Learn Extracted exam questions AP Chemistry 2018 Free Response
2018 Free Response
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A student performs an experiment to determine the value of the enthalpy change, $\Delta H^{\circ}_{rxn}$, for the oxidation-reduction reaction represented by the balanced equation above.
Determine the oxidation number of Cl in $\text{NaOCl}$.
Calculate the number of grams of $\text{Na}_2\text{S}_2\text{O}_3$ needed to prepare 100.00 mL of 0.500 $M$ $\text{Na}_2\text{S}_2\text{O}_3(aq)$.
In the experiment, the student uses the solutions shown in the table below.
| Solution | Concentration ($M$) | Volume (mL) |
|---|---|---|
| $\text{Na}_2\text{S}_2\text{O}_3(aq)$ | 0.500 | 5.00 |
| $\text{NaOCl}(aq)$ | 0.500 | 5.00 |
| $\text{NaOH}(aq)$ | 0.500 | 5.00 |
Using the balanced equation for the oxidation-reduction reaction and the information in the table above, determine which reactant is the limiting reactant. Justify your answer.
The solutions, all originally at 20.0°C, are combined in an insulated calorimeter. The temperature of the reaction mixture is monitored, as shown in the graph below.
[Graph of Temperature (°C) vs. Time. Y-axis: Temperature (°C) from 18.0 to 34.0 in increments of 2.0. X-axis: Time (unlabeled scale). The curve starts flat at about 20.0°C, then rises steeply in an S-shaped curve, leveling off at approximately 32.5°C.]
According to the graph, what is the temperature change of the reaction mixture?
The mass of the reaction mixture inside the calorimeter is 15.21 g.
Calculate the magnitude of the heat energy, in joules, that is released during the reaction. Assume that the specific heat of the reaction mixture is 3.94 J/(g·°C) and that the heat absorbed by the calorimeter is negligible.
Using the balanced equation for the oxidation-reduction reaction and your answer to part (c), calculate the value of the enthalpy change of the reaction, $\Delta H^{\circ}_{rxn}$, in kJ/mol$_{rxn}$. Include the appropriate algebraic sign with your answer.
The student repeats the experiment, but this time doubling the volume of each of the reactants, as shown in the table below.
| Solution | Concentration ($M$) | Volume (mL) |
|---|---|---|
| $\text{Na}_2\text{S}_2\text{O}_3(aq)$ | 0.500 | 10.0 |
| $\text{NaOCl}(aq)$ | 0.500 | 10.0 |
| $\text{NaOH}(aq)$ | 0.500 | 10.0 |
The magnitude of the enthalpy change, $\Delta H^{\circ}_{rxn}$, in kJ/mol$_{rxn}$, calculated from the results of the second experiment is the same as the result calculated in part (e)(ii). Explain this result.
Write the balanced net ionic equation for the given reaction.
A student investigates the reactions of nitrogen oxides. One of the reactions in the investigation requires an equimolar mixture of $\text{NO}(g)$ and $\text{NO}_2(g)$, which the student produces by using the reaction represented above.
The particle-level representation of the equimolar mixture of $\text{NO}(g)$ and $\text{NO}_2(g)$ in the flask at the completion of the reaction between $\text{NO}(g)$ and $\text{O}_2(g)$ is shown below in the box on the right. In the box below on the left, draw the particle-level representation of the reactant mixture of $\text{NO}(g)$ and $\text{O}_2(g)$ that would yield the product mixture shown in the box on the right. In your drawing, represent oxygen atoms and nitrogen atoms as indicated below.
[Key: Oxygen atom = open (white) circle; Nitrogen atom = filled (gray) circle.]
[Two boxes side by side connected by an arrow. Left box labeled "Reactant Mixture" is empty (for the student to draw in). Right box labeled "Product Mixture" shows a particle-level representation containing 6 NO molecules (each one open circle bonded to one filled circle) and 6 NO$_2$ molecules (each one filled circle bonded to two open circles), scattered randomly in the box.]
The student reads in a reference text that $\text{NO}(g)$ and $\text{NO}_2(g)$ will react as represented by the equation below. Thermodynamic data for the reaction are given in the table below the equation.
| $\Delta H^{\circ}_{298}$ | $\Delta S^{\circ}_{298}$ | $\Delta G^{\circ}_{298}$ |
|---|---|---|
| $-40.4$ kJ/mol$_{rxn}$ | $-138.5$ J/(K·mol$_{rxn}$) | $0.87$ kJ/mol$_{rxn}$ |
The student begins with an equimolar mixture of $\text{NO}(g)$ and $\text{NO}_2(g)$ in a rigid reaction vessel and the mixture reaches equilibrium at 298 K.
Calculate the value of the equilibrium constant, $K$, for the reaction at 298 K.
If both $P_{\text{NO}}$ and $P_{\text{NO}_2}$ in the vessel are initially 1.0 atm, will $P_{\text{N}_2\text{O}_3}$ at equilibrium be equal to 1.0 atm? Justify your answer.
The student hypothesizes that increasing the temperature will increase the amount of $\text{N}_2\text{O}_3(g)$ in the equilibrium mixture. Indicate whether you agree or disagree with the hypothesis. Justify your answer.
$\text{N}_2\text{O}_3(g)$ reacts with water to form nitrous acid, $\text{HNO}_2(aq)$, a compound involved in the production of acid rain. The reaction is represented below.
The skeletal structure of the $\text{HNO}_2$ molecule is shown in the box below.
[Box showing the skeletal structure: H O N O, written left to right in a line.]
Complete the Lewis electron-dot diagram of the $\text{HNO}_2$ molecule in the box below, including any lone pairs of electrons.
Based on your completed diagram above, identify the hybridization of the nitrogen atom in the $\text{HNO}_2$ molecule.
To produce an aqueous solution of $\text{HNO}_2$, the student bubbles $\text{N}_2\text{O}_3(g)$ into distilled water. Assume that the reaction goes to completion and that $\text{HNO}_2$ is the only species produced. To determine the concentration of $\text{HNO}_2(aq)$ in the resulting solution, the student titrates a 100. mL sample of the solution with 0.100 $M$ $\text{KOH}(aq)$. The neutralization reaction is represented below.
The following titration curve shows the change in pH of the solution during the titration.
[Titration curve graph: Y-axis pH from 0 to 14 in increments of 2. X-axis "Volume of 0.100 M KOH(aq) Added (mL)" from 0 to 25+ in increments of 5. The curve starts around pH 2.3 at 0 mL, rises very gradually and slightly through a small buffer region to about pH 4 near 15-18 mL, then rises steeply (equivalence point) around 20 mL up to about pH 12, then levels off gradually toward pH 12-13 by 25 mL.]
Use the titration curve and the information above to determine the initial concentration of the $\text{HNO}_2(aq)$ solution.
Use the titration curve and the information above to estimate the value of $\text{p}K_a$ for $\text{HNO}_2(aq)$.
During the titration, after a volume of 15 mL of 0.100 $M$ $\text{KOH}(aq)$ has been added, which species, $\text{HNO}_2(aq)$ or $\text{NO}_2^-(aq)$, is present at a higher concentration in the solution? Justify your answer.
Answer the following questions relating to Fe and its ions, $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$.
Write the ground-state electron configuration of the $\text{Fe}^{2+}$ ion.
| Ion | Ionic Radius (pm) |
|---|---|
| $\text{Fe}^{2+}$ | 92 |
| $\text{Fe}^{3+}$ | 79 |
The radii of the ions are given in the table above. Using principles of atomic structure, explain why the radius of the $\text{Fe}^{2+}$ ion is larger than the radius of the $\text{Fe}^{3+}$ ion.
$\text{Fe}^{3+}$ ions interact more strongly with water molecules in aqueous solution than $\text{Fe}^{2+}$ ions do. Give one reason for this stronger interaction, and justify your answer using Coulomb's law.
A student obtains a solution that contains an unknown concentration of $\text{Fe}^{2+}(aq)$. To determine the concentration of $\text{Fe}^{2+}(aq)$ in the solution, the student titrates a sample of the solution with $\text{MnO}_4^-(aq)$, which converts $\text{Fe}^{2+}(aq)$ to $\text{Fe}^{3+}(aq)$, as represented by the following equation.
Write the balanced equation for the half-reaction for the oxidation of $\text{Fe}^{2+}(aq)$ to $\text{Fe}^{3+}(aq)$.
The student titrates a 10.0 mL sample of the $\text{Fe}^{2+}(aq)$ solution. Calculate the value of $[\text{Fe}^{2+}]$ in the solution if it takes 17.48 mL of added 0.0350 $M$ $\text{KMnO}_4(aq)$ to reach the equivalence point of the titration.
To deliver the 10.0 mL sample of the $\text{Fe}^{2+}(aq)$ solution in part (e), the student has the choice of using one of the pieces of glassware listed below.
- 25 mL buret
- 25 mL graduated cylinder
- 25 mL beaker
- 25 mL volumetric flask
Explain why the 25 mL volumetric flask would be a poor choice to use for delivering the required volume of the $\text{Fe}^{2+}(aq)$ solution.
In a separate experiment, the student is given a sample of powdered $\text{Fe}(s)$ that contains an inert impurity. The student uses a procedure to oxidize the $\text{Fe}(s)$ in the sample to $\text{Fe}_2\text{O}_3(s)$. The student collects the following data during the experiment.
| Mass of $\text{Fe}(s)$ with inert impurity | 6.724 g |
| Mass of $\text{Fe}_2\text{O}_3(s)$ produced | 7.531 g |
Calculate the number of moles of Fe in the $\text{Fe}_2\text{O}_3(s)$ produced.
Calculate the percent by mass of Fe in the original sample of powdered $\text{Fe}(s)$ with the inert impurity.
If the oxidation of the $\text{Fe}(s)$ in the original sample was incomplete so that some of the 7.531 g of product was $\text{FeO}(s)$ instead of $\text{Fe}_2\text{O}_3(s)$, would the calculated mass percent of $\text{Fe}(s)$ in the original sample be higher, lower, or the same as the actual mass percent of $\text{Fe}(s)$? Justify your answer.
[Key: Sulfur atom = large gray circle; Carbon atom = small black circle; Oxygen atom = small white circle.]
| Compound | Molecular Structure | Boiling Point at 1 atm (K) |
|---|---|---|
| $\text{CS}_2$ | gray–black–gray (S=C=S, linear, symmetric) | 319 |
| $\text{COS}$ | gray–black–white (S=C=O, linear, asymmetric) | 223 |
The table above gives the molecular structures and boiling points for the compounds $\text{CS}_2$ and $\text{COS}$.
In terms of the types and relative strengths of all the intermolecular forces in each compound, explain why the boiling point of $\text{CS}_2(l)$ is higher than that of $\text{COS}(l)$.
A 10.0 g sample of $\text{CS}_2(l)$ is put in an evacuated 5.0 L rigid container. The container is sealed and heated to 325 K, at which temperature all of the $\text{CS}_2(l)$ has vaporized. What is the pressure in the container once all of the $\text{CS}_2(l)$ has vaporized?
The ionization of $\text{HF}(aq)$ in water is represented by the equation above. In a 0.0350 $M$ $\text{HF}(aq)$ solution, the percent ionization of HF is 13.0 percent.
Two particulate representations of the ionization of HF molecules in the 0.0350 $M$ $\text{HF}(aq)$ solution are shown below in Figure 1 and Figure 2. Water molecules are not shown. Explain why the representation of the ionization of HF molecules in water in Figure 1 is more accurate than the representation in Figure 2. (The key below identifies the particles in the representations.)
[Key: filled black triple-particle cluster = $\text{H}_3\text{O}^+$; two-particle black-and-gray cluster = HF; single gray circle = $\text{F}^-$.]
[Figure 1: a circle containing a mixture of particles — approximately 2 $\text{H}_3\text{O}^+$ clusters, 2 $\text{F}^-$ single circles, and 6 HF two-particle clusters, scattered with mostly HF molecules present and few ionized species, consistent with a small (13%) percent ionization.]
[Figure 2: a circle containing a denser mixture of particles — approximately 6 $\text{H}_3\text{O}^+$ clusters, 6 $\text{F}^-$ single circles, and 8 HF two-particle clusters, scattered with a much higher proportion of ionized species than Figure 1, inconsistent with only 13% ionization.]
Use the percent ionization data above to calculate the value of $K_a$ for HF.
If 50.0 mL of distilled water is added to 50.0 mL of 0.035 $M$ $\text{HF}(aq)$, will the percent ionization of $\text{HF}(aq)$ in the solution increase, decrease, or remain the same? Justify your answer with an explanation or calculation.
[Diagram of a galvanic cell: a voltmeter connects two electrodes, each dipped into a separate beaker. Left beaker: electrode labeled Cr(s), immersed in a solution labeled $\text{Cr}^{3+}(aq)$. Right beaker: electrode labeled Ag(s), immersed in a solution labeled $\text{Ag}^+(aq)$. The two electrode wires both connect upward to a box labeled "Voltmeter." No salt bridge is shown connecting the two beakers.]
A student sets up a galvanic cell at 298 K that has an electrode of Ag(s) immersed in a 1.0 $M$ solution of $\text{Ag}^+(aq)$ and an electrode of Cr(s) immersed in a 1.0 $M$ solution of $\text{Cr}^{3+}(aq)$, as shown in the diagram above.
The student measures the voltage of the cell shown above and discovers that it is zero. Identify the missing component of the cell, and explain its importance for obtaining a nonzero voltage.
| Half-Reaction | $E^{\circ}$ (V) |
|---|---|
| $\text{Ag}^+(aq) + e^- \rightarrow \text{Ag}(s)$ | $+0.80$ |
| $\text{Cr}^{3+}(aq) + 3\,e^- \rightarrow \text{Cr}(s)$ | ? |
The student adds the missing component to the cell and measures $E^{\circ}_{cell}$ to be $+1.54$ V. As the cell operates, $\text{Ag}^+$ ions are reduced. Use this information and the information in the table above to do the following.
Calculate the value of $E^{\circ}$ for the half-reaction $\text{Cr}^{3+}(aq) + 3\,e^- \rightarrow \text{Cr}(s)$.
Write the balanced net-ionic equation for the overall reaction that occurs as the cell operates.
Calculate the value of $\Delta G^{\circ}$ for the overall cell reaction in J/mol$_{rxn}$.
[Graph: Photoelectron spectrum. Y-axis "Relative Number of Electrons" (unlabeled scale, showing peak heights). X-axis "Binding Energy (MJ/mol)" from 45 down to 0, in increments of 5, decreasing left to right. Peaks: one medium-height peak at approximately 40 MJ/mol; two peaks close together near 2 MJ/mol — a medium-height peak just above 2, and a tall peak (tallest in the spectrum) just below 2, close to 0.]
The complete photoelectron spectrum of an element is represented above.
Identify the element.
A radioactive isotope of the element decays with a half-life of 10. minutes.
Calculate the value of the rate constant, $k$, for the radioactive decay. Include units with your answer.
If 64 atoms of the radioactive isotope are originally present in a sample, what is the expected amount of time that will pass until only one atom of the isotope remains? Show how you arrived at your answer.