College Chemistry Quiz: Galvanic And Electrolytic Cells

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Galvanic And Electrolytic CellsQuestion 1 of 20

Which of the following best explains why a salt bridge or porous membrane is essential in a galvanic cell?

Without it, the electrodes would physically touch and short-circuit the cell, preventing any voltage generation

Without it, charge buildup in each compartment would create an opposing electric field that stops current flow

Without it, the electrode materials would dissolve completely into solution, destroying the physical electrodes

Without it, the redox reactions would reverse spontaneously, making the cell discharge in the wrong direction

Without it, oxygen from air would interfere with the electrode reactions and prevent proper cell operation

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College Chemistry Quiz

College Chemistry Quiz: Galvanic And Electrolytic Cells

Practice Galvanic And Electrolytic Cells in College Chemistry with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Galvanic And Electrolytic Cells, giving you a quick way to practice the rules, question types, and explanations that matter most for College Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Which of the following best explains why a salt bridge or porous membrane is essential in a galvanic cell?

Without it, the electrodes would physically touch and short-circuit the cell, preventing any voltage generation
Without it, charge buildup in each compartment would create an opposing electric field that stops current flow (correct answer)
Without it, the electrode materials would dissolve completely into solution, destroying the physical electrodes
Without it, the redox reactions would reverse spontaneously, making the cell discharge in the wrong direction
Without it, oxygen from air would interfere with the electrode reactions and prevent proper cell operation

Explanation: Galvanic cell questions test your understanding of how electrochemical cells maintain electrical neutrality while allowing current to flow. The key insight is that any functioning electrical circuit must have a complete path for charge movement. In a galvanic cell, oxidation occurs at the anode (producing electrons and positive ions) while reduction occurs at the cathode (consuming electrons and often producing negative ions or consuming positive ions). Without a salt bridge, the anode compartment would accumulate positive charge while the cathode compartment would accumulate negative charge. This charge separation creates an electric field that opposes further electron flow through the external circuit. The salt bridge allows ions to migrate between compartments—anions move toward the anode and cations toward the cathode—maintaining electrical neutrality and allowing continuous current flow. This makes choice B correct. Choice A incorrectly assumes the electrodes would physically touch, but galvanic cells are specifically designed with separate compartments to prevent direct contact. Choice C misunderstands the role of the salt bridge—while some electrode dissolution may occur, this isn't the primary reason salt bridges are essential, and many electrodes remain stable during operation. Choice D confuses thermodynamics with kinetics—the spontaneity of redox reactions depends on the standard potentials, not the presence of a salt bridge. Remember: whenever you see galvanic cell questions, think about charge balance. The salt bridge's job is maintaining electrical neutrality in both compartments, not preventing physical contact or controlling reaction direction.

Question 2

In an electrolytic cell used to plate silver from $AgNO_3$ solution, a current of 2.5 A is passed for 30 minutes. How many grams of silver will be deposited on the cathode? ( $F = 96,485 C/mol$ , atomic mass of Ag = 107.87 g/mol)

2.52 g
5.04 g (correct answer)
10.1 g
15.2 g
20.2 g

Explanation: When you encounter electroplating problems, you're dealing with Faraday's laws of electrolysis, which connect electrical current to the amount of substance deposited. The key relationship is: moles of electrons = charge (C) ÷ Faraday constant. First, calculate the total charge passed:

Q = I \times t = 2.5 \text{ A} \times (30 \times 60) \text{ s} = 4500 \text{ C}

Next, find moles of electrons:

\frac{4500 \text{ C}}{96,485 \text{ C/mol}} = 0.0466 \text{ mol e}^-

The reduction reaction at the cathode is:

Ag^+ + e^- \rightarrow Ag

. This shows a 1:1 ratio between electrons and silver atoms, so 0.0466 mol of electrons produces 0.0466 mol of silver. Finally, convert to grams:

0.0466 \text{ mol} \times 107.87 \text{ g/mol} = 5.03 \text{ g}

This matches answer (B) 5.04 g. (A) 2.52 g represents exactly half the correct answer, likely from mistakenly thinking the silver ion has a +2 charge instead of +1. (C) 10.1 g is roughly double the correct answer, possibly from an error in time conversion or using the wrong electron-to-silver ratio. (D) 15.2 g is about three times too large, suggesting multiple calculation errors or misunderstanding the stoichiometry. Study tip: Always write the reduction reaction first to determine the electron-to-metal ratio. Remember that time must be in seconds when using amperes, and double-check the charge on your metal ion.

Question 3

A student constructs an electrochemical cell using the half-reactions: $Ni^{2+} + 2e^- → Ni$ ( $E° = -0.25 V$ ) and $Ag^+ + e^- → Ag$ ( $E° = +0.80 V$ ). If the $[Ni^{2+}] = 0.10 M$ and $[Ag^+] = 2.0 M$ , what is the cell potential at 25°C? ( $R = 8.314 J/mol·K$ , $F = 96,485 C/mol$ )

0.96 V
1.05 V
1.14 V (correct answer)
1.23 V
1.32 V

Explanation: When you encounter electrochemical cell problems with non-standard conditions, you need to use the Nernst equation to account for concentration effects on cell potential. First, determine the standard cell potential (

E°_{cell}

). Since the silver half-reaction has a higher reduction potential (+0.80 V vs -0.25 V), silver will be reduced at the cathode while nickel is oxidized at the anode. The overall reaction is:

Ni + 2Ag^+ → Ni^{2+} + 2Ag

, giving

E°_{cell} = 0.80 - (-0.25) = 1.05 V

. Next, apply the Nernst equation:

E_{cell} = E°_{cell} - \frac{RT}{nF}\ln Q

, where

n = 2

electrons and

Q = \frac{[Ni^{2+}]}{[Ag^+]^2} = \frac{0.10}{(2.0)^2} = 0.025

. Calculating:

E_{cell} = 1.05 - \frac{(8.314)(298)}{(2)(96,485)}\ln(0.025) = 1.05 - (0.01284)(-3.689) = 1.05 + 0.047 = 1.14 V

Answer A (0.96 V) results from incorrectly subtracting the Nernst correction term, suggesting a misunderstanding of how concentration affects cell potential. Answer B (1.05 V) is simply the standard cell potential, ignoring concentration effects entirely. Answer D (1.23 V) likely comes from calculation errors in the natural logarithm or improper unit conversions. Remember: when product concentrations are low relative to reactants (Q < 1), the actual cell potential will be higher than the standard potential, making the reaction more favorable than under standard conditions.

Question 4

An electrolytic cell contains molten $NaCl$ . When a voltage is applied, chlorine gas is produced at one electrode and sodium metal at the other. Which statement correctly describes this process?

Sodium is reduced at the anode, chlorine is oxidized at the cathode, and the process requires an external voltage source
Chlorine is oxidized at the anode, sodium is reduced at the cathode, and the process requires an external voltage source (correct answer)
Sodium is oxidized at the cathode, chlorine is reduced at the anode, and the process occurs spontaneously
Chlorine is reduced at the cathode, sodium is oxidized at the anode, and the process occurs spontaneously
Both sodium and chlorine undergo oxidation simultaneously at separate electrodes with no net change in oxidation states

Explanation: Electrolytic cells use electrical energy to drive non-spontaneous chemical reactions, which is the key to understanding this molten

NaCl

setup. When you encounter electrolysis problems, remember that an external voltage forces reactions that wouldn't happen naturally. In this cell, the applied voltage breaks down

NaCl

into its elements. At the anode (positive electrode), chloride ions (

Cl^-

) lose electrons to form chlorine gas:

2Cl^- \rightarrow Cl_2 + 2e^-

. This is oxidation (loss of electrons). At the cathode (negative electrode), sodium ions (

Na^+

) gain electrons to form sodium metal:

Na^+ + e^- \rightarrow Na

. This is reduction (gain of electrons). The process requires continuous external voltage because it's energetically unfavorable. Option A incorrectly places sodium reduction at the anode and chlorine oxidation at the cathode, which reverses the electrode identities. Remember: oxidation always occurs at the anode, reduction at the cathode. Option C switches the electrode assignments entirely, claiming sodium oxidation at the cathode and chlorine reduction at the anode. It also incorrectly states the process is spontaneous. Option D correctly identifies that processes can be spontaneous, but this is wrong for electrolysis. Also, it suggests sodium is being oxidized (losing electrons to form

Na^+

), which is backwards since we're converting

Na^+

Na

metal. Study tip: For electrolysis questions, memorize "An Ox, Red Cat" (Anode = Oxidation, Reduction = Cathode) and remember that electrolytic cells always require external energy to drive non-spontaneous reactions.

Question 5

A galvanic cell operates with the reaction: $Zn(s) + Cu^{2+}(aq) → Zn^{2+}(aq) + Cu(s)$ . As the cell discharges, which of the following changes will occur?

The mass of the zinc electrode increases while the copper electrode mass decreases, and $[Cu^{2+}]$ increases
The mass of the zinc electrode decreases while the copper electrode mass increases, and $[Zn^{2+}]$ increases (correct answer)
Both electrode masses remain constant, but $[Cu^{2+}]$ increases and $[Zn^{2+}]$ decreases
The mass of both electrodes increases as metal ions are reduced from solution onto both surfaces
No mass changes occur at either electrode, but the total ion concentration in both compartments increases

Explanation: When analyzing galvanic cell reactions, you need to track what happens at each electrode as electrons flow and the reaction proceeds forward. In this cell, zinc is being oxidized (

Zn(s) → Zn^{2+}(aq) + 2e^-

) at the anode, while copper ions are being reduced (

Cu^{2+}(aq) + 2e^- → Cu(s)

) at the cathode. As the cell discharges, solid zinc metal dissolves into solution as

Zn^{2+}

ions, decreasing the zinc electrode's mass. Simultaneously,

Cu^{2+}

ions from solution gain electrons and deposit as solid copper metal, increasing the copper electrode's mass. The concentration of

Zn^{2+}

increases as more zinc dissolves, while

Cu^{2+}

concentration decreases as these ions are consumed. Answer A incorrectly reverses the mass changes and suggests

[Cu^{2+}]

increases, which contradicts the forward reaction where copper ions are being consumed. Answer C incorrectly states that electrode masses remain constant—this would only be true if no net reaction occurred. Answer D wrongly suggests both electrodes gain mass, which would require reduction at both electrodes simultaneously, violating the principle that galvanic cells have distinct anode and cathode reactions. Answer B correctly identifies that zinc electrode mass decreases (oxidation), copper electrode mass increases (reduction), and

[Zn^{2+}]

increases as the product forms. Study tip: Always trace electron flow in galvanic cells—mass decreases where oxidation occurs (electrons leave) and increases where reduction occurs (electrons arrive).

Question 6

In an electrolytic cell used for copper refining, impure copper serves as the anode and pure copper as the cathode in $CuSO_4$ solution. If 5.0 A of current is passed for 2.0 hours, what mass of copper will be transferred from anode to cathode? (Atomic mass of Cu = 63.55 g/mol, $F = 96,485 C/mol$ )

5.9 g
11.8 g (correct answer)
23.6 g
35.4 g
47.2 g

Explanation: When you encounter electrolysis problems, you're dealing with the quantitative relationship between electrical current and chemical change. The key is using Faraday's laws to connect coulombs of charge to moles of substance transferred. In copper refining,

Cu^{2+}

ions are reduced at the cathode (

Cu^{2+} + 2e^- \rightarrow Cu

), meaning each copper atom requires 2 electrons. Start by calculating total charge:

Q = I \times t = 5.0 \text{ A} \times 2.0 \text{ h} \times 3600 \text{ s/h} = 36,000 \text{ C}

Next, find moles of electrons:

\frac{36,000 \text{ C}}{96,485 \text{ C/mol}} = 0.373 \text{ mol } e^-

Since copper reduction requires 2 electrons per atom, moles of copper =

\frac{0.373 \text{ mol } e^-}{2} = 0.186 \text{ mol Cu}

Finally, convert to mass:

0.186 \text{ mol} \times 63.55 \text{ g/mol} = 11.8 \text{ g}

Answer A (5.9 g) results from forgetting to multiply current by time in seconds, using only 2 hours as the time value. Answer C (23.6 g) comes from assuming copper requires only 1 electron instead of 2, doubling the actual answer. Answer D (35.4 g) represents calculating for 3 times the correct amount, possibly from unit conversion errors. Remember: always identify the balanced half-reaction first to determine electron stoichiometry, convert time to seconds when working with amperes, and use Faraday's constant to bridge electrical and chemical quantities.

Question 7

A student observes that when a galvanic cell operates for an extended period, the voltage gradually decreases even though the external circuit remains unchanged. Which of the following best explains this observation?

The electrodes are physically deteriorating due to corrosion, increasing the internal resistance of the cell
The salt bridge is becoming saturated with ions, blocking further ion migration between compartments
The concentration gradient between the half-cells is decreasing as the cell approaches equilibrium (correct answer)
The temperature of the cell is rising due to current flow, which always decreases cell voltage
Impurities in the electrode materials are being depleted, reducing the efficiency of the redox reactions

Explanation: When analyzing galvanic cell behavior over time, focus on the fundamental driving force: the concentration difference between half-cells that creates the cell potential according to the Nernst equation. As a galvanic cell operates, electrons flow from anode to cathode through the external circuit while ions migrate through the salt bridge to maintain charge neutrality. This process gradually changes the ion concentrations in each half-cell. At the anode, metal atoms are oxidized and enter solution as cations, increasing that compartment's ion concentration. At the cathode, cations are reduced and deposited as metal, decreasing the ion concentration. Over time, these concentration changes reduce the driving force for the reaction. The Nernst equation shows that cell potential depends on the concentration ratio:

E = E° - \frac{RT}{nF}\ln Q

. As the reaction quotient Q approaches the equilibrium constant, the cell potential approaches zero. This explains why choice C correctly identifies the decreasing concentration gradient as the cause of voltage decline. Choice A incorrectly assumes electrode corrosion, but galvanic cells typically involve controlled oxidation-reduction, not destructive corrosion. Choice B misunderstands salt bridge function—it doesn't become "saturated" and block ion flow under normal operating conditions. Choice D makes a false generalization about temperature effects; while temperature can affect cell voltage, it doesn't always decrease it, and heating from current flow is typically minimal in galvanic cells. Remember: galvanic cell voltage decreases over time because the system approaches chemical equilibrium, reducing the concentration-driven potential difference between half-cells.

Question 8

A fuel cell operates by combining hydrogen and oxygen gases to produce water, generating electrical energy. Which statement correctly describes the electrode processes in this cell?

Hydrogen is reduced at the cathode and oxygen is oxidized at the anode, with water formation occurring in the external circuit
Hydrogen is oxidized at the anode and oxygen is reduced at the cathode, with water formation occurring at the cathode (correct answer)
Both hydrogen and oxygen are oxidized at separate anodes, while water is reduced at a third electrode
Hydrogen and oxygen combine directly at a single electrode to form water without separate oxidation and reduction steps
Water is decomposed at the anode to form hydrogen and oxygen, which then recombine at the cathode

Explanation: When analyzing fuel cell operations, focus on identifying the separate oxidation and reduction reactions occurring at different electrodes, just like in any electrochemical cell. In a hydrogen fuel cell, two distinct half-reactions occur simultaneously. At the anode, hydrogen gas undergoes oxidation:

\text{H}_2 \rightarrow 2\text{H}^+ + 2e^-

. This releases electrons that flow through the external circuit, generating electrical current. At the cathode, oxygen gas undergoes reduction by accepting these electrons:

\text{O}_2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\text{O}

. The water product forms at the cathode where oxygen, protons, and electrons combine. Option A incorrectly reverses the electrode processes—hydrogen is oxidized (loses electrons), not reduced, while oxygen is reduced (gains electrons), not oxidized. It also wrongly places water formation in the external circuit rather than at an electrode. Option C describes an impossible scenario with multiple anodes and suggests water undergoes reduction, which contradicts the fundamental fuel cell reaction where water is a product, not a reactant being reduced. Option D misrepresents the electrochemical nature of fuel cells by suggesting direct combination without separate redox processes. Fuel cells specifically rely on spatially separated oxidation and reduction reactions to generate electrical current. Remember the mnemonic "An Ox, Red Cat"—oxidation occurs at the anode, reduction at the cathode. For fuel cells, always identify which reactant loses electrons (gets oxidized) and which gains electrons (gets reduced), then match them to the correct electrodes.

Question 9

During the electrolysis of molten $MgCl_2$ , what mass of magnesium metal can be produced by passing 10.0 A of current for 1.5 hours? (Atomic mass of Mg = 24.31 g/mol, $F = 96,485 C/mol$ )

6.8 g (correct answer)
13.6 g
20.4 g
27.2 g
40.8 g

Explanation: When you encounter electrolysis problems, you're dealing with the relationship between electrical current, time, and the amount of substance produced. The key is understanding that current measures the flow of charge, and specific amounts of charge are needed to produce each mole of product. To solve this, you need to connect current and time to moles of electrons, then to moles of product. First, calculate the total charge:

Q = I \times t = 10.0 \text{ A} \times (1.5 \times 3600 \text{ s}) = 54,000 \text{ C}

Next, convert charge to moles of electrons:

\frac{54,000 \text{ C}}{96,485 \text{ C/mol}} = 0.560 \text{ mol e}^-

The crucial step is understanding the balanced half-reaction. For magnesium production:

\text{Mg}^{2+} + 2\text{e}^- \rightarrow \text{Mg}

. This tells you that 2 moles of electrons produce 1 mole of magnesium. Therefore:

\text{mol Mg} = \frac{0.560 \text{ mol e}^-}{2} = 0.280 \text{ mol Mg}

Finally:

\text{mass} = 0.280 \text{ mol} \times 24.31 \text{ g/mol} = 6.8 \text{ g}

Answer A (6.8 g) is correct. Answer B (13.6 g) likely assumes only 1 electron per magnesium atom instead of 2. Answer C (20.4 g) might involve calculation errors or wrong conversion factors. Answer D (27.2 g) could result from using 1 electron per magnesium and additional computational mistakes. Remember: always identify the balanced half-reaction first to determine the electron-to-product ratio—this is where most electrolysis errors occur.

Question 10

An electrolytic cell is used to electroplate nickel onto a steel object. If the efficiency of the plating process is 85% and 3.0 A of current is passed for 45 minutes, what mass of nickel will actually be deposited? ( $Ni^{2+} + 2e^- → Ni$ , atomic mass of Ni = 58.69 g/mol, $F = 96,485 C/mol$ )

1.5 g
1.8 g (correct answer)
2.1 g
2.5 g
2.9 g

Explanation: Electroplating problems combine electrochemistry with stoichiometry, requiring you to connect current, time, and moles through Faraday's law. The key insight is that efficiency reduces the actual amount deposited below the theoretical maximum. Start by calculating the total charge:

Q = I \times t = 3.0 \text{ A} \times (45 \times 60) \text{ s} = 8,100 \text{ C}

. Next, find moles of electrons transferred:

\text{mol e}^- = \frac{8,100 \text{ C}}{96,485 \text{ C/mol}} = 0.0839 \text{ mol}

. Since the reaction shows

Ni^{2+} + 2e^- → Ni

, you need 2 moles of electrons per mole of nickel, so:

\text{mol Ni} = \frac{0.0839}{2} = 0.0420 \text{ mol}

. The theoretical mass would be

0.0420 \text{ mol} \times 58.69 \text{ g/mol} = 2.46 \text{ g}

. However, the process is only 85% efficient, so the actual mass deposited is

2.46 \text{ g} \times 0.85 = 2.1 \text{ g}

. Choice A (1.5 g) likely results from calculation errors or using wrong conversion factors. Choice C (2.1 g) appears to round incorrectly from our calculation of approximately 2.09 g. Choice D (2.5 g) represents the theoretical maximum without accounting for efficiency. The correct answer is B (1.8 g), which matches our calculated value when properly rounded. For electroplating problems, always follow this sequence: calculate charge → find moles of electrons → apply stoichiometry → calculate theoretical mass → apply efficiency. Don't forget that efficiency always reduces the final answer below the theoretical maximum.

Question 11

Which of the following factors will always increase the voltage output of a galvanic cell compared to standard conditions?

Increasing the temperature of the cell from 25°C to 50°C
Increasing the concentration of reactants while decreasing the concentration of products (correct answer)
Increasing the surface area of both electrodes by a factor of 10
Decreasing the distance between the electrodes to reduce internal resistance
Adding an inert salt to both compartments to increase ionic strength

Explanation: When analyzing galvanic cell voltage, you need to understand how the Nernst equation governs cell potential under non-standard conditions. The cell voltage depends on both the standard cell potential and the reaction quotient (Q), which reflects the concentrations of reactants and products. Option B is correct because increasing reactant concentrations while decreasing product concentrations reduces the reaction quotient Q in the Nernst equation:

E_{cell} = E°_{cell} - \frac{RT}{nF}\ln Q

. Since Q appears with a negative sign, decreasing Q increases the cell voltage above standard conditions. This makes thermodynamic sense—the cell has more "driving force" when reactants are abundant and products are scarce. Option A is wrong because temperature effects on voltage are unpredictable without knowing the entropy change (ΔS) of the reaction. Higher temperature can either increase or decrease voltage depending on whether ΔS is positive or negative. Option C is incorrect because electrode surface area affects reaction rate and current capacity, but not the thermodynamic voltage of the cell. Voltage is an intensive property independent of electrode size. Option D represents a common misconception. While reducing electrode distance decreases internal resistance and improves power delivery, it doesn't change the fundamental electrochemical potential difference between the half-cells, which determines voltage. Study tip: Remember that galvanic cell voltage is determined by thermodynamics (concentrations, temperature effects on equilibrium), not by physical cell design factors like electrode size or spacing. Focus on how concentration changes affect the reaction quotient Q.

Question 12

An electrolytic cell contains aqueous $AgNO_3$ solution with silver electrodes. When current flows, one electrode gains mass while the other loses mass, but the $[Ag^+]$ in solution remains essentially constant. Which statement best explains this observation?

Silver is being oxidized at both electrodes simultaneously, maintaining constant $[Ag^+]$ through competing reactions
Silver is being reduced at both electrodes, with $Ag^+$ being regenerated by water oxidation
Silver is oxidized at the anode and reduced at the cathode in equal amounts, maintaining constant $[Ag^+]$ (correct answer)
The $AgNO_3$ is undergoing electrolytic decomposition to produce nitrogen oxides rather than affecting silver concentration
Impurities in the silver electrodes are dissolving preferentially, while pure silver concentration remains unchanged

Explanation: When you encounter electrolytic cell problems, focus on identifying what happens at each electrode and how these processes affect solution composition. In electrolysis, oxidation occurs at the anode (electrons lost) while reduction occurs at the cathode (electrons gained). In this silver electrolytic cell, the key observation is that one electrode gains mass while the other loses mass, yet

[Ag^+]

remains constant. This tells you that silver metal is being dissolved from one electrode while being deposited on the other in equal amounts. At the anode, silver metal undergoes oxidation:

Ag(s) \rightarrow Ag^+(aq) + e^-

. This electrode loses mass as silver dissolves, adding

Ag^+

ions to solution. Simultaneously, at the cathode, reduction occurs:

Ag^+(aq) + e^- \rightarrow Ag(s)

. This electrode gains mass as

Ag^+

ions are removed from solution and deposited as metal. Since these processes consume and produce

Ag^+

at equal rates, the concentration remains constant. This makes answer C correct. Answer A is wrong because silver cannot be oxidized at both electrodes—one must be the anode (oxidation) and one the cathode (reduction). Answer B incorrectly suggests reduction at both electrodes, which violates basic electrolysis principles. Answer D is incorrect because

AgNO_3

decomposition would change

[Ag^+]

, contradicting the observation. Remember: in electrolytic cells with metal electrodes of the same type as the cation in solution, the metal electrode itself participates in the redox reactions, creating a "refining" effect where metal transfers from anode to cathode.

Question 13

A galvanic cell consists of a standard hydrogen electrode (SHE) connected to an unknown metal electrode. The cell generates 0.85 V with the unknown electrode serving as the cathode. What is the standard reduction potential of the unknown electrode?

-0.85 V
0.00 V
+0.42 V
+0.85 V (correct answer)
+1.70 V

Explanation: Galvanic cells test your understanding of how electrochemical potentials combine to produce cell voltage. When you see a problem involving a standard hydrogen electrode (SHE) and an unknown electrode, remember that the SHE serves as your reference point with a defined potential of 0.00 V. The key insight is that cell voltage equals the difference between cathode and anode potentials:

E_{cell} = E_{cathode} - E_{anode}

. Since the unknown electrode is the cathode and generates 0.85 V, you can substitute:

0.85 \text{ V} = E_{unknown} - E_{SHE}

. Because the SHE has a standard reduction potential of 0.00 V, this becomes:

0.85 \text{ V} = E_{unknown} - 0.00 \text{ V}

. Therefore, the unknown electrode's standard reduction potential is +0.85 V. Choice A (-0.85 V) represents a common sign error—you might get this if you mistakenly treated the unknown as the anode instead of the cathode. Choice B (0.00 V) would be the SHE's potential, not the unknown electrode's. Choice C (+0.42 V) might result from incorrectly dividing the cell voltage by two, perhaps confusing this with concentration cell calculations. The correct answer is D (+0.85 V). Remember this pattern: when one electrode is the SHE (0.00 V), the cell voltage directly equals the other electrode's potential if that electrode is the cathode, or equals the negative of that potential if it's the anode. Always identify which electrode is which before applying the cell voltage equation.

Question 14

Which of the following statements about the salt bridge in a galvanic cell is most accurate?

It provides a pathway for electrons to flow between the half-cells while preventing mixing of solutions
It allows ions to migrate between compartments to maintain electrical neutrality while preventing bulk solution mixing (correct answer)
It serves as a reservoir of additional electrolyte to replace ions consumed during the cell reaction
It acts as a resistor to control the rate of electron flow and prevent the cell from discharging too rapidly
It provides a direct electrical connection between the electrodes to ensure maximum current flow through the cell

Explanation: When you encounter questions about galvanic cells, focus on understanding that these electrochemical devices must maintain both electrical neutrality and physical separation of solutions to function properly. The salt bridge serves a crucial dual purpose in galvanic cells. As electrons flow through the external circuit from anode to cathode, the half-cells develop charge imbalances - the anode compartment becomes positively charged (losing electrons) while the cathode compartment becomes negatively charged (gaining electrons). The salt bridge allows ions to migrate between compartments to neutralize these charges: anions move toward the anode and cations toward the cathode. Simultaneously, the salt bridge prevents the two solutions from mixing completely, which would eliminate the potential difference needed for the cell to operate. This makes option B correct. Option A contains a fundamental error - electrons flow through the external wire, not the salt bridge. The salt bridge is specifically for ion movement. Option C misrepresents the salt bridge's function; while it does contain electrolyte, its primary role isn't as a reservoir but as an ion conductor. The salt bridge doesn't replace consumed ions from the electrode reactions. Option D incorrectly describes the salt bridge as a resistor controlling electron flow rate. While salt bridges do have some resistance, this isn't their primary function, and they don't control electron flow - that's determined by the external circuit. Remember: salt bridges handle ions and charge balance, while the external circuit handles electrons and current flow. Keep these pathways distinct in your mind.

Question 15

An electrolytic cell contains aqueous $NaCl$ solution. When sufficient voltage is applied, the products formed are $H_2$ gas at one electrode and $Cl_2$ gas at the other, with $OH^-$ ions accumulating in solution. Which statement correctly explains this observation?

Water reduction occurs preferentially at the cathode over $Na^+$ reduction, while $Cl^-$ oxidation occurs at the anode (correct answer)
Sodium reduction occurs at the cathode while water oxidation occurs preferentially at the anode over $Cl^-$ oxidation
Both $Na^+$ and $H_2O$ are reduced simultaneously at the cathode, producing both $Na$ metal and $H_2$ gas
Chloride oxidation and water reduction both occur at the same electrode, producing a mixture of gases
The $NaCl$ undergoes thermal decomposition rather than electrolysis, explaining the unexpected products

Explanation: When analyzing electrolytic cells, you need to identify what gets reduced at the cathode (negative electrode) and what gets oxidized at the anode (positive electrode) by comparing standard reduction potentials and considering kinetic factors. In aqueous

NaCl

, four species could potentially react:

Na^+

Cl^-

H_2O

, and

OH^-

. At the cathode, you have two reduction possibilities:

Na^+ + e^- \rightarrow Na

(E° = -2.71 V) or

2H_2O + 2e^- \rightarrow H_2 + 2OH^-

(E° = -0.83 V). Water reduction occurs preferentially because it has a much higher (less negative) reduction potential than sodium reduction. At the anode,

Cl^-

gets oxidized to

Cl_2

gas rather than water being oxidized to oxygen, due to kinetic factors and the relatively high concentration of chloride ions. Choice A correctly identifies that water reduction produces

H_2

at the cathode while

Cl^-

oxidation produces

Cl_2

at the anode. The

OH^-

ions come from the water reduction reaction. Choice B incorrectly suggests sodium metal forms, which won't happen under these conditions due to the unfavorable thermodynamics. Choice C falsely claims simultaneous reduction reactions producing sodium metal, which contradicts the observation that only

H_2

gas forms. Choice D incorrectly places both reactions at the same electrode, violating the fundamental principle that reduction and oxidation must occur at opposite electrodes. Remember: In electrolysis problems, always compare reduction potentials to predict which species will actually react, and consider that kinetic factors can sometimes override thermodynamic predictions.

Question 16

A concentration cell is constructed using two silver electrodes: one in 0.010 M $AgNO_3$ and another in 1.0 M $AgNO_3$ . What is the cell potential at 25°C? ( $R = 8.314 J/mol·K$ , $F = 96,485 C/mol$ )

0.000 V
0.059 V
0.118 V (correct answer)
0.177 V
0.236 V

Explanation: A concentration cell involves identical electrodes in solutions of different concentrations, generating potential from the concentration difference alone. Since both electrodes are silver in

AgNO_3

solutions, the standard cell potential is zero, but the Nernst equation accounts for concentration differences. For this concentration cell, the half-reactions are:

Cathode (higher concentration): $Ag^+ + e^- \rightarrow Ag$
Anode (lower concentration): $Ag \rightarrow Ag^+ + e^-$

Using the Nernst equation:

E_{cell} = E°_{cell} - \frac{RT}{nF}\ln\frac{[Ag^+]_{anode}}{[Ag^+]_{cathode}}

Since

E°_{cell} = 0

for identical electrodes and

n = 1

electron:

E_{cell} = -\frac{(8.314)(298)}{(1)(96,485)}\ln\frac{0.010}{1.0}

E_{cell} = -0.0257\ln(0.010) = -0.0257(-4.605) = 0.118 \text{ V}

Answer A (0.000 V) incorrectly assumes no potential difference exists, ignoring the concentration gradient. Answer B (0.059 V) likely comes from using the simplified form

\frac{0.059}{n}\log\frac{[cathode]}{[anode]}

but making an error in the logarithm calculation or using natural log instead of log base 10. Answer D (0.177 V) represents a calculation error, possibly using incorrect values or mathematical mistakes. The correct answer is C (0.118 V). Study tip: For concentration cells, remember that current flows from low to high concentration, and always use the Nernst equation with careful attention to which concentration goes in the numerator versus denominator of the ratio.

Question 17

In the electrolysis of water using inert electrodes, 560 mL of $H_2$ gas is collected at the cathode under STP conditions. What volume of $O_2$ gas is simultaneously produced at the anode under the same conditions?

140 mL
280 mL (correct answer)
560 mL
1120 mL
2240 mL

Explanation: When you encounter electrolysis problems, focus on the balanced chemical equation and stoichiometric relationships between products. The electrolysis of water follows this reaction:

2H_2O(l) \rightarrow 2H_2(g) + O_2(g)

At the cathode, water molecules gain electrons to produce hydrogen gas, while at the anode, water molecules lose electrons to form oxygen gas. The key insight is the molar ratio from the balanced equation: for every 2 moles of

H_2

produced, 1 mole of

O_2

is generated. Since equal moles of gases occupy equal volumes under identical conditions (Avogadro's law), this 2:1 molar ratio translates directly to a 2:1 volume ratio. If 560 mL of

H_2

is collected, then the volume of

O_2

produced is

560 \text{ mL} \times \frac{1}{2} = 280 \text{ mL}

. Choice A (140 mL) represents a 4:1 ratio, suggesting confusion about the stoichiometry or perhaps mixing up coefficients. Choice C (560 mL) assumes a 1:1 ratio, ignoring the balanced equation entirely. Choice D (1120 mL) inverts the relationship, treating oxygen as if twice as much is produced as hydrogen. The correct answer is B (280 mL). Study tip: For any electrolysis problem, always write the balanced equation first, then use the coefficients to establish molar (and volume) ratios between products. The stoichiometry governs everything in these calculations.

Question 18

During the electrolysis of brine (concentrated $NaCl$ solution), chlorine gas is produced at the anode. If 2.24 L of $Cl_2$ gas is collected at STP, how much electrical charge was passed through the cell? ( $F = 96,485 C/mol$ )

9,650 C
19,300 C (correct answer)
38,600 C
77,200 C
154,400 C

Explanation: When you encounter electrolysis problems, you're dealing with the relationship between electrical charge and the amount of substance produced. The key is connecting moles of gas to electrons transferred using Faraday's constant. Start by finding moles of

Cl_2

produced. At STP, one mole of any gas occupies 22.4 L, so:

\frac{2.24 \text{ L}}{22.4 \text{ L/mol}} = 0.100 \text{ mol } Cl_2

Next, determine electrons needed. The half-reaction at the anode is:

2Cl^- \rightarrow Cl_2 + 2e^-

. This shows that producing 1 mole of

Cl_2

requires 2 moles of electrons. Therefore:

0.100 \text{ mol } Cl_2 \times \frac{2 \text{ mol } e^-}{1 \text{ mol } Cl_2} = 0.200 \text{ mol } e^-

Finally, convert to charge using Faraday's constant:

0.200 \text{ mol } e^- \times 96,485 \text{ C/mol} = 19,297 \text{ C} \approx 19,300 \text{ C}

Answer A (9,650 C) represents using only 1 electron per

Cl_2

molecule instead of 2 - a common mistake when writing the half-reaction incorrectly. Answer C (38,600 C) results from using 4 electrons per

Cl_2

, possibly confusing this with reactions involving

ClO_4^-

or similar species. Answer D (77,200 C) comes from using 8 electrons per

Cl_2

, an unrealistic stoichiometry. The correct answer is B (19,300 C). Study tip: Always write the balanced half-reaction first in electrolysis problems - the electron stoichiometry is crucial for getting the right charge calculation.

Question 19

A galvanic cell has $E°_{cell} = +1.50 V$ under standard conditions. If the cell is operated under conditions where $Q = 100$ (where Q is the reaction quotient), what will be the approximate cell potential at 25°C for a 2-electron transfer reaction?

1.44 V (correct answer)
1.50 V
1.56 V
1.62 V
1.68 V

Explanation: When you encounter a galvanic cell problem with non-standard conditions, you need the Nernst equation to relate the actual cell potential to the standard potential. The Nernst equation is:

E_{cell} = E°_{cell} - \frac{RT}{nF}\ln Q

, where R is the gas constant, T is temperature, n is electrons transferred, F is Faraday's constant, and Q is the reaction quotient. At 25°C, the term

\frac{RT}{F}

equals 0.0257 V, so the equation becomes:

E_{cell} = E°_{cell} - \frac{0.0257}{n}\ln Q

. With

E°_{cell} = 1.50

n = 2

, and

Q = 100

E_{cell} = 1.50 - \frac{0.0257}{2}\ln(100) = 1.50 - 0.01285 × 4.605 = 1.50 - 0.059 = 1.44

V This confirms answer (A) 1.44 V is correct. (B) 1.50 V assumes standard conditions persist, ignoring that

Q ≠ 1

changes the cell potential. (C) 1.56 V and (D) 1.62 V both suggest the cell potential increases when

Q > 1

, but this is backwards—when the reaction quotient exceeds 1, the products are favored over reactants, reducing the driving force and lowering the cell potential below its standard value. Remember:

Q > 1

always decreases cell potential below

E°_{cell}

, while

Q < 1

increases it above

E°_{cell}

. The Nernst equation quantifies exactly how much the potential changes from standard conditions.

Question 20

An electrolytic cell contains a solution with $Cu^{2+}$ , $Ni^{2+}$ , and $Zn^{2+}$ ions all at 1.0 M concentration. Given the standard reduction potentials: $Cu^{2+}/Cu$ (+0.34 V), $Ni^{2+}/Ni$ (-0.25 V), and $Zn^{2+}/Zn$ (-0.76 V), which metal will be deposited first at the cathode when electrolysis begins?

Zinc, because it has the most negative reduction potential and is most easily reduced
Copper, because it has the most positive reduction potential and is most easily reduced (correct answer)
Nickel, because it has an intermediate reduction potential allowing selective deposition
All three metals will deposit simultaneously in proportion to their concentrations
No metal deposition will occur because the solution contains multiple competing ions

Explanation: When you encounter electrolysis problems with multiple metal ions, the key principle is that reduction occurs more readily for species with higher (more positive) reduction potentials. At the cathode, electrons are supplied to reduce cations to metals, so you need to identify which cation accepts electrons most easily. Looking at the standard reduction potentials:

Cu^{2+}/Cu

(+0.34 V) is the highest, followed by

Ni^{2+}/Ni

(-0.25 V), then

Zn^{2+}/Zn

(-0.76 V). The more positive the reduction potential, the greater the tendency for that species to gain electrons and be reduced. Since copper has the most positive reduction potential (+0.34 V),

Cu^{2+}

ions will be reduced first, depositing metallic copper at the cathode. Choice A incorrectly suggests zinc deposits first because of its negative potential. However, negative reduction potentials indicate that a species is harder to reduce, not easier. Zinc actually resists reduction the most among these three metals. Choice C incorrectly assumes that intermediate values somehow enable selective deposition. While nickel will deposit after copper is depleted, its intermediate potential doesn't give it priority over copper's higher potential. Choice D is wrong because the reduction potentials, not concentrations, determine the order of deposition when concentrations are equal. The thermodynamically favored reduction always occurs first. Remember: in electrolysis, higher reduction potentials mean easier reduction at the cathode. Always rank the potentials from highest to lowest to predict deposition order.