What is the sign of the entropy change for the following reaction, which is the principal process taking place when mortar hardens?
Ca(OH)2(s) + CO2(g) ---> CaCO3(s) + H2O(l)

Would the sign of entropy be negative since a solid and gas yields a solid and what does it mean by principal process?

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  1. I assume principle means that there may be intermediate reactions but what is shown is the overall reaction. I agree with you that delta S probably is negative and for the reason you cite. You don't mention the water but that is a liquid and S is lower for liquids than gases.

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  2. From experience, I can say it's negative. Most - if not all - reactions in which an adhesive, glue, cement or whatever you wanna call it hardens, it will be exothermic and entropy goes down. I also asked my chem teacher few months back about that, she said that's pretty much the case. Since the glue becomes more stable and lowers its entropy. Principal process could be what DrBob said, or/and it could also mean there may be completely different reactions going on.

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