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# Third Law of Thermodynamics & Entropy Changes (College)

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## Third Law of Thermodynamics & Entropy Changes

Instructions: The following questions may require the use of a reference table. The questions concern the Third Law of Thermodynamics and calculating entropy changes in a reaction.

1.
Which of the following correctly classifies entropy?
1. It is an intensive property, in that it depends on the identity of the substance
2. It is an extensive property, in that it depends on the number of moles
3. It is neither extensive nor intensive
4. It is an extensive property, in that it depends on the identity of the substance
2.
Which of the following correctly represents trends in Sº, in going from $H_2O(s)$ to $H_2O(l)$?
1. A decrease
2. An increase
3. A decrease, and an increase
4. No changes
3.
Which of the following represents the value for the entropy of a perfect crystal at 0 K?
1. Positive
2. Negative
3. 0
4. 1
4.
Which of the following accounts for the entropy value of a perfect crystal at 0 K?
1. All of the particles are arranged in a regular pattern
2. All of the particles are arranged in a perfect position in the lattice structure
3. All of the particles move past one another with minimal entropy
4. All of the particles are evenly dispersed
5.
Which of the following factors dominates changes in positional entropy for particles in a reaction, when gaseous reactants and products are involved?
1. The temperature of the particles
2. The volume of the particles
3. The physical states of the particles themselves
4. The numbers of gaseous reactants and products
6.
For reactants involving gas molecules, which of the following conditions need to be constant, in order to determine changes in positional entropy?
1. Temperature, only
2. Pressure, only
3. Temperature and Pressure, only
4. Volume, only
7.
Which of the following reactions correctly demonstrates an increase in positional entropy?
1. $2H_2 (g) + O_2 (g) rarr 2H_2O(l)$
2. $N_2(g) + O_2(g) rarr 2NO(g)$
3. $N_2O_4(g) rarr 2NO_2(g)$
4. None of the above
8.
Which of the following represents an increase in positional entropy?
1. $2CO(g) + O_2(g) rarr 2CO_2(g)$
2. $2C_2H_6(g) + 7O_2(g) rarr 4CO_2(g) + 6H_2O(g)$
3. $N_2(g) + O_2(g) rarr 2NO(g)$
4. None of the above
9.
Which of the following can be used to calculate entropy changes in reactions?
1. $∆Gº_{reaction}=∑n_{r}∆Gº_f{"reactants"}-∑n_{p}∆Gº_f{"products"}$
2. $∆Gº_{reaction}=∑n_{r}∆Gº_f{"reactants"}+∑n_{p}∆Gº_f{"products"}$
3. $∆Gº_{reaction}=∑n_{p}∆Gº_f{"products"}-∑n_{r}∆Gº_f{"reactants"}$
4. None of the above
10.
Based on the following reaction:
$SO_3(s) rarr SO_3(l)$
What is the value of $∆Sº_{reaction}$? Show your work.

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