01:160:162 Lecture Notes - Lecture 33: Concentration Cell, Nernst Equation, Electrolytic Cell

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ivoryant268

25 Jul 2017

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Concentration cell voltaic cell that employs the same half-reaction within anode and cathode. Difference in [] between half-cells drives spontaneous e- flow from anode to cathode. Cathode is half-cell with higher [] e- flow continues until [] within both half-cells become equal. [] within anode increases and concentration within cathode decreases. Because same half-reaction is used for both half-cells: Potential of concentration cell can be calculated using nernst equation. Electrolytic cell electrochemical cell that uses electrical current to force a nonspontaneous reaction to occur. Reduction still in cathode e- flow still from anode to cathode. **outside power source is required to drive e- flow. Can be a battery or dc power supply. Anode of electrolytic cell attaches to + terminal of external battery. Cathode of electrolytic cell attaches to - terminal of external battery. Without outside power source nonspontaneous redox reaction will not occur. Electrical current causes water to decompose into its constituent elements.

Related Questions

What are the similarities and differences between Figure 20.3 and Figure 20.4? (b) Why are Na⁺ ions drawn into the cathode half-cell as the voltaic cell shown in Figure 20.5 operates?

One Class Solution:-

Figure 20.3 shows spontaneous reaction when a strip of zinc is placed in contact with a Cu²⁺ solution. Cu²⁺(aq) ions fades and copper metal deposits on the zinc. At the same time, the zinc begins to dissolve.

Same redox reaction: Cu²⁺_(aq) +Zn → Cu +Zn²_(aq)⁺ is occurs in both cases (Figure 20.3 and 20.4). The significant difference between Figure 20.3 and 20.4 is that in the voltaic cell the Zn metal and Cu²⁺(aq) are not in direct contact with each other. Zn metal is in contact with Zn²⁺(aq) in one compartment, and Cu metal is in contact with Cu²⁺(aq) in the other compartment. Cu²⁺(aq) reduction can occur only by the flow of electrons through an external circuit, namely, a wire connecting the Zn and Cu strips. Electrons flowing through a wire and ions moving in solution both constitute an electrical current.

In Figure 20.5, a salt bridge serves this purpose. The salt bridge consists of a U-shaped tube containing NaNO₃(aq), whose ions will not react with other ions in the voltaic cell or with the electrodes. The electrolyte is often incorporated into a paste or gel so that the electrolyte solution does not pour out when the U-tube is inverted. As oxidation and reduction proceed at the electrodes, ions from the salt bridge migrate into the two half-cells—cations migrating to the cathode half-cell and anions migrating to the anode half-cell—to neutralize charge in the half-cell solutions. Whichever device is used to allow ions to migrate between half-cells, anions always migrate toward the anode and cations toward the cathode.

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(a) What are concentration cells?

(b) What is in a concentration cell?

(d) Is the higher or the lower ion concentration solution present at the anode?

(e) When the anode ion concentration is decreased and/or the cathode ion concentration is increased, both give rise to larger cell potentials. Why?

Concentration cells are commonly used to calculate the value of equilibrium constants for various reactions. For example, the silver concentration cell illustrated in Fig. 17-9 can be used to determine the value for AgCl(s). To do so, NaCl is added to the anode compartment until no more precipitate forms. The [Cl^-] in solution is then determined somehow.

(f) What happens to when NaCl is added to the anode compartment? To calculate the value, [Ag⁺]must be calculated.

(g) Given the value of , how is [Ag⁺]determined at the anode?

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01:160:162 Lecture Notes - Lecture 33: Concentration Cell, Nernst Equation, Electrolytic Cell

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