Entropy spontaneous change, direction

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 ... 

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. 

The second law of thermodynamics introduces a new function of state, the entropy, S, in order to quantify the spontaneity and direction of change for natural systems... 

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). 

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 ... 

In contrast to the conservation of internal energy (Eq. 2.1, the first law of thermodynamics), the entropy of the Universe always increases (Eq. 2.5), which is an alternative definition of the second law of thermodynamics. Inherent in the concept of entropy is a preferred direction for spontaneous change (AS rr > 0). For example, at 1 bar pressure, ice melts at 10°C, water freezes at —10°C, and not vice versa. A spontaneous process leads from a state of lower probability to a state of higher probability, and equilibrium is the state of maximum probability (Pitzer, 1995). 

We predict the direction of a spontaneous change from the second iaw of thermodynamics a spontaneous change occurs in the direction that increases the entropy of the universe (system plus surroundings). In other words, a change occurs spontaneously if the energy of the universe becomes more dispersed. 

The standard entropy of reaction, AS°xn, is calculated from S° values. When the amount (mol) of gas (AOgas) increases in a reaction, usually ASfxn > 0- The value of ASsurr is related directly to AH ys and inversely to the T at which the change occurs. In a spontaneous change, the entropy of the system can decrease only if the entropy of the surroundings increases even more. For a system at equilibrium, ASuniv = 0-... 

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 second law of thermodynamics is a revolutionary physical law which applied properly in all branches of sciences. It is exactly well-matched on the direction where the reaction changes spontaneously. This states that the entropy of the universe is continuously increases for a spontaneous change, i.e.. 

AS3, is related directly to SHly, and inversely to the T at which a change occurs. In a spontaneous change, the entropy of the system can decrease only if the entropy of the surroundings increases even more, so that > 0. 

The consequence of the negative entropy change can be found in the familiar rules of thermodynamics. Nature prefers disordered systems of high entropy to ordered systems of low entropy. So the direction of spontaneous change for a system is towards the state of higher entropy. To be more precise, the direction of spontaneous change is towards the state of lower free energy ... 

Processes An Introduction— A process that proceeds without external intervention is said to be a spontaneous process. A nonspontaneous process cannot occur without external intervention. If a process is spontaneous in one direction, then it is nonspontaneous in the reverse direction. Some spontaneous processes are exothermic, and others are endothermic, so the criterion for spontaneous change cannot be based on enthalpy changes alone. The direction of spontaneous change involves changes in another property called entropy. Entropy provides a measure of the number of ways a given quantity of energy can be dispersed, or distributed, among the particles of the system. 

Using Enthalpy and Entropy Changes to Predict the Direction of Spontaneous Change... 

Chapter 13 is a significant revision of Chapter 19 from the tenth edition. It introduces the concept of entropy, the criteria for predicting the direction of spontaneous change, and the thermodynamic equilibrium condition. In Chapters 14-19, we apply and extend concepts introduced in Chapter 13. However, Chapters 14 19 can be taught without explicitly covering, or referring back to. Chapter 13. 

It is more common to find that AH° and AS° have the same sign (Table 17.2, III and IV). When this happens, the enthalpy and entropy factors oppose each other. AG° changes sign as temperature increases, and the direction of spontaneity reverses. At low temperatures, AH° predominates, and the exothermic reaction, which may be either the forward or the reverse reaction, occurs. As the temperature rises, the quantity TAS° increases in magnitude and eventually exceeds AH°. At high temperatures, the reaction that leads to an increase in entropy occurs. In most cases, 25°C is a low temperature, at least at a pressure of 1 atm. This explains why exothermic reactions are usually spontaneous at room temperature and atmospheric pressure. 

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