Cell potential, standard

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

Thus, because the standard cell potential for reaction 15 is positive, the reaction proceeds spontaneously as written. Consequendy, to produce chlorine and hydrogen gas, a potential must be appHed to the cell that is greater than the open-circuit value. This then becomes an example of an electrolytic process. 

The aluminum-air fuel cell is used as a reserve battery in remote locations. In this cell aluminum reacts with the oxygen in air in basic solution, (a) Write the oxidation and reduction half-reactions for this cell, (b) Calculate the standard cell potential. See Box 12.1. 

Calculate the standard cell potential for the reaction between silicon and water in a cell that also produces hydrogen from water and write the balanced equation for the cell reaction. See Box 12.1. 

When the two electrodes are connected, current flows from M to X in the external circuit. When the electrode corresponding to half-reaction 1 is connected to the standard hydrogen electrode (SHE), current flows from M to SHE. (a) What are the signs of ° of the two half-reactions (b) What is the standard cell potential for the cell constructed from these two electrodes ... 

C (298.15 K) and 1 bar. standard cell potential See standard emf. standard emf ( °) The emf when the concentration of each solute taking part in the cell reaction is 1 mol-L 1 (strictly, unit activity) and all the gases are at 1 bar. The standard emf of a galvanic cell is the difference between its two standard potentials E° = E°(cathode) — °(anode). 

A table giving the cell potentials of all possible redox reactions would be immense. Instead, chemists use the fact that any redox reaction can be broken into two distinct half-reactions, an oxidation and a reduction. They assign a potential to every half-reaction and tabulate E ° values for all half-reactions. The standard cell potential for any redox reaction can then be obtained by combining the potentials for its two half-reactions. 

Tabulated standard reduction potentials allow us to determine the potential of any cell under standard conditions. This net standard cell potential is obtained by subtracting the more negative standard reduction potential from the more positive standard reduction potential, giving a positive overall potential. 

V) from the more positive value (-0.447 V) to obtain the standard cell potential ... 

Example provides another illustration of standard cell potentials. 

The calculation o E° for this cell illustrates an important feature of cell potentials. A standard cell potential is the difference between two standard reduction potentials. This difference does not change when one half-reaction is multiplied by 2 to cancel electrons in the overall redox reaction. 

C19-0020. Use standard reduction potentials to determine the net reaction and standard cell potential for cells of two compartments, each containing a 1.00 M solution of the indicated cation in contact with an... 

The linkage between free energy and cell potentials can be made quantitative. The more negative the value of A G ° for a reaction, the more positive its standard cell potential, as the following two examples illustrate ... 

E = E °-------In Q The Nemst equation is used to convert between standard cell potentials and potentials of... 

The standard cell potential for the hydrogen-oxygen reaction is then determined by the free energy of formation of water (gas) by... 

EXAMPLE 14.5. Calculate the standard cell potential of the Danicll cell, shown in Fig. 14-l. 

Because the standard cell potential is negative, we conclude that this cell reaction is nonspontaneous. This would not be a feasible method of producing peroxodisulfate ion. 

Since the standard cell potential is positive, this cell reaction is spontaneous. 

The standard cell potential for this reaction is more positive than that for situation (1) reaction (2) should occur preferentially. Also, if Sn4+(aq) is formed, it should react... 

The reaction is not spontaneous in either direction when Ece]i = 0.000 V. We use the standard cell potential from Example 21-9. 

The standard cell potentials do not tell the whole story, for we might think that the reaction with the more positive value of E°ell would occur. In truth, Al(s) is coated with a thin layer of tightly adhering Al203(s), which protects the metal from attack by water. Na(s) has no such protective coating. 

Thus, when all concentrations are the same, the ion product, Q, equals 1.00. From the negative standard cell potential, it is clear that Keq must be (slightly) less than one. Therefore, all the concentrations cannot be 0.500 M at the same time. 

The production of I2(s) is more likely it results in the larger standard cell potential. 

A2= 96,500 J mole-1 V1 E° = standard cell potential, volts (V) n = number of electrons in the half-reaction... 

Stalactites, 15 29 Stalagmites, 15 29 Stalk fibers, 21 18 Stamp-pad inks, 14 328 Standard cell potential, 15 750 Standard deviation, 13 257, 20 699 Standard electrode potential selected ions, 7 799t... 

We can use this table of standard reduction potentials to write the overall cell reaction and to calculate the standard cell potential, the potential (voltage) associated with the cell at standard conditions. There are a couple of things to remember when using these standard reduction potentials to generate the cell reaction and cell potential ... 

Since oxidation occurs at the anode and reduction at the cathode, the standard cell potential can be calculated from the standard reduction potentials of the two half-reactions involved in the overall reaction by using the equation ... 

Once you have calculated the standard cell potential, then the reaction can be written by reversing the half-reaction associated with the anode (show it as oxidation) and adding the two half-reactions. 

Thus far, we have based all of our calculations on the standard cell potential or standard half-cell potentials—that is, standard state conditions. However, many times the cell is not at standard conditions—commonly the concentrations are not 1 M. We may calculate the actual cell potential, Ecell, by using the Nemst equation ... 

The standard cell potential for the reduction of hydrogen ions to hydrogen gas is, by definition, 0.00 V. This potential is for the standard hydrogen electrode, SHE, which is the reference to which we compare all other cell potentials. All metals above hydrogen on the Activity Series will displace hydrogen gas from acids. (See Chapter 4) Metals below hydrogen will not displace hydrogen gas. 

The standard cell potential for a galvanic cell is a positive value, E° > 0. 

Because one half-reaction must involve oxidation, one of the half-reactions shown in the table of reduction potentials must be reversed to indicate the oxidation. If the half-reaction is reversed, the sign of the standard reduction potential must be reversed. However, this is not necessary to calculate the standard cell potential. 

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