Learn Extracted exam questions AP Chemistry 2019 Free Response
2019 Free Response
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[Complete Lewis electron-dot diagram of urea: H—N(H)—C(=O)—N(H)—H, with the carbon double-bonded to oxygen (two lone pairs on O), and each nitrogen bonded to two H atoms and bearing one lone pair, and the central carbon bonded to both nitrogens]
The compound urea, $\text{H}_2\text{NCONH}_2$, is widely used in chemical fertilizers. The complete Lewis electron-dot diagram for the urea molecule is shown above.
Identify the hybridization of the valence orbitals of the carbon atom in the urea molecule.
Urea has a high solubility in water, due in part to its ability to form hydrogen bonds. A urea molecule and four water molecules are represented in the box below. Draw ONE dashed line (----) to indicate a possible location of a hydrogen bond between a water molecule and the urea molecule.
[Diagram: a box containing one urea molecule (drawn with spheres: white = O, gray = H, black = C, hatched = N) surrounded by four separate water molecules, each water molecule drawn as one white (O) sphere bonded to two gray (H) spheres; legend: white circle = O, gray circle = H, black circle = C, hatched circle = N]
The dissolution of urea is represented by the equation above. A student determines that 5.39 grams of $\text{H}_2\text{NCONH}_2$ (molar mass 60.06 g/mol) can dissolve in water to make 5.00 mL of a saturated solution at 20.$^\circ$C.
Calculate the concentration of urea, in mol/L, in the saturated solution at 20.$^\circ$C.
The student also determines that the concentration of urea in a saturated solution at 25$^\circ$C is 19.8 $M$. Based on this information, is the dissolution of urea endothermic or exothermic? Justify your answer in terms of Le Chatelier's principle.
[Diagram: row of laboratory equipment icons, labelled left to right: Polystyrene Cup, Thermometer, Stirring Rod, Bottle of Urea, Balance, Distilled Water]
The equipment shown above is provided so that the student can determine the value of the molar heat of solution for urea. Knowing that the specific heat of the solution is 4.18 J/(g$\cdot^\circ$C), list the specific measurements that are required to be made during the experiment.
| $S^\circ$ (J/(mol$\cdot$K)) | |
|---|---|
| $\text{H}_2\text{NCONH}_2(s)$ | 104.6 |
| $\text{H}_2\text{NCONH}_2(aq)$ | ? |
The entropy change for the dissolution of urea, $\Delta S^\circ_{soln}$, is 70.1 J/(mol$\cdot$K) at 25$^\circ$C. Using the information in the table above, calculate the absolute molar entropy, $S^\circ$, of aqueous urea.
Using particle-level reasoning, explain why $\Delta S^\circ_{soln}$ is positive for the dissolution of urea in water.
The student claims that $\Delta S^\circ$ for the process contributes to the thermodynamic favorability of the dissolution of urea at 25$^\circ$C. Use the thermodynamic information above to support the student's claim.
Answer the following questions relating to the chemistry of the halogens.
The molecular formulas of diatomic bromine, chlorine, fluorine, and iodine are written below. Circle the formula of the molecule that has the longest bond length. Justify your choice in terms of atomic structure.
A chemistry teacher wants to prepare $\text{Br}_2$. The teacher has access to the following three reagents: $\text{NaBr}(aq)$, $\text{Cl}_2(g)$, and $\text{I}_2(s)$.
| Half-Reaction | $E^\circ$ at 25$^\circ$C (V) |
|---|---|
| $\text{Br}_2 + 2\,e^- \rightarrow 2\,\text{Br}^-$ | 1.07 |
| $\text{Cl}_2 + 2\,e^- \rightarrow 2\,\text{Cl}^-$ | 1.36 |
| $\text{I}_2 + 2\,e^- \rightarrow 2\,\text{I}^-$ | 0.53 |
Using the data in the table above, write the balanced equation for the thermodynamically favorable reaction that will produce $\text{Br}_2$ when the teacher combines two of the reagents. Justify that the reaction is thermodynamically favorable by calculating the value of $E^\circ$ for the reaction.
$\text{Br}_2$ and $\text{Cl}_2$ can react to form the compound $\text{BrCl}$.
The boiling point of $\text{Br}_2$ is 332 K, whereas the boiling point of $\text{BrCl}$ is 278 K. Explain this difference in boiling point in terms of all the intermolecular forces present between molecules of each substance.
The compound $\text{BrCl}$ can decompose into $\text{Br}_2$ and $\text{Cl}_2$, as represented by the balanced chemical equation below.
A 0.100 mole sample of pure $\text{BrCl}(g)$ is placed in a previously evacuated, rigid 2.00 L container at 298 K. Eventually the system reaches equilibrium according to the equation above.
Calculate the pressure in the container before equilibrium is established.
Write the expression for the equilibrium constant, $K_{eq}$, for the decomposition of $\text{BrCl}$.
After the system has reached equilibrium, 42 percent of the original $\text{BrCl}$ sample has decomposed.
Determine the value of $K_{eq}$ for the decomposition reaction of $\text{BrCl}$ at 298 K.
Calculate the bond energy of the Br-Cl bond, in kJ/mol, using $\Delta H^\circ$ for the reaction (1.6 kJ/mol$_{rxn}$) and the information in the following table.
| Bond | Bond Energy (kJ/mol) |
|---|---|
| Br $-$ Br | 193 |
| Cl $-$ Cl | 243 |
| Br $-$ Cl | ? |
A student is given 50.0 mL of a solution of $\text{Na}_2\text{CO}_3$ of unknown concentration. To determine the concentration of the solution, the student mixes the solution with excess 1.0 $M$ $\text{Ca}(\text{NO}_3)_2(aq)$, causing a precipitate to form. The balanced equation for the reaction is shown below.
Write the net ionic equation for the reaction that occurs when the solutions of $\text{Na}_2\text{CO}_3$ and $\text{Ca}(\text{NO}_3)_2$ are mixed.
The diagram below is incomplete. Draw in the species needed to accurately represent the major ionic species remaining in the solution after the reaction has been completed.
[Diagram of a beaker of solution after reaction: floating ion labels $\text{Na}^+$, $\text{NO}_3^-$, $\text{NO}_3^-$, $\text{Na}^+$, $\text{NO}_3^-$, $\text{NO}_3^-$ scattered above a layer of gray solid labelled "Solid $\text{CaCO}_3$" at the bottom of the beaker]
The student filters and dries the precipitate of $\text{CaCO}_3$ (molar mass 100.1 g/mol) and records the data in the table below.
| Volume of $\text{Na}_2\text{CO}_3$ solution | 50.0 mL |
|---|---|
| Volume of 1.0 $M$ $\text{Ca}(\text{NO}_3)_2$ added | 100.0 mL |
| Mass of $\text{CaCO}_3$ precipitate collected | 0.93 g |
Determine the number of moles of $\text{Na}_2\text{CO}_3$ in the original 50.0 mL of solution.
The student realizes that the precipitate was not completely dried and claims that as a result, the calculated $\text{Na}_2\text{CO}_3$ molarity is too low. Do you agree with the student's claim? Justify your answer.
After the precipitate forms and is filtered, the liquid that passed through the filter is tested to see if it can conduct electricity. What would be observed? Justify your answer.
The student decides to determine the molarity of the same $\text{Na}_2\text{CO}_3$ solution using a second method. When $\text{Na}_2\text{CO}_3$ is dissolved in water, $\text{CO}_3^{2-}(aq)$ hydrolyzes to form $\text{HCO}_3^-(aq)$, as shown by the following equation.
The student decides to first determine $[\text{OH}^-]$ in the solution, then use that result to calculate the initial concentration of $\text{CO}_3^{2-}(aq)$.
Identify a laboratory method (not titration) that the student could use to collect data to determine $[\text{OH}^-]$ in the solution.
Explain how the student could use the measured value in part (f)(i) to calculate the initial concentration of $\text{CO}_3^{2-}(aq)$. (Do not do any numerical calculations.)
In the original $\text{Na}_2\text{CO}_3$ solution at equilibrium, is the concentration of $\text{HCO}_3^-(aq)$ greater than, less than, or equal to the concentration of $\text{CO}_3^{2-}(aq)$? Justify your answer.
The student needs to make a $\text{CO}_3^{2-}/\text{HCO}_3^-$ buffer. Is the $\text{Na}_2\text{CO}_3$ solution suitable for making a buffer with a pH of 6? Explain why or why not.
A student is doing experiments with $\text{CO}_2(g)$. Originally, a sample of the gas is in a rigid container at 299 K and 0.70 atm. The student increases the temperature of the $\text{CO}_2(g)$ in the container to 425 K.
Describe the effect of raising the temperature on the motion of the $\text{CO}_2(g)$ molecules.
Calculate the pressure of the $\text{CO}_2(g)$ in the container at 425 K.
In terms of kinetic molecular theory, briefly explain why the pressure of the $\text{CO}_2(g)$ in the container changes as it is heated to 425 K.
The student measures the actual pressure of the $\text{CO}_2(g)$ in the container at 425 K and observes that it is less than the pressure predicted by the ideal gas law. Explain this observation.
The complete photoelectron spectrum of an element in its ground state is represented below.
[Graph: Photoelectron spectrum; y-axis "Relative Number of Electrons" (unscaled, gridlines shown, no numeric values), x-axis "Binding Energy per Electron (J)" plotted with decreasing energy to the right (values labelled at each peak, diagonally, right to left as printed): a short peak at $647 \times 10^{-18}$; two adjacent short peaks near $70.2 \times 10^{-18}$ and $55.7 \times 10^{-18}$ (the taller of the pair, at $55.7\times10^{-18}$, reaches near the top of the graph); two adjacent peaks at $7.10 \times 10^{-18}$ (short) and $4.07 \times 10^{-18}$ (tall, reaching near the top); and a short peak at $0.980 \times 10^{-18}$]
Based on the spectrum, write the ground-state electron configuration of the element.
Based on the spectrum, identify the element.
Calculate the wavelength, in meters, of electromagnetic radiation needed to remove an electron from the valence shell of an atom of the element.
Nitrogen dioxide, $\text{NO}_2(g)$, is produced as a by-product of the combustion of fossil fuels in internal combustion engines. At elevated temperatures $\text{NO}_2(g)$ decomposes according to the equation below.
The concentration of a sample of $\text{NO}_2(g)$ is monitored as it decomposes and is recorded on the graph directly below. The two graphs that follow it are derived from the original data.
[Graph 1: $[\text{NO}_2]$ (mol/L, y-axis 0.00 to 0.08) vs. Time (s, x-axis 0 to 300); data points approximately: (0, 0.075), (60, 0.018), (120, 0.010), (180, 0.008), (240, 0.007), (300, 0.005) — a decaying curve]
[Graph 2: $1/[\text{NO}_2]$ (y-axis 0 to 250) vs. Time (s, x-axis 0 to 300); data points approximately: (0, 13), (60, 58), (120, 105), (180, 152), (240, 200), (300, 245) — an approximately straight increasing line]
[Graph 3: $\ln([\text{NO}_2])$ (y-axis $-6.00$ to $-2.00$) vs. Time (s, x-axis 0 to 300); data points approximately: (0, $-2.6$), (60, $-4.1$), (120, $-4.6$), (180, $-5.05$), (240, $-5.2$), (300, $-5.4$) — a curved, decreasing, concave-up trend]
Explain how the graphs indicate that the reaction is second order.
Write the rate law for the decomposition of $\text{NO}_2(g)$.
Consider two possible mechanisms for the decomposition reaction.
Is the rate law described by mechanism I shown below consistent with the rate law you wrote in part (b)? Justify your answer.
Mechanism I
Step 1: $\text{NO}_2(g) + \text{NO}_2(g) \rightarrow \text{NO}(g) + \text{NO}_3(g)$ $\quad$ slow
Step 2: $\text{NO}_3(g) \rightarrow \text{NO}(g) + \text{O}_2(g)$ $\quad$ fast
Is the rate law described by mechanism II shown below consistent with the rate law you wrote in part (b)? Justify your answer.
Mechanism II
Step 1: $\text{NO}_2(g) + \text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g)$ $\quad$ fast equilibrium
Step 2: $\text{N}_2\text{O}_4(g) \rightarrow 2\,\text{NO}(g) + \text{O}_2(g)$ $\quad$ slow
A student dissolved a 0.139 g sample of oxalic acid, $\text{H}_2\text{C}_2\text{O}_4$, in water in an Erlenmeyer flask. Then the student titrated the $\text{H}_2\text{C}_2\text{O}_4$ solution in the flask with a solution of $\text{KMnO}_4$, which has a dark purple color. The balanced chemical equation for the reaction that occurred during the titration is shown above.
Identify the species that was reduced in the titration reaction. Justify your answer in terms of oxidation numbers.
The student used a 50.0 mL buret to add the $\text{KMnO}_4(aq)$ to the $\text{H}_2\text{C}_2\text{O}_4(aq)$ until a faint lavender color was observed in the flask, an indication that the end point of the titration had been reached. The initial and final volume readings of the solution in the buret are shown below. Write down the initial reading and the final reading and use them to determine the volume of $\text{KMnO}_4(aq)$ that was added during the titration.
[Diagram: two burets side by side. Left buret labelled "Initial", showing the liquid meniscus between the 0 and 10 mL marks, with a magnified circular inset showing the meniscus between the 3 and 4 mL marks (bottom of meniscus close to the 4 mL line). Right buret labelled "Final", showing the liquid meniscus between the 20 and 30 mL marks, with a magnified circular inset showing the meniscus between the 29 and 30 mL marks (bottom of meniscus close to the 30 mL line)]
Given that the concentration of $\text{KMnO}_4(aq)$ was 0.0235 $M$, calculate the number of moles of $\text{MnO}_4^-$ ions that completely reacted with the $\text{H}_2\text{C}_2\text{O}_4$.
The student proposes to perform another titration using a 0.139 g sample of $\text{H}_2\text{C}_2\text{O}_4$, but this time using 0.00143 $M$ $\text{KMnO}_4(aq)$ in the buret. Would this titrant concentration be a reasonable choice to use if the student followed the same procedure and used the same equipment as before? Justify your response.