Which ranking of standard molar entropy is correct based on the chart?

Answer: S°(gas) S°(liquid) S°(solid) Particles in the gas phase are most disordered, then liquid, then solid (most ordered), so S°(gas) S°(liquid) S°(solid).

What is ΔS° for the reaction?

Answer: −89 J/mol·K ΔS° = 2(189) − [2(131) + 205] = 378 − (262 + 205) = 378 − 467 = −89 J/mol·K.

What is the sign of ΔS for vaporizing water?

Answer: Positive; gas has greater disorder Liquid → gas dramatically increases positional disorder, so ΔS 0.

AP Chemistry · Topic 9.2

Absolute Entropy and Entropy Change Practice

Part of Applications of Thermodynamics.(ENE-4.B)

Practice questions

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Sample questions

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Sample 1difficulty 1/5
$Which ranking of standard molar entropy is correct based on the chart?$
- A
  $S°(solid) > S°(liquid) > S°(gas)$
- B
  $All three phases have equal entropy$
- C
  $S°(liquid) > S°(gas) > S°(solid)$
- D
  $S°(gas) > S°(liquid) > S°(solid)$
  check_circle
Why
$Particles in the gas phase are most disordered, then liquid, then solid (most ordered), so S°(gas) > S°(liquid) > S°(solid).$
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Sample 2difficulty 2/5
$Standard entropies (J/mol\cdotK) for the reaction 2 H₂(g) + O₂(g) \to 2 H₂O(g) are tabulated.$
$What is ΔS° for the reaction?$
- A
  $-151 J/mol\cdotK$
- B
  $+89 J/mol\cdotK$
- C
  $+151 J/mol\cdotK$
- D
  $-89 J/mol\cdotK$
  check_circle
Why
$ΔS° = 2(189) - [2(131) + 205] = 378 - (262 + 205) = 378 - 467 = -89 J/mol\cdotK.$
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Sample 3difficulty 2/5
$What is the sign of ΔS for vaporizing water?$
- A
  $Positive; gas has greater disorder$
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- B
  $Zero; same molecule$
- C
  $Cannot be determined$
- D
  $Negative; gas is more ordered$
Why
$Liquid \to gas dramatically increases positional disorder, so ΔS > 0.$
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Sample 4difficulty 2/5
$For 2 H₂O(l) \to 2 H₂(g) + O₂(g), ΔS is$
- A
  $\approx 0$
- B
  $Positive (liquid \to more gas moles)$
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- C
  $Negative (gas \to liquid)$
- D
  $Cannot tell$
Why
$Producing gas from liquid increases disorder \to ΔS > 0.$
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Sample 5difficulty 2/5
$Of solid, liquid, gas (same substance), entropy is largest in$
- A
  $Equal$
- B
  $Liquid$
- C
  $Gas$
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- D
  $Solid$
Why
$Gas has the most molecular motion and positional freedom.$
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