Question

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

Short answer

To construct a galvanic cell from two substances (A and B) with negative standard reduction potentials, first identify which substance has a higher (less negative) reduction potential; this will be Substance B. Construct a cathode half-cell containing Substance B in its reduced and oxidized forms and an anode half-cell containing Substance A in its reduced and oxidized forms. Connect the two half-cells with a salt bridge. The anode is where oxidation occurs (Substance A) and the cathode is where reduction occurs (Substance B). The overall redox reaction in this galvanic cell will be A + B+ → A+ + B.

Step-by-step solution

Identify two substances with negative standard reduction potentials and determine which one has a higher reduction potential

Let's consider the following two substances with negative standard reduction potentials: Substance A: Standard Reduction Potential (E°) = -1.20 V Reduction half-reaction: A+ + e- → A Substance B: Standard Reduction Potential (E°) = -0.75 V Reduction half-reaction: B+ + e- → B Comparing the standard reduction potentials, Substance B has a relatively higher reduction potential (less negative) than Substance A.

Construct the galvanic cell

To construct the galvanic cell, we need to prepare two half-cells. The substance with the higher reduction potential (Substance B) will be used in the cathode half-cell, while the substance with the lower reduction potential (Substance A) will be used in the anode half-cell. The two half-cells will be connected using a salt bridge that will maintain the ionic balance and facilitate the flow of ions between the two half-cells. Cathode Half-cell: Contains Substance B in its reduced and oxidized forms, e.g., B+ ions in solution and a B solid electrode. Anode Half-cell: Contains Substance A in its reduced and oxidized forms, e.g., A+ ions in solution and an A solid electrode.

Define the anode and the cathode

In a galvanic cell, the anode is the electrode where oxidation occurs, and the cathode is the electrode where reduction occurs. Cathode: Substance B - the electrode where Substance B+ will gain an electron and be reduced to B Anode: Substance A - the electrode where Substance A will lose an electron and be oxidized to A+

Identify the overall redox reaction

To identify the overall redox reaction, we first write the two half-reactions. Oxidation half-reaction (Anode): A → A+ + e- (This half-reaction should be reversed because the anode will undergo oxidation) Reduction half-reaction (Cathode): B+ + e- → B Now, add the two half-reactions: A + B+ → A+ + B This is the overall redox reaction in the galvanic cell composed of two substances with negative standard reduction potentials.

Key concepts

Standard Reduction Potential

Understanding the standard reduction potential (SRP) is crucial when exploring the workings of galvanic cells. This concept helps us to compare the tendency of different substances to acquire electrons and undergo reduction. Specifically, it measures the voltage difference when a half-reaction occurs at an electrode compared to a standard hydrogen electrode under standard conditions (1 M concentration, 1 atm pressure, and a temperature of 298 K).

When dealing with substances that possess negative standard reduction potentials, it's vital to consider that although they are less inclined to gain electrons compared to hydrogen, they can still participate in redox reactions. In constructing a galvanic cell using these substances, the one with the less negative potential acts as a cathode because it's relatively better at gaining electrons, while the one with the more negative potential serves as the anode.

Redox Reaction

Galvanic cells operate on the principle of redox reactions, which are a type of chemical reaction where oxidation and reduction occur simultaneously. During these reactions, electrons are transferred from one substance to another. The substance that loses electrons is oxidized, whereas the one that gains electrons undergoes reduction.

In the context of a galvanic cell with negative SRPs, despite both substances being generally reluctant to acquire electrons, a redox reaction still occurs. The cell is engineered to create a situation where the less negative SRP substance is forced to reduce, while the more negative one must oxidize, driving the flow of electrons through the circuit and resulting in electrical energy.

Oxidation and Reduction

To further elucidate, oxidation and reduction are the two halves of a redox reaction. Oxidation involves the loss of electrons, which increases the oxidation number, while reduction involves the gain of electrons, decreasing the oxidation number.

In galvanic cells, the electrode where oxidation takes place is the anode, and the electrode where reduction occurs is the cathode. It is essential to remember that in the case of the galvanic cell built with substances with negative SRPs, the reactions at the electrodes are still founded on these fundamental principles. The substance with the more negative SRP will give up electrons (oxidize) at the anode, while the one with the less negative SRP will gain electrons (reduce) at the cathode.

Half-Cell Setup

A half-cell setup consists of an electrode submerged in an electrolyte solution containing cations and anions of the metal making up the electrode. In a galvanic cell, there are two half-cells, each containing a different electrode/electrolyte combination. These half-cells are linked by a salt bridge, facilitating ion flow and maintaining electrical neutrality.

For substances with negative SRPs, we establish each half-cell with one substance in its oxidized and reduced forms. A solid electrode is generally the reduced form, and the electrolyte solution contains the oxidized form. These half-cells must be carefully designed to ensure that the reactions can proceed under the imposed conditions to generate a spontaneous flow of electrons—ultimately, the goal of constructing a functional galvanic cell.