[yomi]

The reaction: CaO(s) + H2O(l) → Ca(OH)2(s)  is spontaneous at 0oC.
Predict the sign of the reaction entropy (+ or  -)

The answer is: negative, but I don't know why.
The answer should be >0  when the reaction is spontaneous, right?
Can I predict this reation without calculating the data?

[MrTeo]

The answer should be >0  when the reaction is spontaneous, right?

∆G<0 when the reaction is spontaneous and as you know ∆G depends on both ∆H and ∆S. Here to find out the sign of the entropy change you don't really need to know whether the reaction is spontaneous or not (anyway you should know that throwing CaO in water produces enough heat to boil an egg). Simply look at the aggregation state of the reagents and of the products.

[yomi]

Thanks!