Half-cells, galvanics

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

Any galvanic cell may be regarded as consisting of two half-cells, each... 

In galvanic cells it is only possible to determine the potential difference as a voltage between two half-cells, but not the absolute potential of the single electrode. To measure the potential difference it has to be ensured that an electrochemical equilibrium exists at the phase boundaries, e.g., at the electrode/electrolyte interface. At the least it is required that there is no flux of current in the external and internal circuits. 

The potentials of the metals in their 1 mol U salt solution are all related to the standard or normal hydrogen electrode (NHE). For the measurement, the hydrogen half-cell is combined with another half-cell to form a galvanic cell. The measured voltage is called the normal potential or standard electrode potential, E° of the metal. If the metals are ranked according to their normal potentials, the resulting order is called the electrochemi-... 

A student was given a standard Fe(s) Fe2+(aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 M MNO,(aq). When these two half-cells were connected at 25°C, the complete cell functioned as a galvanic cell with E = +1.24 V. The reaction was allowed to continue overnight and the two electrodes were weighed. The iron electrode was found to be lighter and the unknown metal electrode was heavier. What is the standard potential of the unknown MT/M couple ... 

As an example of the numerous combinations of the hydrogen electrode with other electrodes, attention is drawn to Figure 6.10 (C) of a galvanic cell. The two redox couples or the half-cells to make up the cell and reactions taking place at the electrodes or the electrode reactions constituting the cell are presented below ... 

Danlell cell a galvanic cell composed of copper/copperGI) ion and zinc/zinc ion half-cells. 

Define cell, half-cell, anode, cathode, electrolytic cell, and galvanic cell. 

A cell is a complete electroanalytical system consisting of an electrode at which reduction occurs, as well as an electrode at which oxidation occurs, and including the connections between the two. A half-cell is half of a cell in the sense that it is one of the two electrodes (and associated chemistry) in the system, termed either the reduction half-cell or the oxidation half-cell. The anode is the electrode at which oxidation takes place. The cathode is the electrode at which reduction takes place. An electrolytic cell is one in which the current that flows is not spontaneous, but rather due to the presence of an external power source. A galvanic cell is a cell in which the current that flows is spontaneous. 

Figure 11.1 shows one example of a galvanic cell, called the Daniell cell. One half of the cell consists of a piece of zinc placed in a zinc sulfate solution. The other half of the cell consists of a piece of copper placed in a copper(II) sulfate solution. A porous barrier, sometimes called a semi-permeable membrane, separates these two half-cells. It stops the copper(II) ions from coming into direct contact with the zinc electrode. 

The redox reaction takes place in a galvanic cell when an external circuit, such as a metal wire, connects the electrodes. The oxidation half-reaction occurs in one half-cell, and the reduction half-reaction occurs in the other half-cell. For the Daniell cell ... 

Predict whether the cell potentials of galvanic cells depend on the electrodes and electrolytes in the half-cells. Give reasons for your prediction. 

Prepare a 9 x 9 grid in your notebook. Label the nine columns to match the nine half-cells. Label the nine rows in the same way. You will use this chart to mark the positive cell potentials you obtain when you connect two half-cells to build a galvanic cell. You will also record the anode and the cathode for each galvanic cell you build. (You may not need to fill out the entire chart.)... 

In this section, you learned that you can calculate cell potentials by using tables of half-cell potentials. The half-cell potential for a reduction half-reaction is called a reduction potential. The half-cell potential for an oxidation half-reaction is called an oxidation potential. Standard half-cell potentials are written as reduction potentials. The values of standard reduction potentials for half-reactions are relative to the reduction potential of the standard hydrogen electrode. You used standard reduction potentials to calculate standard cell potentials for galvanic cells. You learned two methods of calculating standard cell potentials. One method is to subtract the standard reduction potential of the anode from the standard reduction potential of the cathode. The other method is to add the standard reduction potential of the cathode and the standard oxidation potential of the anode. In the next section, you will learn about a different type of cell, called an electrolytic cell. 

O Look at the half-cells in the table of standard reduction potentials in Appendix E. Could you use two of the standard half-cells to build a galvanic cell witb a standard cell potential of 7 V Explain your answer. 

One half-cell of a galvanic cell has a nickel electrode in a 1 mol/L nickel(II) chloride solution. The other half-cell has a cadmium electrode in a 1 mol/L cadmium chloride solution. 

The two half-cells in a galvanic cell consist of one iron electrode in a 1 mol/L iron(II) sulfate solution, and a silver electrode in a 1 mol/L silver nitrate solution. 

Tabulated E values can be used to calculate the for any reaction, as illustrated in Table 7.2 for the Zn/Cu galvanic cell. The redox reaction is spontaneous when the half-reaction (Cu /Cu) with the larger reduction (+0.34V) acts as the oxidizing agent. In this case, the other half-reaction (Zn /Zn) proceeds as an oxidation. The halfcell potential for this reduction is +0.76 V as it represents the reverse of the half-cell reduction potential as listed in Table 7.2. The sum of the oxidation and reduction half reactions is +0.34V + 0.76 V = +1.10 V. Thus for the galvanic Zn/Cu cell is +1.10V. 

The most energetic galvanic cell (highest i created by pairing the half-cell reaction, which has the largest reduction with the one that has the smallest. Using the entries in Table 7.1, this would involve Co (aq) as the oxidant and Na(s) as the... 

Calculate the EMF of a galvanic cell from the half-cell potentials. 

Ozkaya (76) studied conceptual difficulties experienced by prospective teachers in a number of electrochemical concepts, namely half-cell potential, cell potential, and chemical and electrochemical equilibrium in galvanic cells. The study identified common misconceptions among student teachers from different countries and different levels of electrochemistry. Misconceptions were also identified in relation to chemical equilibrium, electrochemical equilibrium, and the instrumental requirements for die measurement of cell potentials. Learning difficulties were attributed mainly to failure of students to acquire adequate conceptual understanding, and the insufficient explanation of the relevant... 

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

If a galvanic cell has a nonzero voltage, then the net cell reaction is not at equilibrium. We say that equilibrium between the two half-cells has not been established. 

Figure 14-9 This galvanic cell can be used to measure the pH of the left half-cell. 

In the simplest case, analyte is an electroactive species that is part of a galvanic cell. An electroactive species is one that can donate or accept electrons at an electrode. We turn the unknown solution into a half-cell by inserting an electrode, such as a Pt wire, that can transfer electrons to or from the analyte. Because this electrode responds to analyte, it is called the indicator electrode. We then connect this half-cell to a second half-cell by a salt bridge. The second half-cell has a fixed composition, so it has a constant potential. Because of its constant potential, the second half-cell is called a reference electrode. The cell voltage is the difference between the variable potential of the analyte half-cell and the constant potential of the reference electrode. 

Suppose you want to measure the relative amounts of Fe2+ and Fe3+ in a solution. You can make this solution part of a galvanic cell by inserting a Pt wire and connecting the cell to a constant-potential half-cell by a salt bridge, as shown in Figure 15-1. 

Figure 15-1 A galvanic cell that can be used to measure the quotient [Fe2+]/[Fe3+J in the right halt-cell. The Pt wire Is the indicator electrode, and the entire left half-cell plus salt bridge (enclosed by the dashed line) can be considered a reference electrode.

The standard potential of any galvanic cell is the sum of the standard half-cell potentials for oxidation at the anode and reduction at the cathode ... 

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