The Best Answer for Predict the sign of the entropy change, ΔS∘ Help!?
Jan gives correct answers without any explanation.
The way to approach this question is to examine the number of molecules and their states on both sides of the equation. If the numbers of molecules increases, this will generally be associated with an increase in the entropy. Additionally, if more molecules of gas are present in the products compared to the reactants, the entropy will generally increase. If soluble ions or gases form a solid product, entropy will generally decrease.
So, in this reaction: 2NO2 --> N2(g) + 2O2(g) , you are going from 2 moles of a gas to 3 moles of gas. That change will have an increase in entropy.
P4(g)+5O2(g) --> P4O10(s): Here, 6 moles of gas form a single mole of solid, so entropy will decrease.
C5H12(g)+8O2(g) --> 5CO2(g)+6H2O(g): Here, you are going from 9 moles of a gas to 11 moles of gas. Additionally, you are going from a larger molecule C5H12 to smaller molecules. These both will increase the entropy.
Hope that helps..