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Group 2: Question 10

Syllabus 27.1

Multiple choice A2 1 mark

Barium nitrate must be heated to a considerably higher temperature than magnesium nitrate before it decomposes into the metal oxide, nitrogen dioxide and oxygen.

Which explanation for this difference in thermal stability is correct?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Comparing the two cations

Mg2+\text{Mg}^{2+} and Ba2+\text{Ba}^{2+} carry the same 2+2+ charge, but Mg2+\text{Mg}^{2+} has a much smaller ionic radius than Ba2+\text{Ba}^{2+} (magnesium is much higher up Group 2). Charge density (polarising power) depends on charge divided by radius, so the smaller Mg2+\text{Mg}^{2+} ion has a considerably higher charge density. It is the more strongly polarising cation.

How this affects the nitrate ion

In both compounds, the nitrate ion, NO3−\text{NO}_3^-, is the same large, delocalised anion. A strongly polarising cation like Mg2+\text{Mg}^{2+} can approach closely and distort (polarise) the electron cloud of the nitrate ion, weakening its N–O bonds. This makes it easier for the ion to break down into NO2\text{NO}_2 and O2\text{O}_2 on heating, so Mg(NO3)2\text{Mg(NO}_3\text{)}_2 decomposes at a comparatively low temperature.

Ba2+\text{Ba}^{2+}, being much larger and less charge-dense, polarises the nitrate ion far less. Its N–O bonds remain closer to full strength, so a much higher temperature is needed to break the nitrate ion apart, consistent with barium nitrate requiring stronger heating to decompose than magnesium nitrate.

Why the other options are wrong

  • A gets the size comparison backwards (barium’s ion is larger, not smaller, than magnesium’s) and then draws the wrong conclusion about polarising strength.
  • C correctly notes a charge-density difference but assigns it to the wrong ion, and wrongly claims polarising power affects melting point but not thermal decomposition. The same distortion of the nitrate ion is exactly what controls the decomposition temperature.
  • D is incorrect because the nitrate ion itself is identical in both compounds; it is the cation’s polarising power, not the anion’s size, that differs and drives the trend.

Final answer

B. Mg2+\text{Mg}^{2+}‘s higher charge density polarises the nitrate ion more strongly than Ba2+\text{Ba}^{2+} does, weakening its N–O bonds and giving magnesium nitrate a lower decomposition temperature than barium nitrate.