AP Chemistry Unit 6.5: Phase-Change Energy

Practice AP Chemistry Unit 6.5 questions on calculating energy for heating, melting, freezing, vaporization, and condensation across phase transitions.

Exam points

Use electrochemical equations and reduction potentials to analyze cell reactions, electron transfer, and direction.
Calculate cell voltage and relate electrode processes, charge balance, and stoichiometry to observed products.

Covered syllabus points

6.5.A—Explain changes in the heat q absorbed or released by a system undergoing a phase transition based on the amount…

[Maximum number: 1]

A scientist is investigating the properties of a mixture of $\mathrm{CH}_{3} \mathrm{OH}$ and $\mathrm{H}_{2} \mathrm{CO}$ . The scientist

generates the following diagram to represent the mixture.

Figure for Question 6.5 Energy of Phase Changes question 1 — AP Chemistry

The scientist plans to cool a gaseous mixture of $\mathrm{CH}_{3} \mathrm{OH}$ and $\mathrm{H}_{2} \mathrm{CO}$ to form a liquid mixture

and finds data on the two compounds. The data are summarized in the table.

Table for Question 6.5 Energy of Phase Changes question 1 — AP Chemistry

The scientist analyzes the mixture after it is cooled and determines that 8.59 g of

$\mathrm{CH}_{3} \mathrm{OH}(l)$ is present. Calculate the amount of thermal energy, in kJ, that was removed to

condense the 8.59 g of $\mathrm{CH}_{3} \mathrm{OH}$ (molar mass $32.04 \mathrm{~g} / \mathrm{mol}$ ) at its boiling point.

(ii) For the correct calculated value:

Point 04

$8.59 \mathrm{~g} \mathrm{CH}_{3} \mathrm{OH} \times \frac{1 \mathrm{~mol} \mathrm{CH}_{3} \mathrm{OH}}{32.04 \mathrm{~g} \mathrm{CH}_{3} \mathrm{OH}} \times \frac{-37.6 \mathrm{~kJ}}{1 \mathrm{~mol} \mathrm{CH}_{3} \mathrm{OH}}=-10.1 \mathrm{~kJ}$ , so 10.1 kJ are removed.

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Covered syllabus points

6.5 Energy of Phase Changes question 1