AP Chemistry Unit 6.9: Hess Law

Practice AP Chemistry Unit 6.9 questions on combining reaction steps with Hess law by reversing, scaling, and adding equations to calculate overall ΔH.

Exam points

Use stoichiometric and molecular models to analyze reactions, composition, and quantities from chemical evidence.
Calculate mass, moles, concentration, and formula relationships using balanced equations and measured data.

Covered syllabus points

6.9.A—Represent a chemical or physical process as a sequence of steps 6.9.B—Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of…

[Maximum number: 1]

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 6.9 Hess’s Law question 1 — AP Chemistry

Calculate the standard enthalpy of formation of $\mathrm{PCl}_{5}(g)$ represented by equation 5.

\text { Equation 5: } \frac{1}{4} \mathrm{P}_{4}(s)+\frac{5}{2} \mathrm{Cl}_{2}(g) \rightarrow \mathrm{PCl}_{5}(g) \quad \Delta H_{f}^{\circ}=\text { ? }

The following particle-level diagram represents the contents of the vessel in an equilibrium

mixture at 546 K involving equation 4.

E For the correct calculated value:

Using Hess's law:

$\begin{aligned} \Delta H_{f, \mathrm{PCl}_{5}(g)}^{\circ} & =\frac{1}{4} \Delta H_{1}^{\circ}+\Delta H_{2}^{\circ} \\ \Delta H_{f, \mathrm{PCl}_{5}(g)}^{\circ} & =\frac{1}{4}(-1148)+(-88)=-375 \mathrm{~kJ} / \mathrm{mol} \end{aligned}$

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Covered syllabus points

6.9 Hess’s Law question 1