question_answer 107)

  When hydrochloric acid is added to cobalt nitrate solution at room temperature, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture it becomes pink. On the basis of this information mark the correct answer. \[\underset{(pink)}{\mathop{{{[Co{{({{H}_{2}}O)}_{6}}]}^{2+}}}}\,(aq)+4C{{l}^{-}}(aq)\underset{{}}{\leftrightarrows}\underset{(blue)}{\mathop{{{[CoC{{l}_{4}}]}^{2-}}}}\,(aq)\]\[+6{{H}_{2}}O(l)\] (a) \[\Delta H>0\]for the reaction                                             (b) \[\Delta H<0\]for the reaction (c)\[\Delta H=0\] for the reaction                                             (d) The sign of \[\Delta H\] cannot be predicted on the basis of this information

Answer:

  (a) On cooling, the solution becomes pink, i.e., it shifts in backward direction: \[\underset{(pink)}{\mathop{{{[Co{{({{H}_{2}}O)}_{6}}]}^{3+}}}}\,(aq)+4C{{l}^{-}}(aq)\underset{{}}{\leftrightarrows}\]\[\underset{(blue)}{\mathop{{{[CoC{{l}_{4}}]}^{2-}}}}\,(aq)+6{{H}_{2}}O(l)\] It means, reaction will be endothermic, i.e., \[\Delta H>0\].