Question 1

Which formula is used to calculate the entropy change ($\Delta S$) during the vaporization of a liquid at its boiling point?

Accepted Answer

$\Delta S_{vap} = \dfrac{\Delta H_{vap}}{T_{bp}}$

Question 2

Methanol has a normal melting point of $64.7\ ^\circ\mathrm{C}$ and an entropy of fusion of $9.36\ \mathrm{J/K}$. What is the enthalpy of fusion ($\Delta H_{fus}$) in joules?

Accepted Answer

$2412\ \mathrm{J}$

Question 3

Which of the following best describes the physical meaning of entropy change ($\Delta S$) during a phase change?

Accepted Answer

It measures the increase in molecular disorder as a substance changes phase.

Question 4

Given $\Delta H_{vap} = 21.7\ \mathrm{kJ/mol}$ and $T_{bp} = 118\ ^\circ\mathrm{C}$ for acetic acid, what is the entropy of vaporization ($\Delta S_{vap}$) in $\mathrm{J/(mol\cdot K)}$? (Use $T$ in Kelvin.)

Accepted Answer

$54.4\ \mathrm{J/(mol\cdot K)}$

Question 5

A student calculates the entropy of fusion using $\Delta S_{fus} = \dfrac{\Delta H_{fus}}{T_{bp}}$. What is the error in this calculation?

Accepted Answer

The student should use the melting point ($T_{mp}$), not the boiling point ($T_{bp}$), in the denominator.

Question 6

If the entropy of vaporization for a substance is $85\ \mathrm{J/(mol\cdot K)}$ at its boiling point of $350\ \mathrm{K}$, what is the enthalpy of vaporization ($\Delta H_{vap}$) in $\mathrm{kJ/mol}$?

Accepted Answer

$29.8\ \mathrm{kJ/mol}$

Question 7

Which of the following statements is true regarding entropy changes during fusion and vaporization?

Accepted Answer

Entropy increases during both fusion and vaporization.

Question 8

A chemist needs to calculate the entropy change when $8.4\ \mathrm{g}$ of acetic acid ($\mathrm{CH_3COOH}$, molar mass $60.05\ \mathrm{g/mol}$) vaporizes at its boiling point ($T_{bp} = 391\ \mathrm{K}$, $\Delta H_{vap} = 21.7\ \mathrm{kJ/mol}$). What is the entropy change in $\mathrm{J/K}$?

Accepted Answer

$7.6\ \mathrm{J/K}$

Question 9

Why is the entropy of vaporization generally larger than the entropy of fusion for most substances?

Accepted Answer

Because the transition from liquid to gas involves a greater increase in molecular disorder than from solid to liquid.

Question 10

If a substance has $\Delta H_{fus} = 6.0\ \mathrm{kJ/mol}$ and a melting point of $300\ \mathrm{K}$, what is its entropy of fusion ($\Delta S_{fus}$) in $\mathrm{J/(mol\cdot K)}$?

Accepted Answer

$20.0\ \mathrm{J/(mol\cdot K)}$

Question 11

Which of the following best explains why temperature must be in Kelvin when calculating entropy changes for phase transitions?

Accepted Answer

Kelvin is an absolute temperature scale, ensuring entropy calculations are physically meaningful and avoid division by zero.

Question 12

A student is asked to calculate the entropy change for the melting of $10.0\ \mathrm{g}$ of a substance with $\Delta H_{fus} = 5.0\ \mathrm{kJ/mol}$, $T_{mp} = 350\ \mathrm{K}$, and molar mass $50.0\ \mathrm{g/mol}$. What is the correct entropy change in $\mathrm{J/K}$?

Accepted Answer

$2.86\ \mathrm{J/K}$

Entropy Calculations: Phase Changes in Liquids

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