Cell potential nonstandard

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. 

Experiments involving the Nernst equation are primarily concerned with concentrations. One or more of the concentrations in the Q portion of the Nernst equation are calculated by measuring the nonstandard cell potential and comparing this to the standard cell potential. Remember, you calculate the concentration from a measured voltage. Once the concentration is determined, it may be combined with other concentrations and used to calculate an equilibrium constant. 

The Nernst equation gives the relationship between the cell potential and the concentrations of the cell components. This equation is used to determine the emf of a cell constructed from nonstandard electrodes. 

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

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

The Nernst equation allows us to plug in nonstandard concentrations to create Q and find the cell potential. 

Calculating Cell Potentials under Nonstandard Conditions Using the Nernst Equation. 

We can also find the cell potential for a nonstandard cell by first finding or the overall standard cell reaction, and then using the Nernst equation to correct for nonstandard concentrations. The next example illustrates this approach. 

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. 

To do so, let s derive an expression for the relation between cell potential and concentration based on the relation between free energy and concentration. Recall from Chapter 20 (Equation 20.13) that AG equals AG° (the free energy change when the system moves from standard-state concentrations to equilibrium) plus RT In Q (the free energy change when the system moves from nonstandard-state to standard-state concentrations) ... 

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... 

Electrolysis of Water and Nonstandard Half-Cell Potentials Before we can analyze the electrolysis products of aqueous salt solutions, we must examine the electrolysis of water itself. Extremely pure water is difficult to electrolyze because very few ions are present to conduct a current. If we add a small amount of a salt that cannot be electrolyzed in water (such as Na2S04), however, electrolysis proceeds rapidly. A glass electrolytic cell with separated gas compartments is used to keep the H2 and O2 gases from mixing (Figure 21.25). At the anode, water is oxidized as the O.N. of O changes from —2 to 0 ... 

Calculating the nonstandard cell potential (Nernst equation) (703) ... 

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 ... 

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

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 ... 

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... 

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