Practice Test 2 - AP Chemistry

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. A proposed mechanism includes the following single step, which is explicitly identified as an elementary reaction: $\mathrm{Br}(g)+\mathrm{H_2}(g)\rightarrow \mathrm{HBr}(g)+\mathrm{H}(g)$ What is the rate law for this elementary step?
2. The elements C, N, O, and F are consecutive elements across Period 2. Which element has the highest electronegativity?
3. A dissolution in water at $25^\circ\text{C}$ is observed to cool the solution ($\Delta H_{\text{soln}} > 0$). The student also concludes that the overall entropy change is positive ($\Delta S_{\text{soln}} > 0$). Which statement is correct about thermodynamic favorability at $25^\circ\text{C}$?
4. Consider the bonds $\mathrm{C-C}$ in ethane ($\mathrm{H_3C-CH_3}$) and $\mathrm{C=C}$ in ethene ($\mathrm{H_2C=CH_2}$). Which bond is expected to be shorter?
5. At a given temperature, the equilibrium constant for $\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}$ is much less than 1 ($K \ll 1$). What does this imply about which side is favored at equilibrium?
6. When solid ammonium nitrate, $\text{NH}_4\text{NO}_3(s)$, dissolves in water, the solution becomes noticeably colder. Which statement correctly describes the sign and meaning of $\Delta H$ for the dissolving process?
7. When hydrogen sulfide gas is bubbled into a solution containing chlorine gas dissolved in water, the reaction below occurs: $\text{H}_2\text{S}(g) + \text{Cl}_2(aq) \rightarrow 2\text{HCl}(aq) + \text{S}(s)$ Which species acts as the oxidizing agent?
8. In a biochemical pathway, Reaction 1 is spontaneous: $A \rightarrow B$ with $\Delta G = -18\ \text{kJ/mol}$. Reaction 2 is nonspontaneous: $B \rightarrow C$ with $\Delta G = +11\ \text{kJ/mol}$. The cell couples the reactions by using intermediate $B$ so that Reaction 1 drives Reaction 2 in the same location and time. Which statement best describes whether the overall coupled process $A \rightarrow C$ is thermodynamically favorable?
9. A solution is measured in a 1.00 cm cuvette and has absorbance $A=0.66$. The molar absorptivity is $\varepsilon=3.3\times10^3\ \text{L mol}^{-1}\text{cm}^{-1}$. What is the concentration?
10. A weak base, $\mathrm{B(aq)}$, is titrated with a strong acid, $\mathrm{HCl(aq)}$. The titration passes through the initial solution (mostly $\mathrm{B}$), buffering region (mixture of $\mathrm{B}$ and $\mathrm{BH^+}$), equivalence region (all $\mathrm{B}$ converted to $\mathrm{BH^+}$), and post-equivalence (excess $\mathrm{H_3O^+}$). At the equivalence region, which species is expected to predominate (ignoring spectator ions)?
11. Consider the reversible reaction $\mathrm{NH_3(g) + HCl(g) \rightleftharpoons NH_4Cl(s)}$ at a certain temperature, where $K_p = 2.0\times10^{4}$. A mixture is prepared with $P_{\mathrm{NH_3}} = 0.10\,\mathrm{atm}$ and $P_{\mathrm{HCl}} = 0.10\,\mathrm{atm}$, and the system is not initially at equilibrium. Based on comparing $Q_p$ to $K_p$, in which direction will the reaction proceed to reach equilibrium?
12. A salt dissolves in water according to: $\text{MX}(s) \rightarrow \text{M}^+(aq)+\text{X}^-(aq)$. For this dissolution, $\Delta H_{\text{soln}}<0$ and $\Delta S_{\text{soln}}>0$. Under which conditions is the dissolution thermodynamically favored (i.e., $\Delta G<0$)?
13. A reaction-energy diagram is shown for a two-step reaction. Which statement correctly identifies $\Delta H$ for the overall reaction? (Vertical axis: potential energy; horizontal axis: reaction coordinate.)
14. A student has a mixture of sand and an aqueous solution of sodium chloride after stirring beach sand in saltwater. The student wants to obtain the sand as a separate component without changing its composition. Which separation method is most appropriate?
15. A $0.10\,\text{M}$ solution of the weak base $\text{B}$ is prepared at $25^\circ\text{C}$. The base reacts with water according to $\text{B}(aq)+\text{H}_2\text{O}(l)\rightleftharpoons \text{BH}^+(aq)+\text{OH}^-(aq)$ with $K_b=4.0\times10^{-4}$. Compared with pure water at the same temperature, which change occurs as equilibrium is established?
16. For the reversible reaction $\mathrm{PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)}$ at a certain temperature, $K_c = 0.40$. A reaction mixture is prepared such that $Q_c = 3.0$, so the system is not initially at equilibrium. Based on the comparison of $Q_c$ and $K_c$, in which direction will the reaction proceed to reach equilibrium?
17. For a reaction with the general rate law, Rate = $k[A]^m[B]^n$, what do the exponents $m$ and $n$ represent?
18. Nitrogen monoxide reacts with ozone in the gas phase: $\text{NO}(g)+\text{O}_3(g)\rightarrow \text{NO}_2(g)+\text{O}_2(g)$. A second run is performed with a small amount of $\text{Cl}(g)$ present as a catalyst; the overall reactants and products are the same in both runs, and $\text{Cl}(g)$ is not consumed. Which statement best explains why the catalyzed run has a higher reaction rate?
19. A reversible reaction $\text{Br}_2(l) \rightleftharpoons \text{Br}_2(g)$ occurs in a closed container at constant temperature. Over time, the amount of liquid bromine and bromine vapor each becomes constant. Which statement best describes equilibrium for this system?
20. A sample of CO2(s) (dry ice) at −78°C is placed in an open container and sublimates completely to CO2(g) at the same temperature. Considering only the CO2 as the system, does entropy increase, decrease, or remain approximately constant?

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