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Chapter 17: Problem 22

How can one construct a galvanic cell from two substances, each having a negative standard reduction potential?

Short Answer

Expert verified
A galvanic cell can be constructed from two substances, each having a negative standard reduction potential, by selecting the half-reaction with the higher potential as the cathode and the other as the anode. Ensure that the overall cell potential (E掳cell) is positive for the reaction to be spontaneous. Construct the cell by placing the substances in separate beakers with respective metal strips and connecting them with a wire and a salt bridge or porous barrier. The redox reaction will spontaneously generate an electric current once connected.

Step by step solution

01

Understand the galvanic cell and reduction potentials.

A galvanic cell, also commonly known as a voltaic cell, is an electrochemical cell that uses a spontaneous redox (reduction-oxidation) reaction to generate an electric current. A redox reaction consists of two half-reactions, where one substance undergoes reduction, and another undergoes oxidation. Reduction potentials are values used to predict which half-reaction will proceed as a reduction reaction (gain of electrons) and which will proceed as an oxidation reaction (loss of electrons) when substances are combined in a galvanic cell. A higher standard reduction potential substance will have a greater tendency to gain electrons. When comparing the reduction potentials of two substances (E掳 values), the reaction becomes spontaneous if the final E掳 is positive. In this exercise, both substances have negative standard reduction potentials. Let's call these two substances A and B and their respective standard reduction potentials E掳(A) and E掳(B).
02

Determine the half-reaction with the higher potential.

Compare the standard reduction potential of substance A and substance B. Select the one with the higher reduction potential. Let's assume that E掳(A) > E掳(B), so substance A will be the cathode where the reduction reaction will occur, and substance B will be the anode where the oxidation reaction will occur. Cathode half-reaction: A + ne^鈭� 鈫� A(reduced form) Anode half-reaction: B 鈫� B(oxidized form) + ne^鈭�
03

Calculate the overall cell potential and determine spontaneity.

The cell potential for the spontaneous reaction (E掳cell) is given by the difference in the standard reduction potentials of the two half-reactions: E掳cell = E掳(cathode) - E掳(anode) Evaluate E掳cell by substituting the E掳 values of the two substances A and B: E掳cell = E掳(A) - E掳(B) Since both E掳(A) and E掳(B) are negative, and E掳(A) is greater than E掳(B), the E掳cell will be positive, and the reaction will be spontaneous.
04

Construct the galvanic cell.

Construct a galvanic cell using the following steps: 1. Place substance A in a beaker and immerse a clean metal strip (such as platinum) into it as the cathode. 2. Place substance B in a separate beaker and immerse a clean metal strip (such as platinum) into it as the anode. 3. Connect the metal strips with a wire, and a salt bridge or a porous barrier should be used to connect the two solutions allowing the movement of ions while blocking the direct flow of solutions. 4. When the cell is connected, the redox reaction will spontaneously start to generate an electric current through the wire. In this way, one can construct a galvanic cell from two substances, each having a negative standard reduction potential.

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Most popular questions from this chapter

Consider a galvanic cell based on the following half-reactions: $$\begin{array}{ll} & \mathscr{E}^{\circ}(\mathrm{V}) \\ \mathrm{La}^{3+}+3 \mathrm{e}^{-} \longrightarrow \mathrm{La} & -2.37 \\ \mathrm{Fe}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Fe} & -0.44 \\ \hline \end{array}$$ a. What is the expected cell potential with all components in their standard states? b. What is the oxidizing agent in the overall cell reaction? c. What substances make up the anode compartment? d. In the standard cell, in which direction do the electrons flow? e. How many electrons are transferred per unit of cell reaction? f. If this cell is set up at \(25^{\circ} \mathrm{C}\) with \(\left[\mathrm{Fe}^{2+}\right]=2.00 \times 10^{-4} \mathrm{M}\) and \(\left[\mathrm{La}^{3+}\right]=3.00 \times 10^{-3} \mathrm{M},\) what is the expected cell potential?

The ultimate electron acceptor in the respiration process is molecular oxygen. Electron transfer through the respiratory chain takes place through a complex series of oxidationreduction reactions. Some of the electron transport steps use iron-containing proteins called cytochromes. All cytochromes transport electrons by converting the iron in the cytochromes from the +3 to the +2 oxidation state. Consider the following reduction potentials for three different cytochromes used in the transfer process of electrons to oxygen (the potentials have been corrected for \(\mathrm{pH}\) and for temperature): $$\begin{aligned} &\text { cytochrome } \mathrm{a}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \text { cytochrome } \mathrm{a}\left(\mathrm{Fe}^{2+}\right)\ &\mathscr{E}^{\circ}=0.385 \mathrm{V}\\\ &\text { cytochrome } \mathbf{b}\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \text { cytochrome } \mathrm{b}\left(\mathrm{Fe}^{2+}\right)\ &\mathscr{E}^{\circ}=0.030 \mathrm{V}\\\ &\text { cytochrome } c\left(\mathrm{Fe}^{3+}\right)+\mathrm{e}^{-} \longrightarrow \text { cytochrome } \mathrm{c}\left(\mathrm{Fe}^{2+}\right)\ &\mathscr{E}^{\circ}=0.254 \mathrm{V} \end{aligned}$$ In the electron transfer series, electrons are transferred from one cytochrome to another. Using this information, determine the cytochrome order necessary for spontaneous transport of electrons from one cytochrome to another, which eventually will lead to electron transfer to \(\mathrm{O}_{2}\)

What is the maximum work that can be obtained from a hydrogen-oxygen fuel cell at standard conditions that produces \(1.00 \mathrm{kg}\) water at \(25^{\circ} \mathrm{C} ?\) Why do we say that this is the maximum work that can be obtained? What are the advantages and disadvantages in using fuel cells rather than the corresponding combustion reactions to produce electricity?

Consider the following galvanic cell at \(25^{\circ} \mathrm{C}\) : $$ \mathrm{Pt}\left|\mathrm{Cr}^{2+}(0.30 M), \mathrm{Cr}^{3+}(2.0 M)\right|\left|\mathrm{Co}^{2+}(0.20 M)\right| \mathrm{Co} $$ The overall reaction and equilibrium constant value are $$\begin{aligned} 2 \mathrm{Cr}^{2+}(a q)+\mathrm{Co}^{2+}(a q) & \longrightarrow \\ 2 \mathrm{Cr}^{3+}(a q)+\mathrm{Co}(s) & K=2.79 \times 10^{7} \end{aligned}$$ Calculate the cell potential, \(\mathscr{E}\), for this galvanic cell and \(\Delta G\) for the cell reaction at these conditions.

Aluminum is produced commercially by the electrolysis of \(\mathrm{Al}_{2} \mathrm{O}_{3}\) in the presence of a molten salt. If a plant has a continuous capacity of 1.00 million \(A\), what mass of aluminum can be produced in \(2.00 \mathrm{h} ?\)
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