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CMC Medical CMC-Medical VELLORE Solved Paper-2014

  • question_answer
    The free energy change for the following reactions are given below. \[{{C}_{2}}{{H}_{2}}(g)+\frac{5}{2}{{O}_{2}}(g)\xrightarrow[{}]{{}}2C{{O}_{2}}(g)+{{H}_{2}}O(l);\]\[\Delta G{}^\circ =-1234\,kJ\] \[C(s)+{{O}_{2}}(g)\xrightarrow{{}}C{{O}_{2}}(g);\] \[\Delta G{}^\circ =-\,394\,kJ\]\[{{H}_{2}}(g)+\frac{1}{2}{{O}_{2}}(g)\xrightarrow{{}}{{H}_{2}}O(l);\] \[\Delta G{}^\circ =-\,237\,kJ\] What is the standard free energy change for the reaction? \[{{H}_{2}}(g)+2C(s)\xrightarrow{{}}{{C}_{2}}{{H}_{2}}(g)\]

    A)  \[-\,209\text{ }kJ\]                        

    B)  \[-\,2259\text{ }kJ\]

    C)  \[+\,2259\text{ }kJ\]                     

    D)  \[209\text{ }kJ\]

    Correct Answer: D

    Solution :

              \[{{C}_{2}}{{H}_{2}}(g)+\frac{5}{2}{{O}_{2}}(g)\xrightarrow{{}}2C{{O}_{2}}(g)+{{H}_{2}}O\,(l);\] \[\Delta G{}^\circ =-1234\,kJ\]                   ?(i) \[C(s)+{{O}_{2}}(g)\xrightarrow{{}}C{{O}_{2}}(g);\]           \[\Delta G{}^\circ =-\,394\,kJ\]                                        ?(ii) \[{{H}_{2}}(g)+\frac{1}{2}{{O}_{2}}(g)\xrightarrow{{}}{{H}_{2}}O(l);\] \[\Delta G{}^\circ =-\,237\,kJ\]                                  ?(iii) Eq. 2(ii) + (iii) - (i) \[2C(s)+{{H}_{2}}(g)\xrightarrow{{}}{{C}_{2}}{{H}_{2}}(g)\] \[\Delta G{}^\circ =2(-394)+(-237)-(-1234)\] \[\Delta G{}^\circ =209\,kJ\]


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