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?
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.
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.
To determine the sign of the entropy change for a reaction, you need to consider the phase changes and the overall complexity of the system.
In the given reaction, Ca(OH)2(s) and CO2(g) react to form CaCO3(s) and H2O(l). The reactants include a solid and a gas, while the products include a solid and a liquid. When a solid and a gas react to form a solid and a liquid, the overall entropy tends to decrease. This is because solids generally have lower entropy than gases, and liquids have intermediate entropy values.
Therefore, for the reaction presented, the sign of the entropy change would be negative because the reaction leads to a decrease in the overall entropy. Keep in mind that this conclusion is based solely on the phase changes occurring in this specific reaction.
Regarding the term "principal process," it refers to the main sequence of chemical or physical changes that occur during a particular reaction. In the given context, "principal process" refers to the main reaction that takes place when mortar hardens.