Quick chem question- please help!

[poptropica]

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One of the chem review packet had this question and I don't know how to do it.

Predict the sign of S (entropy) for the following reaction.

a) CaCO3(s) -> CaO(s) + CO2(g) answer: (+)

b) N2(g) +3H2(g) -> 2NH3(g) answer: (-)

How do you predict the sign when there is no numerical value given, we can't do products - reactant here.

 

[SYPHARMACY]

entropy is disorder. the first one the disorder increases from 1 molecule to 2 molecules. the difference of the final disorder and to the initial is positive.

the 2nd is 2 molecules to 1. the disorder (entropy) decreases. the final disorder is less than the initial, therefore negative. there is less disorder on the right side.

 

[musicman123]

Entropy is defined as degree of freedom or motion. Anytime you go from solid phase to liquid phase, solid to gas, or liquid to gas, the entropy increases. In (a), your reactant is a solid, but your products include a gas molecule. Thus, entropy increases (positive). In (b), there are a total of 4 gas molecules on the reactant side, compared to 2 gas molecules on the product side. Since the # of gas molecules decreases from reactant to product, entropy decreases (negative).