Q: What is the correct formula for calculating the standard entropy change of a reaction ($$\Delta S^\circ_{rxn}$$)?

$$\Delta S^\circ_{rxn} = \sum n_p S^\circ(\text{products}) - \sum n_r S^\circ(\text{reactants})$$

Question 1

Which statement best describes the Third Law of Thermodynamics?

Accepted Answer

The entropy of a perfect crystal at absolute zero is zero: $$S(0\ K,\ 	ext{perfect crystal}) = 0$$

Question 2

Which of the following substances has the highest standard molar entropy at 298 K?

Accepted Answer

C$$_2H$_4(g)$$

Question 3

For the reaction: $$2\ 	ext{CO}(g) + 2\ 	ext{NO}(g) ightarrow 2\ 	ext{CO}_2(g) + 	ext{N}_2(g)$$, given $$\Delta G_f^\circ$$ values: CO(g) = $$-137.2 kJ/$$mol, NO(g) = $$+87.6 kJ/$$mol, CO$$_2(g$$) = $$-394.4 kJ/$$mol, N$$_2(g$$) = $$0$$, what is $$\Delta G^\circ_{rxn}$$?

Accepted Answer

$$-689.6 kJ$$

Question 4

Which factor most strongly increases the standard molar entropy ($$S^\circ) of a $$substance?

Accepted Answer

Changing the physical state from solid to gas

Question 5

What is the correct formula for calculating the standard entropy change of a reaction ($$\Delta S^\circ_{rxn}$$)?

Accepted Answer

$$\Delta S^\circ_{rxn} = \sum n_p S^\circ(	ext{products}) - \sum n_r S^\circ(	ext{reactants})$$

Question 6

Which of the following statements about standard molar entropy ($$S^\circ) is $$TRUE?

Accepted Answer

Standard molar entropy is always positive for any substance at 25°C.

Question 7

For the reaction $$2\ 	ext{H}_2S(g) + 3\ 	ext{O}_2(g) ightarrow 2\ 	ext{H}_2O(g) + 2\ 	ext{SO}_2(g)$$, given $$S^\circ$$ values: H$$_2S(g) = 205.8 J/$$mol·K, O$$_2(g) = 205.2 J/$$mol·K, H$$_2O(g) = 188.8 J/$$mol·K, SO$$_2(g) = 248.2 J/$$mol·K, what is $$\Delta S^\circ_{rxn}$$?

Accepted Answer

$$-153.2 J/K$$

Question 8

Which of the following is a correct method for calculating the standard free energy change ($$\Delta G^\circ_{rxn}) of a $$reaction?

Accepted Answer

$$\Delta G^\circ_{rxn} = \Delta H^\circ_{rxn} - T\Delta S^\circ_{rxn}$$

Question 9

A reaction has $$\Delta H^\circ_{rxn} \u003c 0$$ and $$\Delta S^\circ_{rxn} \u003c 0. At $$what temperature is the reaction spontaneous?

Accepted Answer

At low temperatures

Question 10

Which of the following best explains why NO(g) has a higher standard molar entropy than Ar(g), even though NO(g) has a lower molar mass?

Accepted Answer

NO(g) is more molecularly complex, allowing more microstates.

Question 11

Which physical state generally has the lowest standard molar entropy ($$S^\circ$$)?

Accepted Answer

Solid

Question 12

For the reaction $$	ext{SO}_2(g) + \frac{1}{2}	ext{O}_2(g) ightarrow 	ext{SO}_3(g)$$, given $$\Delta H^\circ_{rxn} = -98.9 kJ $$and $$\Delta S^\circ_{rxn} = -94.0 J/K$$, what is $$\Delta G^\circ_{rxn} at 25$$°C?

Accepted Answer

$$-70.9 kJ$$

Question 13

Which equation is engraved on the tombstone of Ludwig Boltzmann, reflecting his contribution to entropy?

Accepted Answer

$$S = k \ln W$$

Question 14

Which of the following is NOT a valid method for calculating $$\Delta G^\circ_{rxn}$$?

Accepted Answer

Using the sum of standard molar entropies only

Question 15

Which of the following reactions is likely to have a positive $$\Delta S^\circ_{rxn}$$?

Accepted Answer

A reaction where the number of gas molecules increases

Question 16

Which of the following statements about entropy is FALSE?

Accepted Answer

Entropy of elements in their standard state is zero.

Question 17

Which of the following best describes the effect of dissolution on standard molar entropy?

Accepted Answer

Dissolution increases the standard molar entropy of a substance.

Question 18

Which of the following is the correct unit for standard molar entropy ($$S^\circ$$)?

Accepted Answer

J/mol·K

Question 19

Which allotrope of carbon has a higher standard molar entropy at 298 K?

Accepted Answer

Graphite

Standard Entropy and Free Energy Changes in Chemical Reactions

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