Redox Reactions & Oxidation Numbers practice questions

Question 1

A student places a strip of solid zinc, $\text{Zn}(s)$, into an aqueous solution of silver nitrate, $\text{AgNO}_3(aq)$. Solid silver crystals form on the surface of the zinc strip as zinc ions enter the solution according to the following balanced chemical equation: $$\text{Zn}(s) + 2\text{AgNO}_3(aq) \rightarrow \text{Zn(NO}_3)_2(aq) + 2\text{Ag}(s)$$ Which species acts as the reducing agent in this reaction?

Accepted Answer

$\text{Zn}(s)$

Answer

$\text{Ag}^+(aq)$

Answer

$\text{Ag}(s)$

Answer

$\text{NO}_3^-(aq)$

Question 2

A student constructs a galvanic cell operating under standard conditions using the two half-reactions shown below.

$\text{Ag}^+\text{(aq)} + e^- \rightarrow \text{Ag(s)} \quad E^\circ = +0.80 \text{ V}$
$\text{Zn}^{2+}\text{(aq)} + 2 e^- \rightarrow \text{Zn(s)} \quad E^\circ = -0.76 \text{ V}$

Which of the following balanced net-ionic equations represents the overall spontaneous reaction occurring in the cell?

Accepted Answer

$\text{Zn(s)} + 2\text{Ag}^+\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2\text{Ag(s)}$

Answer

$\text{Zn(s)} + \text{Ag}^+\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Ag(s)}$

Answer

2$\text{Ag(s)} + \text{Zn}^{2+}\text{(aq)} \rightarrow 2\text{Ag}^+\text{(aq)} + \text{Zn(s)}$

Answer

$\text{Ag(s)} + \text{Zn}^{2+}\text{(aq)} \rightarrow \text{Ag}^+\text{(aq)} + \text{Zn(s)}$

Question 3

A student uses concentrated nitric acid to dissolve copper from a metal sample. The net ionic equation for the reaction is shown below.

$\text{Cu(s)} + 4\text{H}^+\text{(aq)} + 2\text{NO}_3^-\text{(aq)} \rightarrow \text{Cu}^{2+}\text{(aq)} + 2\text{NO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$

Which species is the oxidizing agent in the reaction?

Accepted Answer

$\text{NO}_3^-\text{(aq)}$

Answer

$\text{Cu(s)}$

Answer

$\text{H}^+\text{(aq)}$

Answer

$\text{NO}_2\text{(g)}$

Question 4

A student evaluates zinc granules as a low-cost method for recovering copper from wastewater. The student places clean $\text{Zn(s)}$ in a solution containing $\text{Cu}^{2+}\text{(aq)}$. After several minutes, chemical analysis confirms that $\text{Cu(s)}$ has formed and $\text{Zn}^{2+}\text{(aq)}$ is present. The process is represented by the following equation.

$$
\text{Zn(s)}+\text{Cu}^{2+}\text{(aq)}\rightarrow\text{Zn}^{2+}\text{(aq)}+\text{Cu(s)}
$$

Which choice correctly states whether $\text{Zn(s)}$ displaces $\text{Cu}^{2+}\text{(aq)}$ and explains the electron-transfer reason?

Accepted Answer

It does. Zinc atoms are more readily oxidized than copper atoms, so electrons from $\text{Zn(s)}$ reduce $\text{Cu}^{2+}\text{(aq)}$ to $\text{Cu(s)}$.

Answer

It does not. The $\text{Cu}^{2+}\text{(aq)}$ ions have already lost electrons and therefore cannot act as an oxidizing agent.

Answer

It does not. Copper atoms are less readily oxidized than zinc atoms.

Answer

It does. The $\text{Zn(s)}$ atoms gain electrons to form $\text{Zn}^{2+}\text{(aq)}$, while $\text{Cu}^{2+}\text{(aq)}$ loses electrons to form $\text{Cu(s)}$.

Question 5

A student constructs the galvanic cell shown to model the recovery of silver ions from laboratory waste. Both solutions are at standard conditions and the salt bridge contains aqueous potassium nitrate. Relevant standard reduction potentials are given below.

| Reduction half-reaction | $E^\circ$ |
|---|---:|
| $\text{Ag}^+\text{(aq)}+e^-\rightarrow\text{Ag(s)}$ | $+0.80\text{ V}$ |
| $\text{Zn}^{2+}\text{(aq)}+2e^-\rightarrow\text{Zn(s)}$ | $-0.76\text{ V}$ |

Which choice correctly identifies the electrode reactions, direction of electron flow, and salt-bridge ion migration in this cell?

Accepted Answer

At the zinc electrode, $\text{Zn(s)}\rightarrow\text{Zn}^{2+}\text{(aq)}+2e^-$; at the silver electrode, $\text{Ag}^+\text{(aq)}+e^-\rightarrow\text{Ag(s)}$. Electrons flow from zinc to silver, while $\text{NO}_3^-$ migrates toward zinc and $\text{K}^+$ migrates toward silver.

Answer

At the zinc electrode, $\text{Zn(s)}\rightarrow\text{Zn}^{2+}\text{(aq)}+2e^-$; at the silver electrode, $\text{Ag}^+\text{(aq)}+e^-\rightarrow\text{Ag(s)}$. Electrons flow from zinc to silver, while $\text{K}^+$ migrates toward zinc and $\text{NO}_3^-$ migrates toward silver.

Answer

At the silver electrode, $2\text{Ag(s)}\rightarrow2\text{Ag}^+\text{(aq)}+2e^-$; at the zinc electrode, $\text{Zn}^{2+}\text{(aq)}+2e^-\rightarrow\text{Zn(s)}$. Electrons flow from silver to zinc, while $\text{NO}_3^-$ migrates toward silver and $\text{K}^+$ migrates toward zinc.

Answer

At the zinc electrode, $\text{Zn(s)}\rightarrow\text{Zn}^{2+}\text{(aq)}+2e^-$; at the silver electrode, $\text{Ag}^+\text{(aq)}+e^-\rightarrow\text{Ag(s)}$. Electrons flow from silver to zinc, while $\text{NO}_3^-$ migrates toward zinc and $\text{K}^+$ migrates toward silver.

Question 6

A student titrates an acidic solution of oxalic acid, $\text{H}_2\text{C}_2\text{O}_4\text{(aq)}$, with a standardized potassium permanganate solution, $\text{KMnO}_4\text{(aq)}$. The balanced net-ionic equation for the redox titration reaction is shown below:

$$2\text{MnO}_4^-\text{(aq)} + 5\text{H}_2\text{C}_2\text{O}_4\text{(aq)} + 6\text{H}^+\text{(aq)} \rightarrow 2\text{Mn}^{2+}\text{(aq)} + 10\text{CO}_2\text{(g)} + 8\text{H}_2\text{O(l)}$$

Which species acts as the reducing agent in this reaction, and what is the change in the oxidation number of the atom being oxidized?

Accepted Answer

$\text{H}_2\text{C}_2\text{O}_4\text{(aq)}$, in which the oxidation number of $\text{C}$ increases from $+3$ to $+4$

Answer

$\text{MnO}_4^-\text{(aq)}$, in which the oxidation number of $\text{Mn}$ decreases from $+7$ to $+2$

Answer

$\text{H}^+\text{(aq)}$, in which the oxidation number of $\text{H}$ increases from $+1$ to $+2$

Answer

$\text{H}_2\text{C}_2\text{O}_4\text{(aq)}$, in which the oxidation number of $\text{C}$ increases from $+2$ to $+4$

Question 7

A student investigates a treatment process for chromium-containing rinse water. The solution is acidified, and chromium(VI), present as $\text{Cr}_2\text{O}_7^{2-}\text{(aq)}$, is reduced to $\text{Cr}^{3+}\text{(aq)}$.

Which equation represents the correctly balanced reduction half-reaction for $\text{Cr}_2\text{O}_7^{2-}\text{(aq)}$ in acidic aqueous solution?

Accepted Answer

$\text{Cr}_2\text{O}_7^{2-}\text{(aq)} + 14\text{H}^+\text{(aq)} + 6e^- \rightarrow 2\text{Cr}^{3+}\text{(aq)} + 7\text{H}_2\text{O(l)}$

Answer

$\text{Cr}_2\text{O}_7^{2-}\text{(aq)} + 14\text{H}^+\text{(aq)} + 3e^- \rightarrow 2\text{Cr}^{3+}\text{(aq)} + 7\text{H}_2\text{O(l)}$

Answer

$\text{Cr}_2\text{O}_7^{2-}\text{(aq)} + 7\text{H}_2\text{O(l)} + 6e^- \rightarrow 2\text{Cr}^{3+}\text{(aq)} + 14\text{OH}^-\text{(aq)}$

Answer

$2\text{Cr}^{3+}\text{(aq)} + 7\text{H}_2\text{O(l)} \rightarrow \text{Cr}_2\text{O}_7^{2-}\text{(aq)} + 14\text{H}^+\text{(aq)} + 6e^-$

Question 8

A student models the recovery of copper from a battery-recycling solution by passing a constant current through an electrolytic cell. Copper is deposited at the cathode according to the following half-reaction.

$\text{Cu}^{2+}\text{(aq)}+2\text{e}^-\rightarrow\text{Cu(s)}$

The current is $1.93\text{ A}$ and is applied for $1.00\times10^3\text{ s}$. Assume that all the current is used to deposit copper. The Faraday constant is $9.65\times10^4\text{ C/mol }\text{e}^-$, and the molar mass of copper is $63.5\text{ g/mol}$. What mass of copper is deposited on the cathode?

Accepted Answer

$0.635\text{ g}$

Answer

$0.318\text{ g}$

Answer

$1.27\text{ g}$

Answer

$2.54\text{ g}$

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