AP Chemistry Unit 6.6: Reaction Enthalpy

Practice AP Chemistry Unit 6.6 questions on converting calorimetry heat into reaction enthalpy using stoichiometric moles, signs, and system-surroundings energy transfer.

Exam points

Use reaction equations and thermodynamic data to analyze energy changes, reaction direction, and equilibrium.
Calculate or interpret ΔH°, ΔS°, ΔG°, K, and temperature effects from chemical reaction data.

Covered syllabus points

6.6.A—Calculate the heat q absorbed or released by a system undergoing a chemical reaction in relationship to the…

[Maximum number: 2]

White phosphorus is composed of $\mathrm{P}_{4}$ molecules with a tetrahedral structure, as shown in the

diagram on the left. Each P atom is bonded to the other three P atoms by single bonds, as shown

in the incomplete Lewis diagram on the right.

Figure for Question 6.6 Introduction to Enthalpy of Reaction question 1 — AP Chemistry

The chemist carries out the calorimetry experiment and records the following information.

Table for Question 6.6 Introduction to Enthalpy of Reaction question 1 — AP Chemistry

Calculate the value of $\Delta H_{\text {rxn }}^{\circ}$ for equation 2 in $\mathrm{kJ} / \mathrm{mol}_{\text {rxn }}$ . Include the sign in your answer.

(ii) For the correct calculated value, consistent with part C (i):

Point 05

$\begin{aligned} & q_{r x n}=-q_{s u r r}=-0.16 \mathrm{~kJ} \\ & \Delta H_{r x n}^{\circ}=\frac{-0.16 \mathrm{~kJ}}{0.100 \mathrm{~g} \mathrm{P}_{4} \mathrm{O}_{10}} \times \frac{283.9 \mathrm{~g} \mathrm{P}_{4} \mathrm{O}_{10}}{1 \mathrm{~mol} \mathrm{P}_{4} \mathrm{O}_{10}} \times \frac{1 \mathrm{~mol} \mathrm{P}_{4} \mathrm{O}_{10}}{1 \mathrm{~mol}_{r x n}}=-450 \mathrm{~kJ} / \mathrm{mol}_{r x n} \end{aligned}$

For the correct sign:

Point 06

$-450 \mathrm{~kJ} / \mathrm{mol}_{r x n}$

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Covered syllabus points

6.6 Introduction to Enthalpy of Reaction question 1