Half-reactions standard reduction potentials

Standard potentials (reactants and products at unit activity, hydrogen ion activity of 1, i.e. pH = 0), of reduction, oxidation, net reaction, the ith redox-active species, and any half-reaction Standard reduction potential at hydrogen ion activity of 10 7 (pH = 7.0)... 

For each galvanic cell, give the balanced cell equation and determine Standard reduction potentials are found inTable18.1. Give the balanced cell equation and determine %° for the galvanic cells based on the following half-reactions. Standard reduction potentials are found in Table 18.1. 

The reaction can be written as the difference of two half-reactions whose reduction potentials are listed in Table 19-1. The nE° values can be calculated as standard reduction potentials so long as one is reversed to an oxidation. The reversal of the reaction results in the opposite sign for the potential. 

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 ... 

Half-cell reaction Standard reduction potential, E9 (in volts)... 

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. 

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... 

Standard Reduction Potentials for Several Biological Reduction Half-Reactions ... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

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... 

Standard potentials are also called standard electrode potentials. Because they are always written for reduction half-reactions, they are also sometimes called standard reduction potentials. 

In addition to defined standard conditions and a reference potential, tabulated half-reactions have a defined reference direction. As the double arrow in the previous equation indicates, E ° values for half-reactions refer to electrode equilibria. Just as the value of an equilibrium constant depends on the direction in which the equilibrium reaction is written, the values of S ° depend on whether electrons are reactants or products. For half-reactions, the conventional reference direction is reduction, with electrons always appearing as reactants. Thus, each tabulated E ° value for a half-reaction is a standard reduction potential. 

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. 

First, identify the half-reactions. Then look up the standard reduction potentials in a table, and subtract the more negative value from the more positive value. 

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. 

This is a quantitative calculation, so it is appropriate to use the seven-step problem-solving strategy. We are asked to determine an equilibrium constant from standard reduction potentials. Visualizing the problem involves breaking the redox reaction into its two half-reactions ... 

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. 

They are the basis of many products and processes, from batteries to photosynthesis and respiration. You know redox reactions involve an oxidation half-reaction in which electrons are lost and a reduction half-reaction in which electrons are gained. In order to use the chemistry of redox reactions, we need to know about the tendency of the ions involved in the half-reactions to gain electrons. This tendency is called the reduction potential. Tables of standard reduction potentials exist that provide quantitative information on electron movement in redox half-reactions. In this lab, you will use reduction potentials combined with gravimetric analysis to determine oxidation numbers of the involved substances. 

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... 

If the sign of the standard reduction potential, E°, of a half-reaction is positive, the half-reaction is the cathodic (reduction) reaction when connected to the standard hydrogen electrode (SHE). Half-reactions with more positive E° values have greater tendencies to occur in the forward direction. Hence, the magnitude of a halfcell potential measures the spontaneity of the forward reaction. 

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. 

It is not possible to prepare F2 by electrolysis of an aqueous NaF solution. In electrolysis, the most easily oxidized and reduced species are the ones involved. To prepare F2, the oxidation of F would have to occur. However, water is more easily oxidized than is F, as seen by its position in the standard reduction potential chart (Appendix J and below). By inspection, H20 is a stronger reducing agent than F because the reduction half-reaction has a less positive E°. So H20 s oxidation is preferable to F s oxidation. F2 can be prepared from molten NaF, but not aqueous NaF. 

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. 

Since one half-reaction must involve oxidation, we must reverse one of the half-reactions shown in the table of reduction potentials in order to indicate the oxidation. If we reverse the half-reaction, we must also reverse the sign of the standard reduction 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 result of all of this is what has come to be known as the table of standard reduction potentials. An abbreviated such table is given here as Table 14.1. In this table, the half-reactions are listed on the left and the numerical values for each are listed on the right. The heading for the right-hand column is E°, the symbol for standard reduction potential. Notice that all half-reactions are written as reductions and that the half-reaction 2H+ + 21 H2 is the reference half-reaction (0.0000 V). 

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. ... 

Step 2 Locate the half-reactions in a table of standard reduction potentials, such as Table 14.1, and write the E° values adjacent to the respective half-reactions. For the oxidation half-reaction, the sign of the E° must he changed, since the reaction is written in reverse. 

The standard reduction potential for this half-reaction (from Table 14.1) is +0.22233 V. The potential is dependent only on the [CT], as was the potential of the SCE, and once again [CT] is constant because the solution is saturated. Thus this electrode is also appropriate for use as a reference electrode. 

The standard reduction potential, symbolized by E°, is a number reflecting the relative tendency of a reduction half-reaction to occur. In a table of standard reduction potentials, such as Table 14.1, those half-reactions at the top have positive numerical values and a strong tendency to occur. Those near the bottom have negative values and a tendency to go in the reverse direction. 

The half-reactions and their standard reductions potentials are supplied on the exam, not in the problem as given here. You will be expected to find the appropriate half-reactions in a table. 

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

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. 

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. 

As shown above, you can obtain the standard oxidation potential from a table of standard reduction potentials by reversing the reduction halfreaction, and changing the sign of the relevant potential. The reduction and oxidation half-reactions for the previous example are as follows. 

---