Learn Extracted exam questions AP Chemistry 2017 Free Response
2017 Free Response
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Carbon tetrachloride, $\text{CCl}_4(g)$, can be synthesized according to the reaction represented above. A chemist runs the reaction at a constant temperature of $120°\text{C}$ in a rigid 25.0 L container.
Chlorine gas, $\text{Cl}_2(g)$, is initially present in the container at a pressure of 0.40 atm.
How many moles of $\text{Cl}_2(g)$ are in the container?
How many grams of carbon disulfide, $\text{CS}_2(g)$, are needed to react completely with the $\text{Cl}_2(g)$ ?
At $30°\text{C}$ the reaction is thermodynamically favorable, but no reaction is observed to occur. However, at $120°\text{C}$, the reaction occurs at an observable rate.
Explain how the higher temperature affects the collisions between the reactant molecules so that the reaction occurs at an observable rate at $120°\text{C}$.
The graph below shows a distribution for the collision energies of reactant molecules at $120°\text{C}$. Draw a second curve on the graph that shows the distribution for the collision energies of reactant molecules at $30°\text{C}$.
[Graph: Maxwell-Boltzmann-type distribution curve; y-axis "Fraction of Collisions" (unlabeled scale), x-axis "Energy of Collisions" (unlabeled scale); a single curve starts at the origin, rises to a peak, then decreases and levels off toward the x-axis; a vertical dashed line partway down the curve's descending portion is labeled "Activation Energy" with an arrow pointing to where it crosses the curve.]
$\text{S}_2\text{Cl}_2$ is a product of the reaction.
In the box below, complete the Lewis electron-dot diagram for the $\text{S}_2\text{Cl}_2$ molecule by drawing in all of the electron pairs.
[Box showing the unfinished skeletal structure: Cl S S Cl (four atoms in a row, single bonds implied between adjacent atoms, no lone pairs or additional dots drawn yet).]
What is the approximate value of the Cl–S–S bond angle in the $\text{S}_2\text{Cl}_2$ molecule that you drew in part (c)(i) ? (If the two Cl–S–S bond angles are not equal, include both angles.)
$\text{CCl}_4(g)$ can also be produced by reacting $\text{CHCl}_3(g)$ with $\text{Cl}_2(g)$ at $400°\text{C}$, as represented by the equation below.
At the completion of the reaction a chemist successfully separates the $\text{CCl}_4(g)$ from the $\text{HCl}(g)$ by cooling the mixture to $70°\text{C}$, at which temperature the $\text{CCl}_4(g)$ condenses while the $\text{HCl}(g)$ remains in the gaseous state.
Identify all types of intermolecular forces present in $\text{HCl}(l)$.
What can be inferred about the relative strengths of the intermolecular forces in $\text{CCl}_4(l)$ and $\text{HCl}(l)$ ? Justify your answer in terms of the information above.
Answer the following questions about the isomers fulminic acid and isocyanic acid.
Two possible Lewis electron-dot diagrams for fulminic acid, HCNO, are shown below.
[Two Lewis structures side by side: Left: H–C≡N–Ö: (H bonded to C by a single bond, C triple-bonded to N, N bonded to O by a single bond, O has two lone pairs) Right: H–C̈=N=Ö: (H bonded to C by a single bond, C double-bonded to N, N double-bonded to O, C has one lone pair, O has two lone pairs)]
Fulminic acid can convert to isocyanic acid according to the equation below.
| Fulminic Acid | Isocyanic Acid |
|---|---|
| H–C≡N–Ö: (O has two lone pairs) | H–N̈=C=Ö: (N has one lone pair, O has two lone pairs) |
Explain why the diagram on the left is the better representation for the bonding in fulminic acid. Justify your choice based on formal charges.
Using the Lewis electron-dot diagrams of fulminic acid and isocyanic acid shown in the boxes above and the table of average bond enthalpies below, determine the value of $\Delta H°$ for the reaction of $\text{HCNO}(g)$ to form $\text{HNCO}(g)$.
| Bond | Enthalpy (kJ/mol) | Bond | Enthalpy (kJ/mol) | Bond | Enthalpy (kJ/mol) |
|---|---|---|---|---|---|
| N–O | 201 | C=N | 615 | H–C | 413 |
| C=O | 745 | C≡N | 891 | H–N | 391 |
A student claims that $\Delta S°$ for the reaction is close to zero. Explain why the student's claim is accurate.
Which species, fulminic acid (HCNO) or isocyanic acid (HNCO), is present in higher concentration at equilibrium at 298 K? Justify your answer in terms of thermodynamic favorability and the equilibrium constant.
The ammonium salt of isocyanic acid is a product of the decomposition of urea, $\text{CO(NH}_2)_2$, represented below.
A student studying the decomposition reaction runs the reaction at $90°\text{C}$. The student collects data on the concentration of urea as a function of time, as shown by the data table and the graph below.
| Time (hours) | $[\text{CO(NH}_2)_2]$ |
|---|---|
| 0 | 0.1000 |
| 5 | 0.0707 |
| 10 | 0.0500 |
| 15 | 0.0354 |
| 20 | 0.0250 |
| 25 | 0.0177 |
| 30 | 0.0125 |
[Graph: y-axis $[\text{CO(NH}_2)_2]$ from 0.00 to 0.10 in increments of 0.02; x-axis "Time (hours)" from 0 to 30; a smooth exponential-decay curve starting at $(0, 0.10)$ and decreasing to approximately $(30, 0.0125)$, consistent with the data table above.]
The student proposes that the rate law is $rate = k[\text{CO(NH}_2)_2]$.
Explain how the data support the student's proposed rate law.
Using the proposed rate law and the student's results, determine the value of the rate constant, $k$. Include units with your answer.
The student learns that the decomposition reaction was run in a solution with a pH of 13. Briefly describe an experiment, including the initial conditions that you would change and the data you would gather, to determine whether the rate of the reaction depends on the concentration of $\text{OH}^-(aq)$.
At high temperatures, $\text{N}_2(g)$ and $\text{O}_2(g)$ can react to produce nitrogen monoxide, $\text{NO}(g)$, as represented by the equation above.
Write the expression for the equilibrium constant, $K_p$, for the forward reaction.
A student injects $\text{N}_2(g)$ and $\text{O}_2(g)$ into a previously evacuated, rigid vessel and raises the temperature of the vessel to $2000°\text{C}$. At this temperature the initial partial pressures of $\text{N}_2(g)$ and $\text{O}_2(g)$ are 6.01 atm and 1.61 atm, respectively. The system is allowed to reach equilibrium. The partial pressure of $\text{NO}(g)$ at equilibrium is 0.122 atm. Calculate the value of $K_p$.
Nitrogen monoxide, $\text{NO}(g)$, can undergo further reactions to produce acids such as $\text{HNO}_2$, a weak acid with a $K_a$ of $4.0 \times 10^{-4}$ and a $\text{p}K_a$ of 3.40.
A student is asked to make a buffer solution with a pH of 3.40 by using 0.100 $M$ $\text{HNO}_2(aq)$ and 0.100 $M$ $\text{NaOH}(aq)$.
Explain why the addition of 0.100 $M$ $\text{NaOH}(aq)$ to 0.100 $M$ $\text{HNO}_2(aq)$ can result in the formation of a buffer solution. Include the net ionic equation for the reaction that occurs when the student adds the $\text{NaOH}(aq)$ to the $\text{HNO}_2(aq)$.
Determine the volume, in mL, of 0.100 $M$ $\text{NaOH}(aq)$ the student should add to 100. mL of 0.100 $M$ $\text{HNO}_2(aq)$ to make a buffer solution with a pH of 3.40. Justify your answer.
A second student makes a buffer by dissolving 0.100 mol of $\text{NaNO}_2(s)$ in 100. mL of 1.00 $M$ $\text{HNO}_2(aq)$. Which is more resistant to changes in pH when a strong acid or a strong base is added, the buffer made by the second student or the buffer made by the first student in part (c) ? Justify your answer.
A new buffer is made using $\text{HNO}_2(aq)$ as one of the ingredients. A particulate representation of a small representative portion of the buffer solution is shown below. (Cations and water molecules are not shown.) Is the pH of the buffer represented in the diagram greater than, less than, or equal to 3.40 ? Justify your answer.
[Diagram: a rectangular box containing a scattered arrangement of small circles representing particles, with a legend above showing two symbol types: "HNO$_2$ molecule" (drawn as two overlapping circles) and "NO$_2^-$ ion" (drawn as a single circle). The box shows a mixture of both symbol types scattered throughout, with noticeably more HNO$_2$-molecule symbols (two-circle pairs) than NO$_2^-$-ion symbols (single circles) — approximately 8 paired (HNO$_2$) symbols and 4 single (NO$_2^-$) symbols visible.]
[Figure, left group "Chromatography Chambers": two rectangular chambers side by side. Left chamber: a strip of chromatography paper suspended inside, with three spots labeled "Dye A", "Dye B", "Dye C" placed at the same height near the bottom of the paper, each with an arrow pointing down to its spot; the bottom of the paper dips into a reservoir labeled "Nonpolar Solvent". Right chamber: a similar strip with one spot labeled "Unknown" with an arrow pointing to it, also dipping into the same nonpolar solvent reservoir.
Right group "Developed Chromatograms": two rectangular strips after development. Left strip: a horizontal line near the top labeled "Solvent Front" (with an arrow pointing to the line); a horizontal dashed line near the bottom labeled "Origin" (with an arrow pointing to it); three dots between origin and solvent front, from lowest to highest: "Dye B" (lowest dot, closest to origin), "Dye A" (middle dot), "Dye C" (highest dot, closest to solvent front) — each labeled with an arrow. Right strip: same solvent front line at top, dashed origin line near bottom; one dot labeled "Unknown" positioned at a height matching Dye A's position on the left strip.]
A student investigates various dyes using paper chromatography. The student has samples of three pure dyes, labeled A, B, and C, and an unknown sample that contains one of the three dyes. The student prepares the chromatography chambers shown above on the left by putting a drop of each dye at the indicated position on the chromatography paper (a polar material) and standing the paper in a nonpolar solvent. The developed chromatograms are shown above on the right.
Which dye (A, B, or C) is the least polar? Justify your answer in terms of the interactions between the dyes and the solvent or between the dyes and the paper.
Which dye is present in the unknown sample? Justify your answer.
A student performs an experiment to determine the enthalpy of combustion of 2-propanol, $\text{C}_3\text{H}_7\text{OH}(l)$, which combusts in oxygen according to the equation above. The student heats a sample of water by burning some of the $\text{C}_3\text{H}_7\text{OH}(l)$ that is in an alcohol burner, as represented below. The alcohol burner uses a wick to draw liquid up into the flame. The mass of $\text{C}_3\text{H}_7\text{OH}(l)$ combusted is determined by weighing the alcohol burner before and after combustion.
[Two diagrams labeled "Initial" and "Final": each shows a ring stand holding a beaker labeled "Water" above an alcohol burner labeled "$\text{C}_3\text{H}_7\text{OH}(l)$" with a small flame; the Initial diagram additionally labels the "Wick" drawing liquid up into the flame. The setups are otherwise identical, representing the state of the apparatus before and after the combustion/heating process.]
Data from the experiment are given in the table below.
| Quantity | Value |
|---|---|
| Mass of $\text{C}_3\text{H}_7\text{OH}(l)$ combusted | 0.55 g |
| Mass of water heated | 125.00 g |
| Initial temperature of water | $22.0°\text{C}$ |
| Final temperature of water | $51.1°\text{C}$ |
| Specific heat of water | $4.18\ \text{J/(g}\cdot°\text{C)}$ |
Calculate the magnitude of the heat energy, in kJ, absorbed by the water. (Assume that the energy released from the combustion is completely transferred to the water.)
Based on the experimental data, if one mole of $\text{C}_3\text{H}_7\text{OH}(l)$ is combusted, how much heat, in kJ, is released? Report your answer with the correct number of significant figures.
A second student performs the experiment using the same mass of water at the same initial temperature. However, the student uses an alcohol burner containing $\text{C}_3\text{H}_7\text{OH}(l)$ that is contaminated with water, which is miscible with $\text{C}_3\text{H}_7\text{OH}(l)$. The difference in mass of the alcohol burner before and after the combustion in this experiment is also 0.55 g. Would the final temperature of the water in the beaker heated by the alcohol burner in this experiment be greater than, less than, or equal to the final temperature of the water in the beaker in the first student's experiment? Justify your answer.
Answer the following questions about $\text{Mg(OH)}_2$. At $25°\text{C}$, the value of the solubility product constant, $K_{sp}$, for $\text{Mg(OH)}_2(s)$ is $1.8 \times 10^{-11}$.
Calculate the number of grams of $\text{Mg(OH)}_2$ (molar mass 58.32 g/mol) that is dissolved in 100. mL of a saturated solution of $\text{Mg(OH)}_2$ at $25°\text{C}$.
The energy required to separate the ions in the $\text{Mg(OH)}_2$ crystal lattice into individual $\text{Mg}^{2+}(g)$ and $\text{OH}^-(g)$ ions, as represented in the table below, is known as the lattice energy of $\text{Mg(OH)}_2(s)$. As shown in the table, the lattice energy of $\text{Sr(OH)}_2(s)$ is less than the lattice energy of $\text{Mg(OH)}_2(s)$. Explain why in terms of periodic properties and Coulomb's law.
| Reaction | Lattice Energy (kJ/mol) |
|---|---|
| $\text{Mg(OH)}_2(s) \rightarrow \text{Mg}^{2+}(g) + 2\,\text{OH}^-(g)$ | 2900 |
| $\text{Sr(OH)}_2(s) \rightarrow \text{Sr}^{2+}(g) + 2\,\text{OH}^-(g)$ | 2300 |
A student wants to determine the concentration of $\text{H}_2\text{O}_2$ in a solution of $\text{H}_2\text{O}_2(aq)$. The student can use one of two titrants, either dichromate ion, $\text{Cr}_2\text{O}_7^{2-}(aq)$, or cobalt(II) ion, $\text{Co}^{2+}(aq)$. The balanced chemical equations for the two titration reactions are shown below.
Dichromate as titrant:
Cobalt(II) as titrant:
The half-reactions and the $E°$ values for the systems related to the titrations above are given in the following table.
| Half-Reaction | $E°$ (V) at 298 K |
|---|---|
| $\text{Co}^{3+}(aq) + e^- \rightarrow \text{Co}^{2+}(aq)$ | 1.84 |
| $\text{H}_2\text{O}_2(aq) + 2\,\text{H}^+(aq) + 2\,e^- \rightarrow 2\,\text{H}_2\text{O}(l)$ | 1.77 |
| $\text{Cr}_2\text{O}_7^{2-}(aq) + 14\,\text{H}^+(aq) + 6\,e^- \rightarrow 2\,\text{Cr}^{3+}(aq) + 7\,\text{H}_2\text{O}(l)$ | 1.33 |
| $\text{O}_2(g) + 2\,\text{H}^+(aq) + 2\,e^- \rightarrow \text{H}_2\text{O}_2(aq)$ | 0.70 |
Use the information in the table to calculate the following.
$E°$ for the reaction between $\text{Cr}_2\text{O}_7^{2-}(aq)$ and $\text{H}_2\text{O}_2(aq)$ at 298 K
$E°$ for the reaction between $\text{Co}^{2+}(aq)$ and $\text{H}_2\text{O}_2(aq)$ at 298 K
Based on the calculated values of $E°$, the student must choose the titrant for which the titration reaction is thermodynamically favorable at 298 K.
Which titrant should the student choose? Explain your reasoning.
Calculate the value of $\Delta G°$, in $\text{kJ/mol}_{rxn}$, for the reaction between the chosen titrant and $\text{H}_2\text{O}_2(aq)$.