Thermal stability of Group 2 carbonates/nitrates

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Anon-
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Hey guys, I had a question on why thermal stability increases down Group 2? What I learnt in Unit 1 was that the smaller the cation radius was, the more polarizing it is: increasing its covalent character and thus its stability/melting temperature. In a Unit 2 question it asks: Calcium nitrate decomposes in a similar way to magnesium nitrate, but at ahigher temperature. Explain why the two nitrates have different stability to heat. Logically I would think that since the Calcium ion is bigger, it is less polarizing, and thus has a lower m.p. The mark scheme says something similar, but it doesn't really make sense to me:
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Could someone please help me out? Thanks in advance.
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NDVA
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(Original post by Anon-)
Hey guys, I had a question on why thermal stability increases down Group 2? What I learnt in Unit 1 was that the smaller the cation radius was, the more polarizing it is: increasing its covalent character and thus its stability/melting temperature. In a Unit 2 question it asks: Calcium nitrate decomposes in a similar way to magnesium nitrate, but at ahigher temperature. Explain why the two nitrates have different stability to heat. Logically I would think that since the Calcium ion is bigger, it is less polarizing, and thus has a lower m.p. The mark scheme says something similar, but it doesn't really make sense to me:
Name:  sEzl2Nz.png
Views: 516
Size:  57.2 KB
Could someone please help me out? Thanks in advance.
Melting point is a different concept from thermal stability in this case.

'Melting point' is when you separate Ca(NO_3)_2 molecules from each other.

Thermal stability is when you break the N-O bond in each Ca(NO_3)_2 molecule to form CaO, NO_2, and O_2 as products.

Your reasoning is fine, you are just confused between 'melting something' and 'decomposing something'
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TheWiseSalmon
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(Original post by Anon-)
Hey guys, I had a question on why thermal stability increases down Group 2? What I learnt in Unit 1 was that the smaller the cation radius was, the more polarizing it is: increasing its covalent character and thus its stability/melting temperature. In a Unit 2 question it asks: Calcium nitrate decomposes in a similar way to magnesium nitrate, but at ahigher temperature. Explain why the two nitrates have different stability to heat. Logically I would think that since the Calcium ion is bigger, it is less polarizing, and thus has a lower m.p. The mark scheme says something similar, but it doesn't really make sense to me:
Name:  sEzl2Nz.png
Views: 516
Size:  57.2 KB
Could someone please help me out? Thanks in advance.
Have you learnt about Lattice Enthalpies yet?
The explanation is that, the difference in Lattice Enthalpies between the Metal Carbonates and Metal Oxides decreases down the group.

Lattice Enthalpy is proportional to the sum of the ionic radii of the cations and anions. Down the group, the cation radius increases so the lattice enthalpy of both the Oxide and the Carbonate decreases, but this also means the difference in the two enthalpies decreases.

If the difference in lattice enthalpies is small, then there's less of thermodynamic driving force to form the Oxide.
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Anon-
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(Original post by TheWiseSalmon)
Have you learnt about Lattice Enthalpies yet?
The explanation is that, the difference in Lattice Enthalpies between the Metal Carbonates and Metal Oxides decreases down the group.

Lattice Enthalpy is proportional to the sum of the ionic radii of the cations and anions. Down the group, the cation radius increases so the lattice enthalpy of both the Oxide and the Carbonate decreases, but this also means the difference in the two enthalpies decreases.

If the difference in lattice enthalpies is small, then there's less of thermodynamic driving force to form the Oxide.
But if the driving force is less, shouldn't it have a lower thermal stability as well? Or is it because of that small force that the stability is higher?
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charco
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(Original post by Anon-)
But if the driving force is less, shouldn't it have a lower thermal stability as well? Or is it because of that small force that the stability is higher?
The two concepts are best kept separate.

Melting point is a function of the lattice enthalpy, which in turn is a function of ionic charge and radius.

Smaller the ions (and the larger the charge magnitude) the higher the lattice enthalpy and the more energy needed to melt the lattice.

Thermal stability is a chemical process whereby the components of the lattice react causing decomposion to more stable products. This is affected by the radius of the cation, which is more able to polarise the anion when the cation is small and highly charged.
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Anon-
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(Original post by charco)
The two concepts are best kept separate.

Melting point is a function of the lattice enthalpy, which in turn is a function of ionic charge and radius.

Smaller the ions (and the larger the charge magnitude) the higher the lattice enthalpy and the more energy needed to melt the lattice.

Thermal stability is a chemical process whereby the components of the lattice react causing decomposion to more stable products. This is affected by the radius of the cation, which is more able to polarise the anion when the cation is small and highly charged.
Oh ok, it makes sense now, my confusion was caused from thinking the two are the same. Thanks a lot.
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