Cell potential nonstandard conditions

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. 

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

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 ... 

The Nernst equation is also used to calculate the electrode potential for a given half-cell at nonstandard conditions. For example, for the half-cell Fe " " - - e Fe " " which has an = 0.77 V and n= the Nemst equation would be ... 

Cell Potentials Measuring Cell Potential Standard Reduction Potentials 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 ... 

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... 

The cell potential at 25°C under nonstandard-state conditions is given by the Nernst equation ... 

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. 

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 ... 

Nernst equation An equation that correlates chemical energy and the electric potential of a galvanic cell or battery. Links the actual reversible potential of an electrode (measured in volts), E, at nonstandard conditions of concentration or pressure, to the standard reversible potential of the electrode couple, EO, which is a thermodynamic value. The Nernst equation is named after the German physical chemist Walther Nernst. 

Calculating Cell Potentials under Nonstandard Conditions Using the Nernst Equation. 

Cell potential Potential difference, (,ell between reduction and oxidation half-cells may be at nonstandard conditions. 

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. 

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... 

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. 

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. 

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... 

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