estimating an entropy change

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Kayla Danesh 1F
Posts: 38
Joined: Thu Jul 13, 2017 3:00 am

estimating an entropy change

Postby Kayla Danesh 1F » Mon Jan 29, 2018 6:04 pm

Self-Test 9.13B: "Without doing any calculations, estimate the sign of the entropy change of the system for the reaction CaCO3(s) --> CaO(s) + CO2(g) and explain your answer"

Can someone please guide me through this question? Can I just assume that delta S is positive, since there is a gas on the reactants side and gases have a higher entropy than solids?

Hannah Chew 2A
Posts: 76
Joined: Fri Sep 29, 2017 7:05 am
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Re: estimating an entropy change

Postby Hannah Chew 2A » Mon Jan 29, 2018 6:54 pm

Well, there is no gas on the reactants side, but there is a gas on the products side. There is a net increase in gases, which results in a positive change in entropy. The molar entropy of a gas is much larger than that of solids and liquids, so we can assume that entropy's sign is positive without having to look at numerical data.

Ethan-Van To Dis2L
Posts: 50
Joined: Fri Sep 29, 2017 7:05 am

Re: estimating an entropy change

Postby Ethan-Van To Dis2L » Mon Jan 29, 2018 6:56 pm

Just think about the position of solid vs gas molecules. The atoms in a solid are rigid and have little room to move so the entropy is low, while gases have a lot of space to move thus a higher entropy, which explains why delta S is positive.

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