Q: A clean piece of iron weighing 3.752 grams is plated with chromium via electrolysis of an aqueous solution of chromium (VI), Cr$$^{6+}(aq). If 1.312 $$Amperes are used for 20.0 minutes, how many moles of electrons are transferred? (Faraday's constant $$= 96500 C/$$mol $$e^-)$$

$$\text{Moles} = \frac{I \cdot t}{F} = \frac{1.312 \times 1200}{96500} \approx 0.0163$$

Question 1

In the Claus process, sulfur is recovered from natural gas and crude oil by the reaction: $$2\mathrm{H}_2\mathrm{S} + \mathrm{O}_2 \rightarrow 2\mathrm{S} + 2\mathrm{H}_2\mathrm{O}. $$Which element is oxidized and which is reduced in this reaction?

Accepted Answer

Sulfur is oxidized, oxygen is reduced.

Question 2

Which of the following substances is the best oxidizing agent?

Accepted Answer

Cl$$_2(g)$$

Question 3

Consider the following redox reaction: $$\mathrm{MnO}_4^-(aq) + 5\mathrm{Fe}^{2+}(aq) + 8\mathrm{H}^+(aq) \rightarrow \mathrm{Mn}^{2+}(aq) + 5\mathrm{Fe}^{3+}(aq) + 4\mathrm{H}_2\mathrm{O}. $$Which species acts as the reducing agent?

Accepted Answer

Fe$$^{2+}$$

Question 4

Which of the following is NOT a redox reaction?

Accepted Answer

$$\mathrm{OH}^-(aq) + \mathrm{H}^+(aq) \rightarrow \mathrm{H}_2\mathrm{O}(l)$$

Question 5

A galvanic cell is constructed using the following half-reactions: $$\mathrm{Mg}^{2+}(aq) + 2e^- \rightarrow \mathrm{Mg}(s)$$ ($$E^\circ = -2.37 V) $$and $$\mathrm{Ag}^+(aq) + e^- \rightarrow \mathrm{Ag}(s)$$ ($$E^\circ = +1.99 V). $$What is the standard cell potential ($$E^\circ_{cell}$$)?

Accepted Answer

$$E^\circ_{cell} = 1.99 - (-2.37) = 4.36 V$$

Question 6

Which of the following changes will cause the voltage of a galvanic cell to decrease but remain above zero?

Accepted Answer

Adding more product ions to the cathode compartment.

Question 7

Which of the following statements about the relationship between $$\Delta G^\circ$$, $$K_{eq}$$, and $$E^\circ_{cell} is $$correct?

Accepted Answer

$$\Delta G^\circ = -nF E^\circ_{cell}$$

Question 8

A clean piece of iron weighing 3.752 grams is plated with chromium via electrolysis of an aqueous solution of chromium (VI), Cr$$^{6+}(aq). If 1.312 $$Amperes are used for 20.0 minutes, how many moles of electrons are transferred? (Faraday's constant $$= 96500 C/$$mol $$e^-)$$

Accepted Answer

$$	ext{Moles} = \frac{I \cdot t}{F} = \frac{1.312 	imes 1200}{96500} \approx 0.0163$$

Question 9

Which of the following statements is true for an electrolytic cell involving the half-reaction $$\mathrm{AlF}_6$$^{3-}$$ + 3e^- \rightarrow \mathrm{Al} + 6\mathrm{F}^-$$?

Accepted Answer

AlF$$_6$$^{3-}$$ is reduced at the cathode.

Question 10

Which of the following metals will be produced in the greatest mass during electrolysis of their molten salts, assuming the same current and time is applied?

Accepted Answer

Na from NaCl(ℓ)

Question 11

In a hydrogen-oxygen fuel cell, which half-reaction occurs at the cathode?

Accepted Answer

$$\mathrm{O}_2(g) + 4\mathrm{H}^+(aq) + 4e^- \rightarrow 2\mathrm{H}_2\mathrm{O}(l)$$

Question 12

A direct methanol fuel cell operates with the following half-reactions: $$\mathrm{CO}_2(g) + 6\mathrm{H}^+(aq) + 6e^- \rightarrow \mathrm{CH}_3\mathrm{OH}(aq) + \mathrm{H}_2\mathrm{O}(l)$$ ($$E^\circ = 0.02 V) $$and $$\mathrm{O}_2(g) + 4\mathrm{H}^+(aq) + 4e^- \rightarrow 2\mathrm{H}_2\mathrm{O}(l)$$ ($$E^\circ = 1.23 V). $$What is the overall standard cell potential?

Accepted Answer

$$E^\circ_{cell} = 1.23 - 0.02 = 1.21 V$$

Question 13

Which of the following will cause the voltage of a galvanic cell to become zero and remain at zero?

Accepted Answer

Replacing the salt bridge with a platinum wire.

Question 14

Which of the following is true for a spontaneous galvanic cell?

Accepted Answer

$$E^\circ_{cell} \u003e 0$$, $$\Delta G^\circ  1$$

Question 15

If the current is allowed to flow in a galvanic cell for several minutes, what will happen to the cell voltage?

Accepted Answer

Voltage decreases but remains above zero.

Question 16

Which of the following is a practical way to reduce the need for new aluminum production?

Accepted Answer

Recycling aluminum products.

Question 17

In the chloralkali process, which half-reaction occurs at the anode?

Accepted Answer

$$2\mathrm{Cl}^-(aq) \rightarrow \mathrm{Cl}_2(g) + 2e^-$$

Electrochemistry: Redox Reactions, Galvanic and Electrolytic Cells, and Applications

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