Why does a reaction stop when turning into a solid?

Question

I recently was looking at a question about the volume of carbon dioxide produced when reacting calcium carbonate with dilute sulfuric acid versus using nitric acid. In the answer it says that calcium sulfate is produced, which is a solid, which stops the reaction, but I don't get why. Calcium nitrate produces an aqueous solution which causes the reaction to carry on reacting.

The reaction equations are:

$$\ce{CaCO3(s) + H2SO4(aq) -> CaSO4(s) + H2O(l) + CO2(g)}$$

$$\ce{CaCO3(s) + 2HNO3(aq) -> Ca(NO3)2(aq) + H2O(l) + CO2(g)}$$

Answer 1

The reason is due to the surface area of calcium carbonate available to react.

When reacting a block of solid calcium carbonate with dilute sulphuric acid, solid calcium sulphate is produced which is deposited on the surface of the calcium carbonate block (since that is where the reaction occurs). The calcium sulphate is insoluble in dilute sulphuric acid and so you end up with calcium carbonate coated in a layer of calcium sulphate, which stops the acid from getting at the carbonate and so stops the reaction.

In contrast, calcium nitrate, produced from the reaction with nitric acid, is highly soluble in dilute nitric acid and so no solid layer forms around the carbonate block and the reaction can continue until one of the reagents is used up.

Answer 2

Reactions such as these take place in an ionic form where Ca2+ and CO3 2- are formed in the aqueous medium and react with H2 2+ and SO4 2-. When CaSO4 is formed, it precipitates as it is insoluble. Thus it covers the surface of the CaCO3 and the reaction stops. In the second reaction CaNO3 is not insoluble so the reaction continues and CaNO3 solution keeps being formed.

Hope this answers your question!