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Trend in thermal decomposition of the carbonate watch

    • Thread Starter

    I thought when we go down a group 2, charge density decreases( cuz of larger ionic size)

    Therefore ions are less strongly attracted and easier to separate them !!! MS says sth else:rolleyes:

    You're not separating the metal and the carbonate. You're breaking a C-O bond in carbonate. I answered this question a few days ago so I'll copy and paste my reply to that thread:

    On heating, metal carbonates break across a C-O bond to give CO2 and a metal oxide as below. M is a group 2 metal.

    MO-CO2 ---> MO(d-)---CO2(d+) ---> MO + CO2

    Bearing in mind carbonate is 2- with the negative charge delocalised, I used d+ and d- (d = delta) to mean relative to the rest of the carbonate ion.

    Because the group 2 (and group 1) metals are positive ions, they draw negative charge towards them even before the C-O bond begins breaking so the carbonate group is polarised like this:


    So you could say the C-O bond is already on its way to breaking. Can you see why metals with higher charge denisty at the top of the group would make it easier to completely break that bond and thus require less energy?
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Updated: March 20, 2011

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