CELL POTENTIALS UNDER NONSTANDARD CONDITIONS

Calculating Cell Potentials under Nonstandard Conditions Using the Nernst Equation. 

So far, we ve considered cells with all components in their standard states. But most cells don t start at those conditions, and even if they did, the concentrations change after a few moments of operation. Moreover, in all practical voltaic cells, such as batteries, reactant concentrations are far from standard-state values. Clearly, we must be able to determine Ecc h the cell potential under nonstandard conditions. 

Knowledge Required (1) Use of the Nemst Equation to calculate cell potential under nonstandard conditions. 

We calculate cell potentials under nonstandard conditions by using standard cell potentials and the Nernst equation. 

SECTION 20.6 Cell Potentials Under Nonstandard Conditions... 

CELL POTENTIALS UNDER NONSTANDARD CONDITIONS (SECTION 20.6) The emf of a redox reaction varies with temperature and with the concentrations of reactants and products. The Nernst equation... 

One thought you may have about corrosion is that we cannot expect standard conditions in real world applications. What must we do to account for the differences that arise when standard conditions are not present This is a very important question with clear ties to thermodynamics. The equation that describes cell potentials under nonstandard conditions is called the Nemst equation ... 

To determine E, the cell potential under nonstandard conditions, we ll derive an expression for the relation between and concentration based on the relation between AG and concentration. Recall from Chapter 20 (Equation 20.13) that... 

This is a quantitative problem, so we follow the standard strategy. The problem asks about an actual potential under nonstandard conditions. Before we determine the potential, we must visualize the electrochemical cell and determine the balanced chemical reaction. The half-reactions are given in the problem. To obtain the balanced equation, reverse the direction of the reduction half-reaction with the... 

Cell potentials under nonstandard-state conditions can be calculated using the Nernst equation,... 

The Nernst equation says that a cell potential under any conditions depends on the potential at standard-state concentrations and a term for the potential at nonstandard-state concentrations. How do changes in Q affect cell potential From Equation 21.9, we see that... 

The Nernst equation is widely used to estimate the emf of cells under nonstandard conditions. In biology it is used, among other things, to estimate the potential difference across biological cell membranes, such as those of neurons. 

Answer It is important to note that standard conditions do not exist in the cell. The value of AE °, as calculated in this problem, gives an indication of whether a reaction would or would not occur in a cell without additional energy being added (usually from ATP) but AE ° does not tell the entire story. The actual cellular concentrations of the electron donors and electron acceptors contribute significantly to the value of A 0 (e.g., see Problem 25). Under nonstandard conditions, the potential can either add to an already favorable AE ° or be such a large positive number as to overwhelm an unfavorable AE °, making AE favorable. 

Potential difference, Ecell, between oxidation and reduction half-cells under nonstandard conditions. 

In writing the equation this way, we have dropped the subscript cell to indicate that the calculated emf does not necessarily refer to a voltaic cell. Also, we have generalized the standard reduction potentials by using the general terms reduction and oxidation rather than the terms specific to voltaic cells, cathode and anode. We can now make a general statement about the spontaneity of a reaction and its associated emf, E A positive value of E indicates a spontaneous process a negative value of E indicates a nonspontaneous process. We use E to represent the emf under nonstandard conditions and E° to indicate the standard emf. 

I use standard reduction potentials to calculate cell potentials under both standard and nonstandard conditions. 

Since cell potential depends not only on the half-reactions occurring in the cell, but also on the concentrations of the reactants and products in those half-reactions, we can construct a voltaic cell in which both half-reactions are the same, but in which a difference in concentration drives the current flow. For example, consider the electrochemical cell shown in Figure 18.12 , in which copper is oxidized at the anode and copper ions are reduced at the cathode. The seeond part of Figure 18.12 depicts this cell under nonstandard conditions, with [Cu ] = 2.0 M in one half-cell and [Cu ] = 0.010 M in the other ... 

By convention, the potentials of all half-reactions, E°, are found tabulated for the reduction process under standard conditions of temperature (298.15 K), pressure (1 atm), and solute concentrations (1 molar). For nonstandard conditions, the reduction potentials, and hence the cell voltage, will differ. The concentration dependence on the cell voltage is given by the Nemst equation ... 

The Nernst equation allows determination of the cell potential for a galvanic cell at nonstandard conditions. Write out the Nernst equation. What are nonstandard conditions What do %, n, and Q stand for in the Nernst equation What does the Nernst equation reduce to when a redox reaction is at equilibrium What are the signs of AG° and when < 1 When > 1 When = 1 Explain the following statement % determines spontaneity, while determines the equilibrium position. Under what conditions can you use to predict spontaneity ... 

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