Redox reactions spontaneity under standard-state conditions

For each of the following redox reactions, (i) write the half-reactions (ii) write a balanced equation for the whole reaction, (iii) determine in which direction the reaction will proceed spontaneously under standard-state conditions ... 

This example illustrates how we can use the sign of the emf of a cell to predict the spontaneity of a redox reaction. Under standard-state conditions for reactants and products, the redox reaction is spontaneous in the forward direction if the standard emf of the cell is positive. If it is negative, the reaction is spontaneous in the opposite direction. It is important to keep in mind that a negative °en does not mean that a reaction will not occur if the reactants are mixed at 1 M concentrations. It merely means that the equilibrium of the redox reaction, when reached, will lie to the left. We will examine the relationships among AG°, and K later in this chapter. 

The relative stability of the oxidation states is extremely important and is usually discussed in terms of standard electrode potentials (Chapter 19). Standard electrode potentials can be used to establidi whether a redox reaction is feasible or spontaneous (under standard thermodynamic conditions). 

Spontaneity of Redox Reactions Under Standard-State Conditions... 

Standard Reduction Potentials Spontaneity of Redox Reactions Under Standard-State Conditions Spontaneity of Redox Reactions Under Conditions Other Than Standard State... 

The positive value of suggests that the oxidation of hydroquinone by soluble Fe should occur spontaneously xn6sr standard-state conAitxom. However, the standard-state activities of dissolved ions (in this case Fe ", Fe, and H" ) of unity correspond to concentrations that are absurdly high for soil solution (on the order of 1 molar). The standard-state potential, then, has little use in predicting the favorability of the reaction under conditions likely to prevail in soil solutions. It is necessary to use the Nemst equation (see Chapter 7) to calculate the adjusted redox potential, E, for more realistic reaction conditions ... 

The decrease in free energy of the system in a spontaneous redox reaction is equal to the electrical work done by the system on the surroundings, or AG = nFE. The equilibrium constant for a redox reaction can be found from the standard electromotive force of a cell. 10. The Nernst equation gives the relationship between the cell emf and the concentrations of the reactants and products under non-standard-state conditions. Batteries, which consist of one or more galvanic cells, are used widely as self-contained power sources. Some of the better-known batteries are the dry cell, such as the Leclanche cell, the mercury battery, and the lead storage battery used in automobiles. Fuel cells produce electrical energy from a continuous supply of reactants. 

The force of gravity always causes a diver to fall downward to a lower energy state, never upward to a higher energy state. When a diver steps off a diving board, his or her spontaneous motion is always downward. Similarly, in the zinc-copper cell, under standard conditions, copper(II) ions at the cathode accept electrons more readily than the zinc ions at the anode. Thus, the redox reaction occurs spontaneously only when the electrons flow from the zinc to the copper. 

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