Entropy question

[r1b0s0m3]

CO2(g) + C(s) --> 2CO(g)
The question is asking if entropy change is positive or negative.

I know that going from 1 mol of gas to 2 moles of gas is more disorder, and therefore an increase in entropy. But how come the solid is not included in this question? Isn't going from 1 mol of solid to 0 mol of solid (in the products) a decrease (negative) entropy?

The answer is +S, but I was wondering why the solid was not included.

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[justhanging]

CO2(g) + C(s) --> 2CO(g)
The question is asking if entropy change is positive or negative.

I know that going from 1 mol of gas to 2 moles of gas is more disorder, and therefore an increase in entropy. But how come the solid is not included in this question? Isn't going from 1 mol of solid to 0 mol of solid (in the products) a decrease (negative) entropy?

The answer is +S, but I was wondering why the solid was not included.

I don't think that pure substances have negative entropy, it is defined to be 0 at absolute zero, I believe. The entropy of carbon solid (graphite) is 5.74, this am sure cause I just checked it in my pchem book. So there is no decrease of negative entropy like you said but rather solids have very low positive entropy. The only negative is the change in entropy. The entropy of CO gas is 196.7 and CO2 gas is 213, these are molar entropy values.

 

[Cosmic]

CO2(g) + C(s) --> 2CO(g)
The question is asking if entropy change is positive or negative.

I know that going from 1 mol of gas to 2 moles of gas is more disorder, and therefore an increase in entropy. But how come the solid is not included in this question? Isn't going from 1 mol of solid to 0 mol of solid (in the products) a decrease (negative) entropy?

The answer is +S, but I was wondering why the solid was not included.

Gases are considerably more entropic than solids or liquids. The question did ask about the entropy of the solid form, just not directly. The final conditions (total disorder of products) have higher entropy than the initial conditions (total disorder of reactants) so overall the entropy should be positive. When they refer to an increase in disorder, they are referring to ALL the initial conditions (reactants) or ALL the final conditions (products), not one specific phase or another.

 

[Cosmic]

I don't think that pure substances have negative entropy, it is defined to be 0 at absolute zero, I believe. The entropy of carbon solid (graphite) is 5.74, this am sure cause I just checked it in my pchem book. So there is no decrease of negative entropy like you said but rather solids have very low positive entropy. The only negative is the change in entropy. The entropy of CO gas is 196.7 and CO2 gas is 213, these are molar entropy values.

Entropy itself is never a negative value. It's either zero or positive. At 0K, a perfect crystal is said to have 0 entropy. All entropic values are defined relative to that (second law of thermodynamics). However, the change in entropy could be negative or positive. Anytime they are asking about the change in entropy of a reaction, we aren't just looking at one phase conversion to another -- we are comparing all the phases of the reactants to all the phases of the products. If the products are less random, change in entropy would be a negative value though each individual component would never be negative since by default entropy values are equal or greater than zero.