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JEE Advanced levelSingle correct optionDifficulty 3 of 3sb104

The thermal decomposition temperatures of the nitrates , , and and the reason for the trend are best described by

  1. Aincrease in the order Ba < Sr < Ca < Mg, because the larger cation polarises the nitrate ion more strongly
  2. Bincrease in the order Ba < Sr < Ca < Mg, because the lattice enthalpy of the oxide falls down the group
  3. Cincrease in the order Mg < Ca < Sr < Ba, because the hydration enthalpy of the cation rises down the group
  4. Dincrease in the order Mg < Ca < Sr < Ba, because the smaller cation polarises the nitrate ion more strongly

Answer: D

Decomposition to , and is aided by distortion of the anion by the cation, and the polarising power falls as the cation gets larger. Thus stability rises down the group (Mg least, Ba most). Hydration enthalpy actually decreases down the group, so the third option is wrong.
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