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NEET Chemistry Metallurgy / धातुकर्म Question Bank Self Evaluation Test - General Principals and Process of Isolation of Elements

  • question_answer
    The value of \[{{\Delta }_{f}}G{}^\circ \] for formation of \[C{{r}_{2}}{{O}_{3}}\] is \[-540\text{ }kJmo{{l}^{-1}}\]  and that of \[A{{l}_{2}}{{O}_{3}}\] is \[-827\text{ }kJ\text{ }mo{{l}^{-1}}\]  What is the value of \[{{\Delta }_{f}}G{}^\circ \] for the reaction? \[\frac{4}{3}Al(s)+\frac{2}{3}C{{r}_{2}}{{O}_{3}}(s)\to \frac{2}{3}A{{l}_{2}}{{O}_{3}}(s)+\frac{4}{3}Cr(s).\]

    A) \[-574\text{ }kJ\,mo{{l}^{-1}}\]           

    B) \[-287\,kJ\,mo{{l}^{-1}}\]

    C) \[+574\text{ }kJ\,mo{{l}^{-1}}\]   

    D) \[+287kJ\,mo{{l}^{-1}}\]

    Correct Answer: B

    Solution :

    [b] The two equation are: \[\frac{4}{3}Al(s)+{{O}_{2}}(g)\to \frac{2}{3}A{{l}_{2}}{{O}_{3}}(s),\] \[{{\Delta }_{f}}G{}^\circ =-827kJ\,mo{{l}^{-1}}\]      .... (1) \[\frac{4}{3}Cr(s)+{{O}_{2}}(g)\to \frac{2}{3}C{{r}_{2}}{{O}_{3}}(s),\] \[{{\Delta }_{f}}G{}^\circ =-540kJ\,mo{{l}^{-1}}\]   .... (2) Subtracting equation (ii) from equation (i) we have, \[\frac{4}{3}Al(s)+\frac{2}{3}C{{r}_{2}}{{O}_{3}}(s),\to \frac{2}{3}A{{l}_{2}}{{O}_{3}}(s)+\frac{4}{3}Cr(s),\] \[{{\Delta }_{r}}{{G}^{{}^\circ }}=-287kJ\text{ }mo{{l}^{-1}}\]


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