Here's the reaction:
$\ce{H2SO4(aq) + CaCO3(s) -> CaSO4(s) + H2O(l) + CO2(g)}$
I don't understand why calcium carbonate decomposes in sulphuric acid. It's not soluble in water. I could understand the hydrogen ions oxidizing the calcium, yet the reaction isn't a redox reaction; the oxidation states remain the same!
I don't understand why calcium carbonate decomposes... It's not soluble in water.
It's more soluble in water than you might think. If it weren't soluble, we wouldn't have awesome things such as stalagmites in caves.
Calcium sulfate is more soluble in water than the carbonate, but that isn't saying much. It's solubility is only about $\pu{2 g/L}$. So writing the product as $\ce{CaSO4(s)}$ is only correct if you are talking about dilute solutions. Anything too concentrated would decompose to form precipitated calcium sulfate.
I could understand the hydrogen ions oxidizing the calcium...
Now that would be a very curious reaction. The calcium in calcium carbonate is already in the +2 oxidation state. Calcium has only two valence electrons and further oxidation would require disrupting an intact octet. It would be very unfavorable. $\ce{Ca^{2+} -> Ca^{3+} + e-}$ would be far too unfavorable a reaction for protons to carry out. $\ce{Ca^{3+}}$ would be a much stronger oxidant than aqueous protons.
Why does calcium carbonate decompose in sulphuric acid?
The decomposition can still happen even with insoluble, solid-phase calcium carbonate because reactions can happen at surfaces. An acidic proton could be attacked by a solid-phase carbonate ion at the crystal surface, forming bicarbonate. The bicarbonate could attack another acidic proton, forming a surface-bound $\ce{H2CO3}$ molecule, which could spontaneously transform into $\ce{CO2}$ and water, which would be very loosely bound to the surface, allowing them to escape and exposing fresh not-yet-reacted calcium carbonate molecules to the aqueous surface, and the process begins anew...
Since carbon dioxide is released out, leading to decrease in concentration of product ,hence the reaction goes in forward direction to attain equilibrium leading to breakdown of calcium carbonate.
