A level chemistry why does reaction of decomposing BaCO3 take longer than MgCO3

I thought that because Mg2+ has a smaller ionic radius, it will bond more strongly to the carbonate ion from its higher charge density and therefore the bond between them is more stable.

Apparently that is wrong because Mg2+ polarises the carbonate ion more, but I still don't fully understand why the bond is easier to break in MgCO3.
I guess it must be because there are multiple atoms in CO3 and it's not like NaCl where there would be no polarising effect between Na+ and Cl- ? Is that all there is to it? It just feels like I am missing something because I've never come across this before

Reply 1

Bookworm524

Original post by jjeeeeeea

I thought that because Mg2+ has a smaller ionic radius, it will bond more strongly to the carbonate ion from its higher charge density and therefore the bond between them is more stable.

If this is right then decomposing MgCO3 would take longer instead?

I don't know either, but if Mg2+ and CO3 2- has covalent character, does that mean their bonding is not as strong as purely ionic perfect sphere? So it's easier broken than an ionic lattice as in BaCO3

Reply 2

jjeeeeeea

Original post by Bookworm524

Yeah, my initial thought was that decomposing MgCO3 should take longer.. How can MgCO3 have covalent character and BaCO3 doesn't though?

Wouldn't the CO3 part have less covalent character in MgCO3 than BaCO3 if the higher charge density of Mg2+ disrupts the covalent character of CO3 2- to a higher extent?

Reply 3

Bookworm524

Original post by jjeeeeeea

Yeah, my initial thought was that decomposing MgCO3 should take longer.. How can MgCO3 have covalent character and BaCO3 doesn't though?

I think Mg2+ is a smaller ion so it attracts negative charge on CO3 2- stronger,
Ba2+ is similar size with CO3 2- so not as much covalent character (similar electronegativity)

Reply 4

jjeeeeeea

I also thought if there were both covalent and ionic character then the bond would be stronger, if BaCO3 does have more covalent character then I suppose maybe that could explain it. Not really sure at all tbh

Original post by jjeeeeeea

Wouldn't the CO3 part have less covalent character in MgCO3 than BaCO3 if the higher charge density of Mg2+ disrupts the covalent character of CO3 2- to a higher extent?

Reply 5

jjeeeeeea

Original post by Bookworm524

I think Mg2+ is a smaller ion so it attracts negative charge on CO3 2- stronger,
Ba2+ is similar size with CO3 2- so not as much covalent character (similar electronegativity)

Wouldn't similar electronegativity only change strength of the ionic bond ?

Reply 6

charco

Study Forum Helper

Original post by jjeeeeeea

Wouldn't similar electronegativity only change strength of the ionic bond ?

There are two facets here.

In the reaction it is the carbonate ion that is decomposing:

CO₃^2- ==> CO₂ + O^2-It is easier for this to happen if the C-O bond in the carbonate ion is distorted (polarised) by the nearby presence of a high charge-density positive ion.

Charge density is a function of charge/volume.

Both barium ions and magnesium ions have the same charge, BUT the barium ion is much larger.

Hence, the magnesium ion has a greater polarising effect on the carbonate ion, weakening the C-O bond and making decomposition easier.