Cells standard reduction potentials

You have learned that the standard hydrogen electrode has an assigned standard reduction potential of exactly 0 V, and is the reference for all half-cell standard reduction potentials. What would happen to cell potentials if a different reference were used You will address this question in the following ThoughtLah. 

Thinking it Through Standard cell potentials for redox reactions can be calculated from the proper combination of half-cell standard reduction potentials. Looking at the target equation, the liquid bromine in changing to ... 

Standard, reduction potentials are determined by measuring the voltages generated in reaction half-cells (Figure 21.2). A half-cell consists of a solution containing 1 M concentrations of both the oxidized and reduced forms of the substance whose reduction potential is being measured, and a simple electrode. 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials... 

E° values have been measured for many reactions and tabulated as standard half-cell potentials. Table 9.3 summarizes half-cell potentials as standard reduction potentials for a select set of reactions.aa In the tabulations, E° for... 

Note that, because the right side of the cell diagram corresponds to reduction, E° = °(for reduction) — E°(for oxidation) where both values of E° are the standard reduction potentials. 

In any galvanic cell that is under standard conditions, electrons are produced by the half-reaction with the more negative standard reduction potential and consumed by the half-reaction with the more positive standard reduction potential. In other words, the half-reaction with the more negative E ° value occurs as the oxidation, and the half-reaction with the more positive E ° value occurs as the reduction. Figure 19-15 summarizes the conventions used to describe galvanic cells. 

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. 

The overall voltage generated by a standard galvanic cell is always obtained by subtracting one standard reduction potential from the other in the way that gives a positive value for E (.gH Example applies this reasoning to zinc and iron. 

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... 

A battery must use cell reactions that generate and maintain a large electrical potential difference. This requires two half-reactions with substantially different standard reduction potentials. The ideal battery would be compact, inexpensive, rechargeable, and environmentally safe. This is a stringent set of requirements. No battery meets all of them, and only a few come close. 

In Table 7-1 the relative tendencies of certain elements to react were listed qualitatively. We can give a quantitative measure of relative tendency to react, called standard reduction potential, as shown in Table 14-2. In this table, the standard half-cell potential for each half-reaction, as a reduction, is tabulated in order with the highest potential first. If we turn these half-reactions around, we change the signs of the potentials and we get oxidation potentials. We thus have half-reactions including both elementary metals and elementary nonmetals in the same table, as well as many half-reactions that do... 

Standard reduction potential reduction potential for a cell in which all solutes arc 1.00 M and all gases are at 1.00 atm. 

FIGURE3.7 The potential window for the redox chemistry of life. Redox chemistry in living cells is approximately limited by the standard potentials for reduction and oxidation of the solvent water at neutral pH. Approximate standard reduction potentials are also indicated for the commonly used oxidant ferricyanide and reductants NADH and dithionite. 

The overall cell potential is +0.96 V, showing that the redox reaction is indeed spontaneous. The standard reduction potential for the half cell Ag2S(s) + 2e - 2Ag(s) + S2 (aq) was obtained from the American Society for Metals (ASM) Handbook, available on the internet. 

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

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 ... 

The more the two half-reactions are separated in the table, the greater is the tendency for the net reaction to occur. This tendency for an overall redox reaction to occur, whether by direct contact or in an electrochemical cell, is determined from the standard reduction potentials, E° values, of the half-reactions involved, and the value of this potential are indications of the tendency of the overall redox reaction to occur. We will now present a scheme for determining this potential, which is symbolized E"d. ... 

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. 

Using measurements of different half-cell combinations, a set of standard reduction potentials may be constructed. This set will be similar to a table of standard reduction potentials. The solutions used in the half-cells must be of known concentration. These solutions are produced by weighing reagents and diluting to volume. The measurements will require a balance and a volumetric flask. It is also possible to produce known concentrations by diluting solutions. This method requires a pipette and a volumetric flask. Review the Solutions and Colligative Properties chapter for solution techniques. 

Standard reduction potentials are used to calculate the cell potential under standard conditions. All half-reactions are shown in the reduction form. 

A voltaic cell converts chemical energy into electrical energy. It consists of two parts called half-cells. When two different metals, one in each half-cell, are used in the voltaic cell, a potential difference is produced. In this experiment, you will measure the potential difference of various combinations of metals used in voltaic cells and compare these values to the values found in the standard reduction potentials table. 

Applying Concepts Write the half-reactions for the anode and cathode in each of the voltaic cells in the data table. Look up the half-reaction potentials from the standard reduction potentials table (Table 21-1) and record these in the data table. 

The numerical values of cell potentials and half-cell potentials depend on various conditions, so tables of standard reduction potentials are true when ions and molecules are in their standard states. These standard states are the same as for tables of standard enthalpy changes. Aqueous molecules and ions have a standard concentration of 1 mol/L. Gases have a standard pressure of 101.3 kPa or 1 atm. The standard temperature... 

One method to calculate the standard cell potential is to subtract the standard reduction potential of the anode from the standard reduction potential of the cathode. 

For a Daniell cell, you know that copper is the cathode and zinc is the anode. The relevant half-reactions and standard reduction potentials from Table 11.1 are as follows. 

To summarize, the standard cell potential can also be calculated as the sum of a standard reduction potential and a standard oxidation potential. 

Step 3 Subtract the standard reduction potentials to calculate the cell potential. 

Write the two half-reactions for the following redox reaction. Subtract the two standard reduction potentials to find the standard cell potential for the reaction. 

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