Question
\[
\mathrm{Li}_3 \mathrm{~N}(s)+2 \mathrm{H}_2(g) \rightleftarrows \mathrm{LiNH}_2(s)+2 \mathrm{LiH}(s) \quad \Delta H^{\circ}=-192 \mathrm{~kJ} / \mathrm{mol}_{r x n}
\]
Because pure \(\mathrm{H}_2\) is a hazardous substance, safer and more cost effective techniques to store it as a solid for shipping purposes have been developed. One such method is the reaction represented above, which occurs at \(200^{\circ} \mathrm{C}\).
Which of the following happens to H atoms in the forward reaction?
(A) H atoms are oxidized only.
(B) H atoms are reduced only.
(C) H atoms are both oxidized and reduced.
(D) H atoms are neither oxidized nor reduced.
▶️Answer/Explanation
Ans:A
In the given reaction:
\[
\mathrm{Li}_3 \mathrm{~N}(s) + 2 \mathrm{H}_2(g) \rightleftarrows \mathrm{LiNH}_2(s) + 2 \mathrm{LiH}(s)
\]
The oxidation states of hydrogen in the reactants (\( \mathrm{H}_2(g) \)) and products (\( \mathrm{LiNH}_2(s) \) and \( \mathrm{LiH}(s) \)) are:
In \( \mathrm{H}_2(g) \), hydrogen is in its elemental form, so its oxidation state is \( 0 \).
In \( \mathrm{LiNH}_2(s) \), hydrogen has an oxidation state of \( +1 \).
In \( \mathrm{LiH}(s) \), hydrogen also has an oxidation state of \( +1 \).
In the forward reaction, hydrogen undergoes a change in oxidation state from \( 0 \) to \( +1 \). This indicates that hydrogen atoms are oxidized in the forward reaction.
Therefore, the correct answer is:(A) H atoms are oxidized only.
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