AP Chemistry: 4.9 Oxidation – Reduction (Redox) Reactions – Exam Style questions with Answer- MCQ

                                       \(5Fe_2+(aq)+MnO_4^−(aq)+8H^+(aq)→5Fe_3^+(aq)+Mn_2+(aq)+4H_2O(l)\)

Which of the following represents the oxidation half-reaction based on the balanced ionic equation shown above?
A \(Fe2^+(aq)+e^−→Fe_3+(aq)\)
B \(Fe_2^+(aq)→Fe_3^+(aq)+e^−\)
C \(MnO_4^−(aq)→Mn_2^+(aq)+5e^−\)
D \(MnO4^−(aq)+5e^−→Mn_2^+(aq)\)

▶️Answer/Explanation

Ans:B
Oxidation is the loss of electrons or the increase of oxidation number. In the reaction, each \(Fe_2^+\) ion loses an electron to form an Fe3+ ion; thus, the equation correctly represents the oxidation half-reaction.

Question

                                                            \(Al(s)→Al_3+(aq)+3e^−\)

                                                            \(Sn_2^+(aq)+2e^−→Sn(s)\)
Based on the half-reactions represented above, which of the following is the balanced ionic equation for the oxidation-reduction reaction between Al(s) and \(Sn_2^+\)(aq)?
A \(Al(s)+Sn_2+(aq)→Al_3+(aq)+Sn(s)+e^−\)
B \(3Al(s)+2Sn_2+(aq)→3Al_3^+(aq)+2Sn(s)\)
C \(2Al(s)+3Sn_2+(aq)→2Al_3^+(aq)+3Sn(s)\)
D \(Al(s)+Sn2+(aq)+2e−→Al3+(aq)+Sn(s)+3e−\)

▶️Answer/Explanation

Ans:C
The number of electrons in the two half-reactions must be equal. Six electrons are lost by 2 Al atoms in the oxidation half-reaction, and six electrons are added to 3 \(Sn_2^+\) ions in the reduction half-reaction. Hence, in the balanced equation for the reaction, the coefficients for Al and Al3+ are 2 and the coefficients for Sn2+ and Sn are 3.

Question

                    \(Zn(s)+CuSO_4(aq)→Cu(s)+ZnSO_4(aq)\)

When a zinc plate is placed in an aqueous solution of copper sulfate, elemental copper forms, as represented by the equation above. Which of the following represents the reduction half-reaction of the reaction?
A \(Cu_2^+(aq)+2e^−→Cu(s)\)
B \(Cu(s)→Cu_2+(aq)+2e^−\)
C \(Zn(s)+2e^−→Zn2^−(aq)\)
D \(Zn(s)→Zn_2^+(aq)+2e^−\)

▶️Answer/Explanation

Ans: A

Reduction is the gain of electrons or the decrease in oxidation number. In the reaction, each Cu2+ ion gains two electrons to form a Cu atom; thus, the equation represents the half-reaction for the reduction of Cu.

Question

\(  M^{+}\) is an unknown metal cation with a +1 charge. A student dissolves the chloride of the unknown  metal, MCI, in enough water to make 100.0 mL of    solution. The student then mixes the solution with   excess \(AgNO_3\) solution, causing AgCl to precipitate.   The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass of AgCl is 143 g/mol.)

Mass of unknown chloride, MCI                        0.74 g

Mass of filter paper                                            0.80 g

Mass of filter paper plus AgCl precipitate          2.23 g

 During the course of the experiment, which of the following happens to the \(NO_{3}^{-}\) ions?

 (A) They are oxidized by CF ions.

 (B) They are reduced to NOE ions.

(C) They are decomposed by reacting with\( M^{+} \)ions

(D) They remain dissolved in the filtrate solution.

▶️Answer/Explanation

Ans:D

Question

\(  M^{+}\) is an unknown metal cation with a +1 charge. A student dissolves the chloride of the unknown  metal, MCI, in enough water to make 100.0 mL of    solution. The student then mixes the solution with   excess \(AgNO_3\) solution, causing AgCl to precipitate.   The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass of AgCl is 143 g/mol.)

Mass of unknown chloride, MCI                        0.74 g

Mass of filter paper                                            0.80 g

Mass of filter paper plus AgCl precipitate          2.23 g

 During the course of the experiment, which of the following happens to the \(NO_{3}^{-}\) ions?

 (A) They are oxidized by CF ions.

 (B) They are reduced to NOE ions.

(C) They are decomposed by reacting with\( M^{+} \)ions

(D) They remain dissolved in the filtrate solution.

▶️Answer/Explanation

Ans:D

Question

\(5 H_2O_2(aq) + 2 MnO_{4} ̄(aq) + 6 H^{+}(aq)→2 Mn^{2+}(aq) + 8 H_{2}O(l) + 5 O_{2}(g)\)

In a titration experiment,\( H_{2}O_{2}(aq)\) reacts with aqueous\( MnO_4^{+}(aq) \)as represented by the equation above. The dark purple \(KMnO_{4}\) solution is added from a buret to a colorless, acidified solution of\( H_{2}O_{2}\)(aq) in an Erlenmeyer flask. (Note: At the end point of the titration, the solution is a pale pink color.)

Which element is being oxidized during the titration, and what is the element’s change in oxidation number?

(A) Oxygen, which changes from -1 to 0

(B) Oxygen, which changes from 0 to -2

(C) Manganese, which changes from -1 to +2

(D) Manganese, which changes from +7 to +2

▶️Answer/Explanation

Ans:A

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