NEET Chemistry NEET PYQ-Chemical Kinetics

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    Consider the reaction \[{{N}_{2}}(g)+3{{H}_{2}}(g)\xrightarrow{{}}2N{{H}_{3}}(g)\]
                The equality relationship between \[\frac{d[N{{H}_{3}}]}{dt}\] and \[-\frac{d[{{H}_{2}}]}{dt}\] is:                  [AIPMT (S) 2006]

    A) \[\frac{d\,[N{{H}_{3}}]}{dt}=-\frac{1}{3}\,\frac{d\,[{{H}_{2}}]}{dt}\]

    B) \[+\frac{d\,[N{{H}_{3}}]}{dt}=-\frac{2}{3}\,\frac{d\,[{{H}_{2}}]}{dt}\]           

    C) \[+\frac{d\,[N{{H}_{3}}]}{dt}=-\frac{3}{2}\,\frac{d\,[{{H}_{2}}]}{dt}\]

    D) \[\frac{d\,[N{{H}_{3}}]}{dt}=-\,\frac{d\,[{{H}_{2}}]}{dt}\]

    Correct Answer: B

    Solution :

    [b] For the reaction,
                \[{{N}_{2}}(g)+3{{H}_{2}}(g)\xrightarrow[{}]{{}}2N{{H}_{3}}(g)\]
                The rate of reaction w.r.t. \[{{N}_{2}}=-\frac{d\,[{{N}_{2}}]}{dt}\]
                The rate of reaction w.r.t \[{{H}_{2}}=-\frac{1}{3}\,\frac{d\,[{{H}_{2}}]}{dt}\]
                The rate of reaction w.r.t \[N{{H}_{3}}=+\frac{1}{2}\,\frac{d\,[N{{H}_{3}}]}{dt}\]
                Hence, at a fixed time
                \[-\frac{d[{{N}_{2}}]}{dt}=-\frac{1}{3}\,\frac{d\,[{{H}_{2}}]}{dt}\]
                                        \[=+\frac{1}{2}\,\frac{d\,[N{{H}_{3}}]}{dt}\]
                \[or+\frac{d\,[N{{H}_{3}}]}{dt}=-\frac{2}{3}\,\frac{d\,[{{H}_{2}}]}{dt}\]
                \[or-\frac{2d\,[{{N}_{2}}]}{dt}\]


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