Nonstandard conditions

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  

The electroplating industry is built upon the idea that metal coatinp on objects can provide useful properties, including corrosion resistance. 

Here Q is the reaction quotient we worked with in Chapter 12 a ratio of concentrations of reactants and products, each raised to the power of its stoichiometric coefficients. Thus for the generic reaction 

F is the Faraday constant, and n is the number of electrons transferred in the redox reaction. F, named after Michael Faraday, has a value of 96,485 J mol or 96,485 C moLh (Remember that 1 J = 1 C V.) Equation 13.3 bears a resemblance to Equation 12.7, which we used to derive the relationship between AG° and the equilibrium constant. We can apply the Nernst equation to estimate the potential of the electrochemical system in the corrosion of steel at more realistic concentrations. Example Problem 13.2 explores the use of the Nernst equation in an electrochemical cell that models corrosion. 

Suppose that you work for a company that designs the drive mechanisms for large ships. The materials in this mechanism will obviously come into contact with environments that enhance corrosion. To estimate the difficulties that corrosion 

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This method is suitable for processing routine orders however, where there are nonstandard conditions a more variable process needs to be developed. Providing you define the approach you intend to take in your procedures, you should be able to demonstrate that your methods provide an adequate degree of control. 

In cases where a lot of proplnt has successfully passed the one-year exposure to methyl violet paper, only a 3% inspection of the exposed test strips need to be made thereafter until such inspection indicates progressive deterioration of the proplnt or other nonstandard condition, at which time 100% inspection and test of the lot under suspicion will be resumed It is considered desirable that succeeding annual inspections include at least 1/3 of the containers included in the original 3% inspection as outlined above. In this manner, such containers may be considered as a basic comparative control with reference to the balance of the lot under test... 

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. 

This is a quantitative thermod mamics problem that asks us to determine entropy under nonstandard conditions using standard thermod3mamic data, so we apply the seven-step approach. The problem asks for two quantities the molar entropy of O2 gas in the pressurized tank and the entropy change when a sample of that gas undergoes a pressure change. 

In most laboratories, electrochemistiy is practiced under nonstandard conditions. That is, concentrations of dissolved solutes often are not 1 M, and gases are not necessarily at 1 bar. Recall from Chapter 14 that ZlG changes with concentration and pressure. The equation that links A G ° with free energy changes under nonstandard conditions is Equation AG = AG° + i 7 lng Here, Q is the reaction quotient. 

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 electrochemical series corresponds only to the standard condition, i.e., for unit activity of the ions, since a change to another ionic concentration can alter the order of the electrode potentials of the elements very markedly. The case of nickel plating mentioned earlier may be taken as typically illustrative of the many practical examples of the effects and the consequences of nonstandard conditions. It must also be mentioned in the context of the examples of displacement reactions provided earlier that the concentrations and the electrode potentials frequently vary during a displacement reaction. 

D) AG° represents the free energy at standard conditions 25°C and 1 atm pressure. AG represents the free energy at nonstandard conditions. In this problem, we have the nonstandard condition of 100°C. In order to solve for the free energy of this reaction, you must use the following equation ... 

C) The question concerns the effect of changing standard conditions of a cell to nonstandard conditions. To calculate the voltage of a cell under nonstandard conditions, use the Nernst equation... 

What is the mathematical relationship necessary to adjust the Gibbs free energy to nonstandard conditions. 

Thus far, we have considered only situations under standard conditions. But how do we cope with nonstandard conditions The change in Gibbs free energy under nonstandard conditions is ... 

From thermodynamic principles, chemists have demonstrated that the free energy change at nonstandard conditions, Ac , is related to the free energy change imder standard conditions, Ac °, by... 

The Nernst equation allows calculation of the cell em/under nonstandard conditions, E. If the cell reaction is given by a general equation aA+bB+... +/ e=AX+jY+... 

The density of any substance is mass divided by volume. For the ammonia sample, the mass is 0.672 g but the volume of the gas is given under nonstandard conditions and must first be converted to STP. Because the amount of sample n is constant, we can set the quantity PV/RT measured under nonstandard conditions equal to PV/RT at STP and then solve for V at STP. 

The amount of gas in the 1.000 L bulb under the measured nonstandard conditions would have a volume of only 0.884 L at STP. Dividing the given mass by this volume gives the density of ammonia at STP ... 

A strict approximation should take into consideration that in vivo conditions are usually nonstandard conditions, and the actual concentrations of the reactants (oxidants and antioxidants) are also important. This last situation will be considered in the subsequent kinetic analysis. 

After calculating < °, the Nemst equation can be used to adjust the potential to nonstandard conditions. 

Self, S. G., Liang, K.-Y., Asymptotic properties of maximum likelihood estimators and likelihood ratio tests under nonstandard conditions,/. Am. Statist. Assoc. 1987, 82 605-610. 

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 ... 

Be able to calculate free energies from equilibrium constants and redox potentials and do so under nonstandard conditions using the appropriate equations involving reactant and product concentrations at the beginning of the reaction. 

To calculate the new electrode potential for nonstandard condition, use the Nemst equation ... 

The Nernst potential (Vn) of Eq. 13 accounts for changes in the activity of the reaction and for nonstandard conditions,... 

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

Corrects standard electrode potentials for nonstandard conditions. 

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