The direction of spontaneous change

To understand the processes occurring in organisms, we need to identify the factors 

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The development presented in Example 17.6 is an important one from a practical standpoint It tells us the temperature at which the direction of spontaneity changes. In the reduction of Fe203 by hydrogen, this temperature is approximately 700 K. At lower temperatures, the reaction does not occur at standard conditions recall from Example 17.5 that AG° at 500 K is +27.6 kj. At temperatures above 700 K, AG° has a negative sign and the reaction... 

As we have already emphasized, to use the entropy to judge the direction of spontaneous change, we must consider the change in the entropy of the system plus the entropy change in the surroundings ... 

FIGURE 7.24 At constant temperature and pressure, the direction of spontaneous change is toward lower Gibbs free energy. The equilibrium state of a system corresponds to the lowest point on the curve. 

Show that, if two copper blocks with different temperatures are placed in contact, then the direction of spontaneous change is toward the equalization of temperatures. Do so by considering the transfer of 1 J of energy as heat from one to the other and assessing the sign of the entropy change. Assume that the temperatures of the blocks remain constant. 

A gas in the frictionless, three-dimensional equivalent to the apparatus of the figure would tend to expand spontaneously. For a film, however, the direction of spontaneous change is contraction. 

FIGURE 7.1 The direction of spontaneous change is for a hot block of metal (top) to cool to the temperature of its surroundings (bottom). A block at the same temperature as its surroundings does not spontaneously become hotter. 

FIGURE 7.2 The direction of spontaneous change is for a gas to fill its container. A gas that already fills its container does not collect spontaneously in a small region of the container. A glass cylinder containing a yellow gas (upper piece of glassware in the top illustration) is attached to an empty flask. When the stopcock between them is opened, the yellow gas fills both upper and lower vessels (bottom illustration). The yellow gas is nitrogen dioxide. 

The minus sign in this equation means that an increase in total entropy corresponds to a decrease in free energy. Therefore, at constant pressure and temperature, the direction of spontaneous change is the direction of decreasing free energy (Fig. 7.23). 

Equation 16 summarizes the factors that determine the direction of spontaneous change at constant temperature and pressure we look for... 

Since some spontaneous reactions are exothermic and others are endothermic, enthalpy alone can t account for the direction of spontaneous change a second factor must be involved. This second thermodynamic driving force is nature s tendency to move to a condition of maximum randomness or disorder (Section 8.13). 

The second law, however, provides a clear-cut criterion of spontaneity. It says that the direction of spontaneous change is always determined by the sign of the total entropy change ... 

When A// and AS have opposite signs (classes 1 and 4), they act in the same direction, so the direction of spontaneous change does not depend on temperature. When Aff and AS have the same signs (classes 2 and 3), their effects oppose one another, so changes in temperature can cause one factor or the other to dominate, and spontaneity depends on temperature. For class 2, decreasing the temperature decreases the importance of the ur a-vorable T AS term, so the reaction becomes product-favored at lower temperatures. For class 3, increasing the temperature increases the importance of the favorable T AS term, so the reaction becomes product-favored at higher temperatures. 

Given sufficient time, chemical substances in contact with each other tend to come to chemical equilibrium. Chemical equilibrium is the time-invariant, most stable state of a closed system (the. state of minimum Gibbs free energy). We study chemical equilibrium concepts so as to learn the direction of spontaneous change of chemical reactions in any system, especially for conditions of constant temperature and pressure. We want to be able to compute the hypothetical equilibrium stale of a system. We would like to predict the conditions for equilibrium in different systems and at different temperatures and pressures without having to measure them. 

We need to understand the concepts of Gibbs free enei gy (G) and chemical potential i/i) in order to know the direction of spontaneous change of a reaction or system. These concepts can also be used to define or predict the most stable (equilibrium) assemblage and gas, fluid, or rock compositions expected in a system at a given pressure and temperature. Some phases and aqueous species in a system may be out of equilibrium with that system. Free-energy calculations permit us to decide which substances are out of equilibrium, and, therefore, which concentrations may be governed by chemical kinetics. 

Which way is the direction of spontaneous change for this system What would be the value of AG if the given data were equilibrium pressures What would be the value of Qp in that case ... 

What is the direction of spontaneous change in all concentration cells ... 

The free energy of a reaction is a measure of the tendency for a chemical reaction to take place (see Section S3.2). The direction of spontaneous change in a process is that of decreasing free energy. If... 

What is the direction of spontaneous change in all concentration eells 19.37 (a) The total charge passing through the circuit is... 

In other words Gibbs saw that the calculated change in entropy for the system and surroundings predicted the direction of spontaneous change in any chemical reaction. Thus with pencil and paper— and not one drop of solution or sweat— we can calculate whether a laboratory or industrial reaction should occur. 

The ability to reverse the direction of spontaneous change in nature is a common theme in movies. Describe a scene from a movie or television show that fits this description. 

Gibbs free energy (G) (10.6) A thermodynamic state function that can be used to predict the direction of spontaneous change at constant temperature and pressure. AG = AH - TAS, and AG < 0 for any spontaneous process. 

Reaction quotient (Q) (12.3) Expression identical in form to the equilibrium constant, but in which the concentrations do not correspond to equilibrium values. Comparison of the reaction quotient to the equihbrium constant predicts the direction of spontaneous change. 

Equation 8.30 provides an extremely useful criterion for determining the direction of spontaneous changes and the nature of physical and chemical equilibrium, hi addition, the changes in the Gibbs free energy function enable us to determine the amount of work that can be done in a process at constant temperature and pressure. To see this, start with the expression for AG for a process at constant temperature and pressure (Equation 8.29) ... 

State the first law of thermodynamics in terms of (a) the energy of the universe (b) the creation or destruction of energy (c) the energy change of system and surroundings. Does the first law reveal the direction of spontaneous change Explain. 

Many of those who found themselves unable to accept chemical oscillation as a reality based their refusal on the Second Law of Thermodynamics. The power of the Second Law lies in its ability to predict the direction of spontaneous change from the deceptively simple condition that... 

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