AP Chemistry Unit 4 Practice Test & Exam | 20 Questions

1. [Skill: 6.A | Topic: 4.1] A student adds 1.00 g of solid $CaCl_2$ to 25.0 mL of water in a cup calorimeter. The solid disappears, and the temperature of the solution increases from 22.0°C to 28.5°C. No bubbles are observed. Which of the following best supports the claim that the process is primarily a physical change?

(A)Because heat is released, a chemical reaction must have occurred.(B)Because the composition of the particles present is the same before and after (the $Ca^{2+}$ and $Cl^-$ ions are simply dispersed in water), the change is physical.(C)Because the solid disappears, the $CaCl_2$ has been converted into a new substance.(D)Because the temperature increases, a gas must have been produced but not observed.

2. [Skill: 3.A | Topic: 4.1] A student performs the following procedure at room temperature: Step 1: Combine 5.0 mL of 0.10 M $Pb(NO_3)_2(aq)$ with 5.0 mL of 0.10 M $KI(aq)$. Observation: A bright yellow solid forms immediately. Which of the following is the best conclusion based on the observation?

(A)A physical change occurred because the ions are still present in solution and no new substance formed.(B)A chemical change occurred because the formation of a precipitate is evidence that a new substance formed.(C)A physical change occurred because the mixture contains the same elements as before mixing.(D)A chemical change occurred because the total volume of the mixture increased when the solutions were combined.

Refer to the figure below.

3. [Skill: 1.A | Topic: 4.1] A student heats a sample of a pure substance and records the temperature as a function of time. [Image Cue]: Graph, "Heating Curve of a Pure Substance", x-axis: Time (min), y-axis: Temperature (°C). The curve shows: (1) temperature rising from 20°C to 60°C, (2) a flat plateau at 60°C, (3) temperature rising from 60°C to 120°C, (4) a flat plateau at 120°C, (5) temperature rising above 120°C. Based on the heating curve, which statement best describes what is occurring during the plateau at 60°C?

(A)A chemical change is occurring because temperature remains constant while energy is added.(B)A physical change is occurring because the substance is undergoing a phase change at constant temperature.(C)A chemical change is occurring because heat is absorbed during the plateau.(D)A physical change is occurring because the chemical composition of the substance is changing gradually with time.

4. [Skill: 2A | Topic: 4.2] A student mixes 25.0 mL of 0.100 M $Na_2SO_4(aq)$ with 25.0 mL of 0.100 M $BaCl_2(aq)$. A white solid forms immediately. Which of the following is the correct net ionic equation for the reaction that occurs?

(A)$Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$(B)$2Na^{+}(aq) + SO_4^{2-}(aq) + Ba^{2+}(aq) + 2Cl^{-}(aq) \rightarrow BaSO_4(s) + 2Na^{+}(aq) + 2Cl^{-}(aq)$(C)$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$(D)$Ba^{2+}(aq) + 2Cl^{-}(aq) + 2Na^{+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) + 2NaCl(s)$

5. [Skill: 1B | Topic: 4.2] A beaker of water is stirred while solid ammonium chloride is added. The solid disappears and the solution becomes uniformly clear. No gas is produced and no other products are observed. Which balanced equation best represents this physical process?

(A)$NH_4Cl(s) \rightarrow NH_4^{+}(aq) + Cl^{-}(aq)$(B)$NH_4Cl(aq) \rightarrow NH_3(g) + HCl(g)$(C)$NH_4^{+}(aq) + Cl^{-}(aq) \rightarrow NH_4Cl(s)$(D)$NH_4Cl(s) + H_2O(l) \rightarrow NH_4OH(aq) + HCl(aq)$

6. [Skill: 2A | Topic: 4.2] A student performs a neutralization by mixing equal volumes of $0.200\,M$ $HCl(aq)$ and $0.200\,M$ $NaOH(aq)$. The temperature of the mixture increases. Which of the following equations best represents the chemical change responsible for the temperature increase?

(A)$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$(B)$H^{+}(aq) + Cl^{-}(aq) + Na^{+}(aq) + OH^{-}(aq) \rightarrow Na^{+}(aq) + Cl^{-}(aq) + H_2O(l)$(C)$H^{+}(aq) + OH^{-}(aq) \rightarrow H_2O(l)$(D)$Na^{+}(aq) + Cl^{-}(aq) \rightarrow NaCl(s)$

Refer to the figure below.

7. [Skill: 1.A | Topic: 4.3] A student models the reaction of nitrogen monoxide and oxygen forming nitrogen dioxide using particulate diagrams. The balanced chemical equation is $2\,NO(g) + O_2(g) \rightarrow 2\,NO_2(g)$ Which particulate diagram correctly represents the reactants and products for one reaction event that follows the balanced equation? (In each diagram, a red sphere represents an O atom and a blue sphere represents an N atom; molecules are shown as spheres touching.)

(A)Reactants: 2 NO molecules and 1 O2 molecule; Products: 2 NO2 molecules(B)Reactants: 1 NO molecule and 1 O2 molecule; Products: 1 NO2 molecule(C)Reactants: 2 NO molecules and 2 O2 molecules; Products: 2 NO2 molecules(D)Reactants: 2 NO molecules and 1 O2 molecule; Products: 1 NO2 molecule

8. [Skill: 1.A | Topic: 4.3] A particulate diagram represents a reaction between propane and oxygen. In the diagram, the reactants contain 1 molecule of propane ($C_3H_8$) and 5 molecules of oxygen ($O_2$). The products contain 3 molecules of carbon dioxide ($CO_2$) and 4 molecules of water ($H_2O$). Which balanced chemical equation is consistent with the particulate diagram?

(A)$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$(B)$C_3H_8 + 4O_2 \rightarrow 3CO_2 + 4H_2O$(C)$C_3H_8 + 5O_2 \rightarrow 3CO + 4H_2O$(D)$2C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$

Refer to the figure below.

9. [Skill: 3.A | Topic: 4.4] A student heats a 25.0 g sample of liquid water at 1 atm using a constant-power hot plate. The student records the temperature of the water over time. [Image Cue]: Line graph, "Temperature of Water vs. Time During Heating", x-axis: time (min), y-axis: temperature (°C). Curve increases from 20°C to 100°C, then shows a horizontal plateau at 100°C for several minutes while heating continues. Which of the following best explains the molecular-level reason the temperature remains constant during the plateau at 100°C?

(A)Energy added during the plateau is used primarily to break O–H covalent bonds within water molecules.(B)Energy added during the plateau is used primarily to overcome intermolecular attractions between water molecules so that molecules can separate into the gas phase.(C)Energy added during the plateau is released as new hydrogen bonds form, preventing the temperature from increasing.(D)Energy added during the plateau is used primarily to ionize water molecules into H+ and OH−, keeping the temperature constant.

10. [Skill: 3.B | Topic: 4.4] A student adds several crystals of NaCl(s) to water in a beaker and stirs. The crystals disappear, the solution conducts electricity, and the beaker feels slightly cooler to the touch. Which statement best describes what is happening at the particulate level as NaCl dissolves in water?

(A)Covalent Na–Cl bonds are broken and new covalent Na–O and Cl–H bonds form, producing a new molecular substance in solution.(B)Only hydrogen bonds between water molecules are broken; the NaCl(s) remains intact but becomes invisible in water.(C)Attractions between Na+ and Cl− ions in the ionic lattice are overcome, and new ion–dipole interactions form between the ions and water molecules; the process can be argued as involving both bond breaking and formation.(D)Na and Cl atoms are converted into different elements as they dissolve, which causes the solution to conduct electricity.

11. **1. [Skill: 4.A | Topic: 4.5]** A student mixes two aqueous solutions to form a precipitate according to the balanced equation below. $BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$ The student combines 25.0 mL of 0.150 M $BaCl_2(aq)$ with 40.0 mL of 0.100 M $Na_2SO_4(aq)$. Assume volumes are additive and the reaction goes to completion. Which of the following is the best estimate of the theoretical mass of $BaSO_4(s)$ formed?

(A)0.875 g(B)0.934 g(C)1.75 g(D)0.438 g

12. **2. [Skill: 4.A | Topic: 4.5]** In a laboratory investigation, solid potassium chlorate decomposes when heated according to the balanced equation below. $2KClO_3(s) \rightarrow 2KCl(s) + 3O_2(g)$ A student collects 425 mL of $O_2(g)$ at 755 mmHg and 25°C. Assume the oxygen gas is dry (no water vapor) and behaves ideally. Which of the following is the best estimate of the mass of $KClO_3(s)$ that decomposed?

(A)1.41 g(B)3.18 g(C)16.8 g(D)1.07 × 10^3 g

13. [Skill: 2A | Topic: 4.6] A student titrates 25.00 mL of 0.0800 M HCl(aq) with 0.100 M NaOH(aq) using phenolphthalein indicator. The reaction goes to completion: $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$ Selected buret readings show the total volume of NaOH added at different times during the titration: - 18.00 mL added: solution is colorless - 20.00 mL added: solution is colorless - 20.10 mL added: solution is faint pink (persistent) - 22.00 mL added: solution is pink Which volume of NaOH added corresponds most closely to the equivalence point?

(A)18.00 mL(B)20.00 mL(C)20.10 mL(D)22.00 mL

14. [Skill: 5A | Topic: 4.6] A student performs a titration in which an aqueous solution of sulfuric acid reacts completely with aqueous sodium hydroxide: $H_2SO_4(aq) + 2\,NaOH(aq) \rightarrow Na_2SO_4(aq) + 2\,H_2O(l)$ The flask initially contains 0.00150 mol of $H_2SO_4(aq)$. The student adds 0.100 M NaOH(aq) from a buret. Which volume of NaOH added corresponds to the equivalence point?

(A)15.0 mL(B)30.0 mL(C)45.0 mL(D)60.0 mL

15. 1. [Skill: 2A | Topic: 4.7] A student mixes 10.0 mL of 0.10 M $AgNO_3(aq)$ with 10.0 mL of 0.10 M $NaCl(aq)$ in a test tube. The student observes that the mixture immediately becomes cloudy and a white solid forms. The balanced molecular equation is shown. $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$ Which of the following best classifies the reaction?

(A)Acid-base reaction, because $H^+$ is transferred between reactants(B)Oxidation-reduction reaction, because electrons are transferred between Ag and Cl(C)Precipitation reaction, because an insoluble ionic compound forms when aqueous ions are mixed(D)Combustion reaction, because an oxide is produced in the products

16. 2. [Skill: 1B | Topic: 4.7] A student places a strip of aluminum metal into 50.0 mL of 0.50 M $CuCl_2(aq)$. After several minutes, the student observes a reddish-brown solid forming on the metal strip and the blue color of the solution fades. The net ionic equation is shown. $2Al(s) + 3Cu^{2+}(aq) \rightarrow 2Al^{3+}(aq) + 3Cu(s)$ Which of the following statements correctly identifies the reaction type and the species oxidized?

(A)Acid-base reaction; $Cu^{2+}$ is oxidized because it gains electrons(B)Precipitation reaction; $Cl^-$ is oxidized because it forms a solid compound(C)Oxidation-reduction reaction; $Al(s)$ is oxidized because its oxidation number increases from 0 to +3(D)Oxidation-reduction reaction; $Cu^{2+}$ is oxidized because its oxidation number decreases from +2 to 0

17. [Skill: 1.A | Topic: 4.8] A student mixes aqueous ammonia with water and observes that the solution becomes basic. The reaction is represented below. $NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$ Which of the following correctly identifies the Brønsted-Lowry acid and the Brønsted-Lowry base in the forward reaction, and one conjugate acid-base pair?

(A)Acid: $NH_3$; Base: $H_2O$; Conjugate pair: $NH_3/NH_4^+$(B)Acid: $H_2O$; Base: $NH_3$; Conjugate pair: $NH_3/NH_4^+$(C)Acid: $NH_4^+$; Base: $OH^-$; Conjugate pair: $H_2O/OH^-$(D)Acid: $OH^-$; Base: $NH_4^+$; Conjugate pair: $NH_4^+/NH_3$

18. [Skill: 5.A | Topic: 4.8] Two monoprotic acids, $HX$ and $HY$, are each dissolved in water to make separate 0.10 M solutions at the same temperature. After each solution reaches equilibrium, the hydronium ion concentrations are measured as shown. - Solution 1 (0.10 M $HX$): $[H_3O^+] = 0.095\,\text{M}$ - Solution 2 (0.10 M $HY$): $[H_3O^+] = 0.010\,\text{M}$ Which of the following conclusions is most consistent with the data?

(A)$HX$ is the stronger acid, and $X^-$ is the weaker conjugate base than $Y^-$.(B)$HY$ is the stronger acid, and $Y^-$ is the weaker conjugate base than $X^-$.(C)$HX$ is the stronger acid, and $X^-$ is the stronger conjugate base than $Y^-$.(D)$HX$ and $HY$ have equal acid strength because they have the same initial concentration.

19. [Skill: 3.A | Topic: 4.9] A student is given the following half-reactions occurring in acidic aqueous solution: Reduction: $MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$ Oxidation: $Fe^{2+} \rightarrow Fe^{3+} + e^-$ Which of the following is the correctly balanced net ionic equation for the redox reaction in acidic solution?

(A)$MnO_4^- + 5Fe^{2+} + 8H^+ \rightarrow Mn^{2+} + 5Fe^{3+} + 4H_2O$(B)$MnO_4^- + Fe^{2+} + 8H^+ \rightarrow Mn^{2+} + Fe^{3+} + 4H_2O$(C)$MnO_4^- + 5Fe^{2+} \rightarrow Mn^{2+} + 5Fe^{3+}$(D)$2MnO_4^- + 10Fe^{2+} + 8H^+ \rightarrow 2Mn^{2+} + 10Fe^{3+} + 4H_2O$

20. [Skill: 3.A | Topic: 4.9] A student is balancing a redox reaction in basic aqueous solution using the half-reaction method. The student has already obtained the following half-reactions (in basic solution): Reduction: $OCl^- + H_2O + 2e^- \rightarrow Cl^- + 2OH^-$ Oxidation: $I^- + 6OH^- \rightarrow IO_3^- + 3H_2O + 6e^-$ Which of the following is the correct net ionic equation after combining the half-reactions and canceling species that appear on both sides?

(A)$I^- + 3OCl^- \rightarrow IO_3^- + 3Cl^-$(B)$I^- + OCl^- + H_2O \rightarrow IO_3^- + Cl^- + 2OH^-$(C)$3I^- + OCl^- \rightarrow 3IO_3^- + Cl^-$(D)$I^- + 3OCl^- + 6OH^- \rightarrow IO_3^- + 3Cl^- + 3H_2O$

AP Chemistry Unit 4: Chemical Reactions Practice Exam

Exam Details

A. Multiple-Choice Questions (20 Questions)

AP Chemistry Unit 4: Chemical Reactions Practice Exam

Exam Details

A. Multiple-Choice Questions (20 Questions)