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A voltaic cell is constructed with two \(\mathrm{Zn}^{2+}-\mathrm{Zn}\) electrodes. The two half-cells have \(\left[\mathrm{Zn}^{2+}\right]=1.8 \mathrm{M}\) and \(\left[\mathrm{Zn}^{2+}\right]=\) \(1.00 \times 10^{-2} M\), respectively. (a) Which electrode is the anode of the cell? (b) What is the standard emf of the cell? (c) What is the cell emf for the concentrations given? (d) For each electrode, predict whether \(\left[\mathrm{Zn}^{2+}\right]\) will increase, decrease, or stay the same as the cell operates.

Short Answer

Expert verified
a) The anode is the half-cell with \(\left[\mathrm{Zn}^{2+}\right]=1.8 M\). b) The standard emf of the cell is 0 V. c) The cell emf for the given concentrations is approximately -0.044 V. d) As the cell operates, the concentration of \(\left[\mathrm{Zn}^{2+}\right]\) will increase at the anode and decrease at the cathode.

Step by step solution

01

Identify the anode and cathode

In a voltaic cell, oxidation occurs at the anode and reduction occurs at the cathode. Since both electrodes are made of Zinc, we will look at their respective concentrations to identify the anode and the cathode. The anode is the half-cell with the higher concentration, while the cathode is the half-cell with the lower concentration. In this case, the half-cell with \(\left[\mathrm{Zn}^{2+}\right]=1.8 M\) is the anode, and the half-cell with \(\left[\mathrm{Zn}^{2+}\right]=1.00 \times 10^{-2} M\) is the cathode.
02

Determine the standard emf of the cell

The standard emf (electromotive force, also known as cell potential) of the cell can be determined using the standard reduction potentials of the half-cells. However, since both half-cells have the same redox couple (Zn虏鈦/Zn), their standard reduction potentials are the same and cancel each other out. Thus, the standard emf of the cell is 0 V.
03

Calculate the cell emf for the given concentrations

To calculate the cell emf for the given concentrations, we will use the Nernst equation: \[E = E^0 - \frac{RT}{nF} \ln Q\] Where: \(E\) is the cell potential \(E^0\) is the standard cell potential \(R\) is the gas constant (\(8.314 J K^{-1} mol^{-1}\)) \(T\) is the temperature (assumed to be 298 K) \(n\) is the number of electrons transferred in the half-cell reaction \(F\) is the Faraday constant (\(96485 C mol^{-1}\)) \(Q\) is the reaction quotient For the Zn/Zn虏鈦 redox couple, the balanced half-reaction is: \[\mathrm{Zn^{2+}} + 2\mathrm{e^-} \rightarrow \mathrm{Zn}\] The reaction quotient \(Q\) for this cell can be calculated as: \[Q = \frac{[\mathrm{Zn^{2+}}]_{cathode}}{[\mathrm{Zn^{2+}}]_{anode}} = \frac{1.00 \times 10^{-2} M}{1.8 M}\] Now, substitute all the values into the Nernst equation. Note that the standard cell potential, \(E^0\), is 0 V: \[E = 0 - \frac{8.314 J K^{-1} mol^{-1} \times 298 K}{2 \times 96485 C mol^{-1}} \ln \frac{1.00 \times 10^{-2} M}{1.8 M}\] Calculate the cell potential: \[E \approx -0.044 V\]
04

Predict the change in Zn虏鈦 concentration

As the cell operates, oxidation will continue to occur at the anode, and reduction will occur at the cathode. Therefore: - At the anode, Zn will continue to oxidize to Zn虏鈦, increasing the concentration of Zn虏鈦. - At the cathode, Zn虏鈦 will continue to reduce, forming Zn, thereby decreasing the concentration of Zn虏鈦. In summary: a) The anode is the half-cell with \(\left[\mathrm{Zn}^{2+}\right]=1.8 M\). b) The standard emf of the cell is 0 V. c) The cell emf for the given concentrations is approximately -0.044 V. d) As the cell operates, the concentration of \(\left[\mathrm{Zn}^{2+}\right]\) will increase at the anode and decrease at the cathode.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electrode
Electrodes are crucial components of a voltaic cell, acting as sites where oxidation and reduction reactions occur. In a voltaic cell, there are two types of electrodes: the anode and the cathode.
The anode is the electrode where oxidation happens; it loses electrons to the external circuit. Conversely, the cathode is where reduction takes place; it gains electrons from the circuit.
Each electrode is involved in its own half-cell reaction, which contributes to the overall operation of the voltaic cell. In the example provided, both electrodes are zinc electrodes with different Zn虏鈦 concentrations. The higher concentration electrode acts as the anode, while the lower concentration one serves as the cathode.
Nernst Equation
The Nernst Equation is a fundamental formula in electrochemistry. It allows us to determine the cell potential (emf) at any conditions away from standard state.
It is given by the formula: \[E = E^0 - \frac{RT}{nF} \ln Q\]where:
  • E: the cell potential under non-standard conditions
  • E0: the standard cell potential
  • R: the universal gas constant (8.314 J路K鈦宦孤穖ol鈦宦)
  • T: the temperature in Kelvin
  • n: the number of electrons exchanged in the half-cell reaction
  • F: the Faraday constant (96485 C/mol)
  • Q: the reaction quotient, calculated from the concentrations of reactants and products
For the provided example, E0 is 0 V because both half-cells have the same redox couple, Zn虏鈦/Zn; thus the value of Q directly influences the emf calculation.
Cell Potential
Cell potential, or electromotive force (emf), is the driving force behind the flow of electrons in a voltaic cell. It represents the difference in potential energy between the two electrodes.
In a voltaic cell, cell potential is positive when the reaction is spontaneous. Factors affecting cell potential include the concentration of ion species in each half-cell and temperature. The standard cell potential is calculated under standard conditions (1 M concentration, 1 atm pressure, and 298 K temperature) but can change under different conditions. Through the Nernst Equation, we adjust the standard potential to account for variable conditions, such as the initial concentration of Zn虏鈦 in our example.
Oxidation-Reduction Reaction
Oxidation-reduction reactions, or redox reactions, are chemical reactions involving the transfer of electrons between two species. These reactions are at the heart of how a voltaic cell generates electrical energy.
In the context of a voltaic cell:
  • The substance that loses electrons is oxidized, occurring at the anode.
  • The substance that gains electrons is reduced, occurring at the cathode.
For the zinc electrodes in our example, the oxidation reaction is:\[\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-\]At the cathode, the reduction reaction is:\[\text{Zn}^{2+} + 2e^- \rightarrow \text{Zn}\]These coupled reactions ensure the continuous flow of electrons, as long as the cell operates.

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Most popular questions from this chapter

During a period of discharge of a lead-acid battery, \(402 \mathrm{~g}\) of \(\mathrm{Pb}\) from the anode is converted into \(\mathrm{PbSO}_{4}(s) .\) (a) What mass of \(\mathrm{PbO}_{2}(s)\) is reduced at the cathode during this same period? (b) How many coulombs of electrical charge are transferred from \(\mathrm{Pb}\) to \(\mathrm{PbO}_{2} ?\)

(a) Which electrode of a voltaic cell, the cathode or the anode, corresponds to the higher potential energy for the electrons? (b) What are the units for electrical potential? How does this unit relate to energy expressed in joules? (c) What is special about a standard cell potential?

A plumber's handbook states that you should not connect a brass pipe directly to a galvanized steel pipe because electrochemical reactions between the two metals will cause corrosion. The handbook recommends you use instead an insulating fitting to connect them. Brass is a mixture of copper and zinc. What spontaneous redox reaction(s) might cause the corrosion? Justify your answer with standard emf calculations.

From each of the following pairs of substances, use data in Appendix \(\mathrm{E}\) to choose the one that is the stronger reducing agent: (a) \(\mathrm{Fe}(s)\) or \(\mathrm{Mg}(s)\) (b) \(\mathrm{Ca}(s)\) or \(\mathrm{Al}(s)\) (c) \(\mathrm{H}_{2}(g,\) acidic solution \()\) or \(\mathrm{H}_{2} \mathrm{~S}(g)\) (d) \(\mathrm{BrO}_{3}^{-}(a q)\) or \(\mathrm{IO}_{3}^{-}(a q)\)

(a) Assuming standard conditions, arrange the following in order of increasing strength as oxidizing agents in acidic solution: \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}, \mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{Cu}^{2+}, \mathrm{Cl}_{2}, \mathrm{O}_{2} .\) (b) Arrange the fol- lowing in order of increasing strength as reducing agents in acidic solution: \(\mathrm{Zn}, \mathrm{I}^{-}, \mathrm{Sn}^{2+}, \mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{Al}\).

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