AP Chemistry 6.9: Hess Law

Practice AP Chemistry questions on combining reaction steps with Hess law by reversing, scaling, and adding equations to calculate an overall enthalpy change.

Exam points

Explain how intermolecular forces and molecular structure influence physical properties and phase behavior.
Apply particle models to predict boiling point, solubility and vapor pressure from molecular interactions.

[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.B—Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of… 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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6.9.B—Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of… question 1