii) Lower the activation energy for the second chain propagation step, more reactive is the halogen. (iv) More negative is the overall heat of the reaction (                                                                                      \[\Delta H_{r}^{\circ }\]  of- halogenation of alkane) more reactive is the halogen.

    A) I and II

    B) I, II and III

    C) II and IV

    D) II, III and IV

    Correct Answer: C

    Solution :

    There are two factors that determine the reactivity of the halogenation of alkanes. (i) Lesser the positive value .of                                                                                               \[{{E}_{act}}\]  of the first chain propagating step, more reactive is the halogen. (ii) Higher the negative value of overall heat of reaction                                                                                 \[(\Delta \Eta _{r}^{\circ })\] , more reactive is the halogen. This explains high reactivity and explosive violence with which                                                                                                 \[{{F}_{2}}\]  reacts with                                                                           \[\text{C}{{\text{H}}_{\text{4}}}\] .


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