Learn Extracted exam questions AP Chemistry 2016 Free Response
2016 Free Response
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A student investigates the enthalpy of solution, $\Delta H_{soln}$, for two alkali metal halides, $\text{LiCl}$ and $\text{NaCl}$. In addition to the salts, the student has access to a calorimeter, a balance with a precision of $\pm 0.1\text{ g}$, and a thermometer with a precision of $\pm 0.1^{\circ}\text{C}$.
To measure $\Delta H_{soln}$ for $\text{LiCl}$, the student adds 100.0 g of water initially at $15.0^{\circ}\text{C}$ to a calorimeter and adds 10.0 g of $\text{LiCl}(s)$, stirring to dissolve. After the $\text{LiCl}$ dissolves completely, the maximum temperature reached by the solution is $35.6^{\circ}\text{C}$.
Calculate the magnitude of the heat absorbed by the solution during the dissolution process, assuming that the specific heat capacity of the solution is $4.18\ \text{J/(g}\cdot^{\circ}\text{C)}$. Include units with your answer.
Determine the value of $\Delta H_{soln}$ for $\text{LiCl}$ in $\text{kJ/mol}_{rxn}$.
To explain why $\Delta H_{soln}$ for $\text{NaCl}$ is different than that for $\text{LiCl}$, the student investigates factors that affect $\Delta H_{soln}$ and finds that ionic radius and lattice enthalpy (which can be defined as the $\Delta H$ associated with the separation of a solid crystal into gaseous ions) contribute to the process. The student consults references and collects the data shown in the table below.
| Ion | Ionic Radius (pm) |
|---|---|
| $\text{Li}^+$ | 76 |
| $\text{Na}^+$ | 102 |
Write the complete electron configuration for the $\text{Na}^+$ ion in the ground state.
Using principles of atomic structure, explain why the $\text{Na}^+$ ion is larger than the $\text{Li}^+$ ion.
Which salt, $\text{LiCl}$ or $\text{NaCl}$, has the greater lattice enthalpy? Justify your answer.
Below is a representation of a portion of a crystal of $\text{LiCl}$. Identify the ions in the representation by writing the appropriate formulas ($\text{Li}^+$ or $\text{Cl}^-$) in the boxes below.
[Diagram: a cluster of packed spheres representing a portion of a LiCl crystal lattice, showing larger light-gray spheres and smaller dark-gray spheres packed together. Two empty boxes flank the cluster, each connected by an arrow pointing to a sphere in the cluster (one arrow points to a dark-colored sphere, one arrow points to a light-colored sphere), for the student to label which ion (Li+ or Cl-) each represents.]
The lattice enthalpy of $\text{LiCl}$ is positive, indicating that it takes energy to break the ions apart in $\text{LiCl}$. However, the dissolution of $\text{LiCl}$ in water is an exothermic process. Identify all particle-particle interactions that contribute significantly to the dissolution process being exothermic. For each interaction, include the particles that interact and the specific type of intermolecular force between those particles.
A student designs an experiment to study the reaction between $\text{NaHCO}_3$ and $\text{HC}_2\text{H}_3\text{O}_2$. The reaction is represented by the equation above. The student places 2.24 g of $\text{NaHCO}_3$ in a flask and adds 60.0 mL of 0.875 $M$ $\text{HC}_2\text{H}_3\text{O}_2$. The student observes the formation of bubbles and that the flask gets cooler as the reaction proceeds.
Identify the reaction represented above as an acid-base reaction, precipitation reaction, or redox reaction. Justify your answer.
Based on the information above, identify the limiting reactant. Justify your answer with calculations.
The student observes that the bubbling is rapid at the beginning of the reaction and gradually slows as the reaction continues. Explain this change in the reaction rate in terms of the collisions between reactant particles.
In thermodynamic terms, a reaction can be driven by enthalpy, entropy, or both.
Considering that the flask gets cooler as the reaction proceeds, what drives the chemical reaction between $\text{NaHCO}_3(s)$ and $\text{HC}_2\text{H}_3\text{O}_2(aq)$? Answer by drawing a circle around one of the choices below.
Enthalpy only Entropy only Both enthalpy and entropy
Justify your selection in part (d)(i) in terms of $\Delta G^{\circ}$.
The $\text{HCO}_3^-$ ion has three carbon-to-oxygen bonds. Two of the carbon-to-oxygen bonds have the same length and the third carbon-to-oxygen bond is longer than the other two. The hydrogen atom is bonded to one of the oxygen atoms. In the box below, draw a Lewis electron-dot diagram (or diagrams) for the $\text{HCO}_3^-$ ion that is (are) consistent with the given information.
[Empty box provided for the student to draw the Lewis electron-dot diagram(s).]
A student prepares a solution containing equimolar amounts of $\text{HC}_2\text{H}_3\text{O}_2$ and $\text{NaC}_2\text{H}_3\text{O}_2$. The pH of the solution is measured to be 4.7. The student adds two drops of 3.0 $M$ $\text{HNO}_3(aq)$ and stirs the sample, observing that the pH remains at 4.7. Write a balanced, net-ionic equation for the reaction between $\text{HNO}_3(aq)$ and the chemical species in the sample that is responsible for the pH remaining at 4.7.
To determine the molar mass of an unknown metal, M, a student reacts iodine with an excess of the metal to form the water-soluble compound $\text{MI}_2$, as represented by the equation above. The reaction proceeds until all of the $\text{I}_2$ is consumed. The $\text{MI}_2(aq)$ solution is quantitatively collected and heated to remove the water, and the product is dried and weighed to constant mass. The experimental steps are represented below, followed by a data table.
[Diagram: a sequence of process illustrations. First row: a beaker on a balance reading 125.457 g; an arrow labeled "Add M" points to a beaker on a balance reading 126.549 g (with solid M added); an arrow labeled "Add I2" points to a beaker on a balance reading 127.570 g (with solid I2 added, shown as a darker layer on top of the M). Second row: an arrow labeled "Add H2O" points from the beaker with M and I2 to a beaker where "reaction occurs" (labeled MI2(aq) at the bottom with "leftover M" also labeled); an arrow labeled "solution collected" points to a test tube containing MI2(aq); an arrow labeled "heat" points to a test tube; an arrow labeled "reheat" points to a final test tube containing solid MI2(s) at the bottom.]
| Data for Unknown Metal Lab | |
|---|---|
| Mass of beaker | 125.457 g |
| Mass of beaker + metal M | 126.549 g |
| Mass of beaker + metal M + $\text{I}_2$ | 127.570 g |
| Mass of $\text{MI}_2$, first weighing | 1.284 g |
| Mass of $\text{MI}_2$, second weighing | 1.284 g |
Given that the metal M is in excess, calculate the number of moles of $\text{I}_2$ that reacted.
Calculate the molar mass of the unknown metal M.
The student hypothesizes that the compound formed in the synthesis reaction is ionic.
Propose an experimental test the student could perform that could be used to support the hypothesis. Explain how the results of the test would support the hypothesis if the substance was ionic.
The student hypothesizes that $\text{Br}_2$ will react with metal M more vigorously than $\text{I}_2$ did because $\text{Br}_2$ is a liquid at room temperature.
Explain why $\text{I}_2$ is a solid at room temperature whereas $\text{Br}_2$ is a liquid. Your explanation should clearly reference the types and relative strengths of the intermolecular forces present in each substance.
While cleaning up after the experiment, the student wishes to dispose of the unused solid $\text{I}_2$ in a responsible manner. The student decides to convert the solid $\text{I}_2$ to $\text{I}^-(aq)$ anion. The student has access to three solutions, $\text{H}_2\text{O}_2(aq)$, $\text{Na}_2\text{S}_2\text{O}_3(aq)$, and $\text{Na}_2\text{S}_4\text{O}_6(aq)$, and the standard reduction table shown below.
| Half reaction | $E^{\circ}$ (V) |
|---|---|
| $\text{S}_4\text{O}_6^{2-}(aq) + 2e^- \rightarrow 2\text{S}_2\text{O}_3^{2-}(aq)$ | 0.08 |
| $\text{I}_2(s) + 2e^- \rightarrow 2\text{I}^-(aq)$ | 0.54 |
| $\text{O}_2(g) + 2\text{H}^+(aq) + 2e^- \rightarrow \text{H}_2\text{O}_2(aq)$ | 0.68 |
Which solution should the student add to $\text{I}_2(s)$ to reduce it to $\text{I}^-(aq)$? Circle your answer below. Justify your answer, including a calculation of $E^{\circ}$ for the overall reaction.
$\text{H}_2\text{O}_2(aq)$ $\text{Na}_2\text{S}_2\text{O}_3(aq)$ $\text{Na}_2\text{S}_4\text{O}_6(aq)$
Write the balanced net-ionic equation for the reaction between $\text{I}_2$ and the solution you selected in part (e).
Phenol is a weak acid that partially dissociates in water according to the equation above.
What is the pH of a 0.75 $M$ $\text{C}_6\text{H}_5\text{OH}(aq)$ solution?
For a certain reaction involving $\text{C}_6\text{H}_5\text{OH}(aq)$ to proceed at a significant rate, the phenol must be primarily in its deprotonated form, $\text{C}_6\text{H}_5\text{O}^-(aq)$. In order to ensure that the $\text{C}_6\text{H}_5\text{OH}(aq)$ is deprotonated, the reaction must be conducted in a buffered solution. On the number scale below, circle each pH for which more than 50 percent of the phenol molecules are in the deprotonated form ($\text{C}_6\text{H}_5\text{O}^-(aq)$). Justify your answer.
[Number scale from 1 to 14, in increments of 1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14]
At high temperatures the compound $\text{C}_4\text{H}_6$ (1,3-butadiene) reacts according to the equation above. The rate of the reaction was studied at 625 K in a rigid reaction vessel. Two different trials, each with a different starting concentration, were carried out. The data were plotted in three different ways, as shown below.
[Graph 1: "$[\text{C}_4\text{H}_6]$ vs. Time" — x-axis Time (s) from 0 to 20; y-axis $[\text{C}_4\text{H}_6]$ (mol/L) from 0 to 0.020. Trial 1 (open circles) starts at approximately (0, 0.0183) and decays smoothly to approximately (20, 0.0065). Trial 2 (filled squares) starts at approximately (0, 0.0096) and decays smoothly to approximately (20, 0.0049). Both curves show a decreasing, concave-up (leveling-off) shape.]
[Graph 2: "$\ln[\text{C}_4\text{H}_6]$ vs. Time" — x-axis Time (s) from 0 to 20; y-axis $\ln[\text{C}_4\text{H}_6]$ from -5.5 to -3.5. Trial 1 (open circles) starts at approximately (0, -4.0) and decreases roughly linearly-curved to approximately (20, -5.0). Trial 2 (filled squares) starts at approximately (0, -4.65) and decreases to approximately (20, -5.3). Both curves are slightly concave, not perfectly straight lines.]
[Graph 3: "$1/[\text{C}_4\text{H}_6]$ vs. Time" — x-axis Time (s) from 0 to 20; y-axis $1/[\text{C}_4\text{H}_6]$ (L/mol) from 0 to 250. Trial 1 (open circles) starts at approximately (0, 55) and increases in a straight line to approximately (20, 155). Trial 2 (filled squares) starts at approximately (0, 104) and increases in a straight line to approximately (20, 205). Both trials appear linear in this plot.]
For trial 1, calculate the initial pressure, in atm, in the vessel at 625 K. Assume that initially all the gas present in the vessel is $\text{C}_4\text{H}_6$.
Use the data plotted in the graphs to determine the order of the reaction with respect to $\text{C}_4\text{H}_6$.
The initial rate of the reaction in trial 1 is 0.0010 $\text{mol/(L}\cdot\text{s)}$. Calculate the rate constant, $k$, for the reaction at 625 K.
The polyatomic ion $\text{C}_{10}\text{H}_{12}\text{N}_2\text{O}_8^{4-}$ is commonly abbreviated as $\text{EDTA}^{4-}$. The ion can form complexes with metal ions in aqueous solutions. A complex of $\text{EDTA}^{4-}$ with $\text{Ba}^{2+}$ ion forms according to the equation above. A 50.0 mL volume of a solution that has an $\text{EDTA}^{4-}(aq)$ concentration of 0.30 $M$ is mixed with 50.0 mL of 0.20 $M$ $\text{Ba(NO}_3)_2$ to produce 100.0 mL of solution.
Considering the value of $K$ for the reaction, determine the concentration of $\text{Ba(EDTA)}^{2-}(aq)$ in the 100.0 mL of solution. Justify your answer.
The solution is diluted with distilled water to a total volume of 1.00 L. After equilibrium has been reestablished, is the number of moles of $\text{Ba}^{2+}(aq)$ present in the solution greater than, less than, or equal to the number of moles of $\text{Ba}^{2+}(aq)$ present in the original solution before it was diluted? Justify your answer.
A student is given a 25.0 mL sample of a solution of an unknown monoprotic acid and asked to determine the concentration of the acid by titration. The student uses a standardized solution of 0.110 $M$ $\text{NaOH}(aq)$, a buret, a flask, an appropriate indicator, and other laboratory equipment necessary for the titration.
The images below show the buret before the titration begins (below left) and at the end point (below right). What should the student record as the volume of $\text{NaOH}(aq)$ delivered to the flask?
[Diagram: two burets side by side, each with a magnified circular inset showing the meniscus reading. Left buret (before titration): the magnified inset shows graduation marks labeled 5 (above) and 6 (below), with the liquid meniscus sitting between them, closer to the 6 mark (approximately 5.8 mL). Right buret (at end point): the magnified inset shows graduation marks labeled 37 (above) and 38 (below), with the liquid meniscus sitting between them, closer to the 38 mark (approximately 37.7 mL).]
Based on the given information and your answer to part (a), determine the value of the concentration of the acid that should be recorded in the student's lab report.
In a second trial, the student accidentally added more $\text{NaOH}(aq)$ to the flask than was needed to reach the end point, and then recorded the final volume. Would this error increase, decrease, or have no effect on the calculated acid concentration for the second trial? Justify your answer.