Learn Extracted exam questions AP Chemistry 2023 Free Response
2023 Free Response
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Answer the following questions related to manganese compounds.
Manganese has several common oxidation states.
Write the complete electron configuration for an $\text{Mn}$ atom in the ground state.
When manganese forms cations, electrons are lost from which subshell first? Identify both the number and letter associated with the subshell.
A student performs an experiment to produce a manganese salt of unknown composition, $\text{Mn}_x\text{Cl}_y(aq)$, and determine its empirical formula. The student places a sample of $\text{Mn}(s)$ in a beaker containing excess $\text{HCl}(aq)$, as represented by the following equation.
The student heats the resulting mixture until only $\text{Mn}_x\text{Cl}_y(s)$ remains in the beaker. The data are given in the following table.
| Quantity | Value |
|---|---|
| Mass of empty beaker | 60.169 g |
| Mass of beaker and $\text{Mn}(s)$ | 61.262 g |
| Mass of beaker and $\text{Mn}_x\text{Cl}_y$ after heating to constant mass | 62.673 g |
Calculate the mass of Cl in the sample of $\text{Mn}_x\text{Cl}_y(s)$ remaining in the beaker.
Calculate the number of moles of Cl in the sample of $\text{Mn}_x\text{Cl}_y(s)$ remaining in the beaker.
The student determines that $0.0199\ \text{mol}$ of Mn was used in the experiment. Use the data to determine the empirical formula of the $\text{Mn}_x\text{Cl}_y(s)$.
The student repeats the experiment using the same amounts of Mn and HCl and notices that some of the $\text{Mn}_x\text{Cl}_y$ splatters out of the beaker as it is heated to dryness. Will the number of moles of Cl calculated for this trial be greater than, less than, or equal to the number calculated in part (c)? Justify your answer.
Another compound of manganese, $\text{MnO}_2$, is used in alkaline batteries, represented by the following diagram.
[Diagram of an alkaline battery: a graphite rod runs through the center, surrounded by MnO$_2$ paste, then KOH paste, enclosed by a zinc case.]
Some half-reactions are given in the table.
| Reduction Half-Reaction | $E^\circ$ (V) |
|---|---|
| $\text{Zn}^{2+}(aq) + 2\,e^- \rightarrow \text{Zn}(s)$ | $-0.76$ |
| $\text{ZnO}(s) + \text{H}_2\text{O}(l) + 2\,e^- \rightarrow \text{Zn}(s) + 2\,\text{OH}^-(aq)$ | $-1.28$ |
| $2\,\text{MnO}_2(s) + \text{H}_2\text{O}(l) + 2\,e^- \rightarrow \text{Mn}_2\text{O}_3(s) + 2\,\text{OH}^-(aq)$ | $0.15$ |
Based on the half-reactions given in the table, write the balanced net ionic equation for the reaction that has the greatest thermodynamic favorability.
Calculate the value of $E^\circ_{cell}$ for the overall reaction.
Calculate the value of $\Delta G^\circ$ in $\text{kJ/mol}_{rxn}$.
A student claims that the total mass of an alkaline battery decreases as the battery operates because the anode loses mass. Do you agree with the student's claim? Justify your answer.
In the gas phase, $\text{AlCl}_3$ is a molecular substance. A reaction of gaseous $\text{AlCl}_3$ at high temperature is represented by the following balanced equation.
Reaction 1: $\text{AlCl}_3(g) \rightarrow \text{Al}(g) + 3\,\text{Cl}(g) \qquad \Delta H_1^\circ = ?$
How many grams of $\text{Cl}(g)$ can be formed from $1.25\ \text{mol}$ of $\text{AlCl}_3(g)$?
Additional reactions that involve Al or Cl are shown in the following table.
| Reaction Number | Equation | $\Delta H^\circ_{rxn}$ (kJ/mol$_{rxn}$) |
|---|---|---|
| 2 | $\text{Al}(s) + \dfrac{3}{2}\text{Cl}_2(g) \rightarrow \text{AlCl}_3(g)$ | $-583$ |
| 3 | $\text{Al}(s) \rightarrow \text{Al}(g)$ | $+326$ |
| 4 | $\text{Cl}_2(g) \rightarrow 2\,\text{Cl}(g)$ | $+243$ |
Calculate the value of $\Delta H_1^\circ$, in $\text{kJ/mol}_{rxn}$, for reaction 1 above using reactions 2, 3, and 4.
A potential energy diagram for $\text{Cl}_2$ is shown in the following graph.
[Graph: potential energy (kJ/mol) vs. internuclear distance (picometers) for Cl$_2$; y-axis from $-500$ to $500$ kJ/mol in steps of 100; x-axis from 0 to 500 picometers in steps of 100; dashed curve starts high near $x=0$, plunges to a minimum of about $-250$ kJ/mol near $x \approx 200$ pm, then rises and levels off near 0 kJ/mol as $x$ increases toward 500 pm.]
Based on the graph, what is the bond length, in picometers, for $\text{Cl}_2$? _________
A student finds that the average $\text{Al}-\text{Cl}$ bond length is 220 picometers and the average bond energy is 425 kJ/mol. Draw the potential energy curve for the average $\text{Al}-\text{Cl}$ bond on the preceding graph.
Three proposed Lewis diagrams for the $\text{AlCl}_3(g)$ molecule are shown.
[Diagram 1: Cl bonded to Al with a single bond above (Cl with 3 lone pairs) and two Cl atoms bonded to Al with single bonds on either side (each Cl with 3 lone pairs) — trigonal arrangement, all single Al–Cl bonds, Al with no lone pair shown.] [Diagram 2: same trigonal arrangement of three Cl atoms singly bonded to Al, each Cl shown with 3 lone pairs — visually similar layout to Diagram 1.] [Diagram 3: two Cl atoms singly bonded to Al (each with 3 lone pairs) and one Cl atom double-bonded to Al (shown with 2 lone pairs) — Cl=Al on the right side.]
Diagram 1, Diagram 2, Diagram 3 (labeled beneath each structure).
The $\text{AlCl}_3(g)$ molecule has a trigonal planar geometry. Which diagram (1, 2, or 3) can be eliminated based on geometry? Justify your choice based on VSEPR theory.
Which of the three diagrams is the best representation for the bonding in $\text{AlCl}_3$? Justify your choice based on formal charges.
$\text{AlCl}_3$ is known to dimerize reversibly in the gas phase. The dimerization equilibrium is represented by the following equation.
Write the expression for the equilibrium constant, $K_p$, for this reaction.
A particle-level diagram of an equilibrium mixture of $\text{AlCl}_3(g)$ and $\text{Al}_2\text{Cl}_6(g)$ at $400^\circ\text{C}$ in a 25 L closed container is shown.
[Particle diagram inside a rectangular box: several clusters of gray/white circles representing AlCl$_3$ monomers (isolated groupings of 1 white Al + 3 gray Cl) and Al$_2$Cl$_6$ dimers (larger fused clusters of 2 white Al + 6 gray Cl), scattered throughout the container. Legend: white circle = Al, gray circle = Cl.]
Using the particle-level diagram, calculate the value of $K_p$ for the reaction if the total pressure in the container is 22.1 atm.
Answer the following questions about an experiment in which $\text{CaCO}_3(s)$ is combined with $\text{HCl}(aq)$, represented by the following balanced equation.
Write the balanced net ionic equation for the reaction.
A student performs an investigation to study factors that affect the rate of the reaction. In each trial the student combines 50.0 mL of $\text{HCl}(aq)$ at $21.2^\circ\text{C}$ with 1.00 g of $\text{CaCO}_3(s)$ and measures the time required for the reaction to go to completion. The data are given in the following table.
| Trial | Concentration of HCl$(aq)$ (M) | Particle Size of CaCO$_3(s)$ | Time of Reaction (s) |
|---|---|---|---|
| 1 | 1.00 | Fine powder | 67 |
| 2 | 1.00 | Small chunks | 112 |
| 3 | 1.00 | Large chunk | 342 |
| 4 | 3.00 | Fine powder | 22 |
| 5 | 3.00 | Small chunks | 227 |
| 6 | 3.00 | Large chunk | 114 |
The student correctly identifies that trial 5 is inconsistent with the other trials. Explain why the student's claim is correct using the data in the table.
Based on the reaction conditions and the collisions that occur between particles, explain the reason for the difference in the reaction times for trial 2 and trial 3.
The student claims that the reaction is zero order with respect to $\text{HCl}(aq)$. Do you agree or disagree with the student's claim? Justify your answer using the student's data.
The $\text{HCl}(aq)$ was present in excess in all trials of the experiment. Determine the molarity of the $\text{HCl}(aq)$ in the beaker after the reaction is complete in trial 2. Assume that the volume of the mixture remains constant at 50.0 mL throughout the trial. (The molar mass of $\text{CaCO}_3$ is $100.09\ \text{g/mol}$.)
In order to measure the enthalpy of the reaction shown, the student repeats trial 1 by mixing 50.0 mL of $\text{HCl}(aq)$ with 1.00 g of $\text{CaCO}_3(s)$ using a coffee cup calorimeter. The student records the temperature of the system every 20 seconds. The data are given in the following table.
| Time (s) | Measured Temperature of Solution ($^\circ$C) |
|---|---|
| 0 | 21.20 |
| 20 | 21.51 |
| 40 | 21.70 |
| 60 | 21.85 |
| 80 | 21.90 |
| 100 | 21.90 |
Is the reaction endothermic or exothermic? Justify your answer using the information in the table.
Based on the experimental data, the mass of the system is 51.0 g, and the specific heat of the reaction mixture is $4.0\ \text{J}/(\text{g}\cdot{}^\circ\text{C})$.
Calculate the magnitude of heat transfer, $q$, in joules.
Calculate the enthalpy of reaction in units of $\text{kJ/mol}_{rxn}$. Include the algebraic sign on your answer.
A student is asked to prepare a buffer solution made with equimolar amounts of $\text{CH}_3\text{NH}_2(aq)$ and $\text{CH}_3\text{NH}_3\text{Cl}(s)$. The student uses 25.00 mL of $0.100\ M\ \text{CH}_3\text{NH}_2(aq)$, which contains 0.00250 mol of $\text{CH}_3\text{NH}_2$, to make the buffer.
Calculate the mass of $\text{CH}_3\text{NH}_3\text{Cl}(s)$ that contains 0.00250 mol of $\text{CH}_3\text{NH}_3\text{Cl}$.
The student has the following materials and equipment available.
- Distilled water
- Electronic balance
- 50 mL beaker
- Pipets
- $0.100\ M\ \text{CH}_3\text{NH}_2(aq)$
- Weighing paper
- 10.0 mL graduated cylinder
- pH meter
- Solid $\text{CH}_3\text{NH}_3\text{Cl}$
- 50.00 mL buret
- Small spatula
The following table contains a partial procedure for making the buffer solution. Fill in steps 1 and 4 to complete the procedure using only materials and equipment selected from the choices given. (Not all materials listed will be used. Assume that all appropriate safety measures are already in place.)
| Step | Procedure |
|---|---|
| 1 | |
| 2 | Place the solid in the 50 mL beaker. |
| 3 | Clean the buret and rinse with distilled water. |
| 4 | |
| 5 | Use the buret to add 25.00 mL of $0.100\ M\ \text{CH}_3\text{NH}_2(aq)$ to the beaker. |
| 6 | Mix well. |
| 7 | Check the pH with the pH meter. |
The value of $K_b$ for $\text{CH}_3\text{NH}_2(aq)$ is $4.4 \times 10^{-4}$, and the pH of the buffer the student prepared is 10.64.
The student prepares a second buffer solution. The student uses 25.00 mL of $0.050\ M\ \text{CH}_3\text{NH}_2(aq)$ instead of 25.00 mL of $0.100\ M\ \text{CH}_3\text{NH}_2(aq)$, and half the mass of $\text{CH}_3\text{NH}_3\text{Cl}(s)$ that was used in the first buffer. Is the pH of the second buffer greater than, less than, or equal to the pH of the first buffer? Justify your answer.
HCl is a molecular gas as a pure substance but acts as an acid in aqueous solution.
A sample of $\text{HCl}(g)$ is stored in a rigid 6.00 L container at 7.45 atm and 296 K.
Calculate the number of moles of $\text{HCl}(g)$ in the container.
The rigid 6.00 L container of $\text{HCl}(g)$ is cooled to a temperature of 271 K. Calculate the new pressure, in atm, of the $\text{HCl}(g)$.
When HCl ionizes in aqueous solution, $\text{Cl}^-(aq)$ ions are formed. In the following box, draw three water molecules with proper orientation around the $\text{Cl}^-$ ion. Use [a water-molecule symbol: a large circle (O) with two small circles (H) attached] to represent water molecules.
[Empty square box with a small circle labeled Cl$^-$ at its center, for the student to draw three water molecules around.]
| Acid (HA) | Anion (A$^-$) | $K_a$ Value |
|---|---|---|
| $\text{HNO}_2$ | $\text{NO}_2^-$ | $5.6 \times 10^{-4}$ |
| HCl | $\text{Cl}^-$ | $2.0 \times 10^{7}$ |
| $\text{HClO}_4$ | $\text{ClO}_4^-$ | $1.6 \times 10^{15}$ |
The $K_a$ values for three acids are shown in the preceding table.
The following particulate diagram represents the ionization of one of the acids in the data table. Water molecules have been omitted for clarity. Which acid ($\text{HNO}_2$, HCl, or $\text{HClO}_4$) is represented in the diagram? Justify your answer using the information in the table.
[Particle diagram: a large circle containing several small particle pairs; legend shows three symbols: a three-circle cluster (two small white circles bonded to one larger white circle) = $\text{H}_3\text{O}^+$; a gray circle with a small black dot attached = HA; a plain gray circle = $\text{A}^-$. Inside the large circle: two $\text{H}_3\text{O}^+$-like clusters, several HA (gray-with-dot) particles, and several bare $\text{A}^-$ (plain gray) particles scattered about — roughly equal numbers of HA and $\text{A}^-$ particles with two $\text{H}_3\text{O}^+$ clusters, suggesting near-complete/extensive ionization.]
Answer the following questions related to $\text{HBr}(l)$ and $\text{HF}(l)$.
In the following table, list all of the types of intermolecular forces present in pure samples of $\text{HBr}(l)$ and $\text{HF}(l)$.
| Liquid | HBr$(l)$ | HF$(l)$ |
|---|---|---|
| Intermolecular forces present |
The enthalpy of vaporization, $\Delta H^\circ_{vap}$, for each liquid is provided in the following table.
| Liquid | HBr$(l)$ | HF$(l)$ |
|---|---|---|
| $\Delta H^\circ_{vap}$ | 17.3 kJ/mol | 25.2 kJ/mol |
Based on the types and relative strengths of intermolecular forces, explain why $\Delta H^\circ_{vap}$ of $\text{HF}(l)$ is greater than that of $\text{HBr}(l)$.
Calculate the amount of thermal energy, in kJ, required to vaporize 6.85 g of $\text{HF}(l)$.
Based on the arrangement of electrons in the Br and F atoms, explain why the bond length in an HBr molecule is greater than that in an HF molecule.
Strontium hydroxide dissolves in water according to the following equation. The $K_{sp}$ expression for strontium hydroxide is provided.
[Particle diagram inside a square box: two "Sr$^{2+}$" gray circles and several "OH$^-$" circles (open circle with a small black dot) scattered inside — roughly 3 gray Sr$^{2+}$ circles and 4 OH$^-$ circles shown. Legend beside the box: gray circle = $\text{Sr}^{2+}$; open circle with dot = $\text{OH}^-$.]
A student draws the particulate diagram shown to represent the ions present in an aqueous solution of $\text{Sr(OH)}_2$. (Water molecules are intentionally omitted.) Identify the error in the student's drawing.
The student prepares a saturated solution by adding excess $\text{Sr(OH)}_2(s)$ to distilled water and stirring until no more solid dissolves. The student then determines that $[\text{Sr}^{2+}] = 0.043\ M$ in the solution.
Calculate the value of $[\text{OH}^-]$ in the solution.
Calculate the value of $K_{sp}$ for $\text{Sr(OH)}_2$.
The student prepares a second saturated solution of $\text{Sr(OH)}_2$ in aqueous $0.10\ M\ \text{Sr(NO}_3)_2$ instead of water. Will the value of $[\text{OH}^-]$ in the second solution be greater than, less than, or equal to the value in the first solution? Justify your answer. (Assume constant temperature.)