System and Surroundings

We need to understand the unique and precise vocabulary of thermodynamics before applying it to the study of bioenergetics. 

An open system can exchange both energy and matter with its surroundings. A closed system is a system that can exchange energy but not matter with its surroundings. 

An example of an open system is a flask that is not stoppered and to which various substances can be added. A biological cell is an open system because nutrients and waste can pass through the cell wall. You and 1 are open systems we ingest, respire, perspire, and excrete. An example of a closed system is a stoppered flask energy can be exchanged with the contents of the flask because the walls may be able to conduct heat. An example of an isolated system is a sealed flask that is thermally, mechanically, and electrically insulated from its surroundings. 

If matter is not able to pass across the boundary, then the system is said to be closed otherwise, it is open. A closed system may still exchange energy with the surroundings unless the system is an isolated one, in which case neither matter nor energy can pass across the boundary. The tea in a closed Thermos bottle approximates a closed system over a short time interval. 

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The words system and surroundings are similarly coupled. A system is taken to be any object, any quantity of matter, any region, and so on, selected for study and set apart (mentally) from everything else, which is called the surroundings. The imaginary envelope which encloses the system and separates it from its surroundings is called the boundary of the system. 

The second item means that heat exchange between system and surroundings must occur at the temperature of the surroundings, presumed to constitute a heat reservoir at a constant and uniform temperature... 

Since heat transfer with respec t to the surroundings and with respect to the system are equal but of opposite sign, = —Q. Moreover, the second law requires for a reversible process that the entropy changes of system and surroundings be equalbut of opposite sign AS = —AS Equation (4-356) can therefore be written Q = TcAS In terms of rates this becomes... 

The total rate of entropy increase (in both system and surroundings) as a result of a process is... 

The law of conservation of energy states that energy ( ) can be neither created nor destroyed it can only be transferred between system and surroundings. That is,... 

Notice that the second law refers to the total entropy change, involving both system and surroundings. For many spontaneous processes, the entropy change for the system is a negative quantity. Consider, for example, the rusting of iron, a spontaneous process ... 

Work (w) Any form of energy except heat exchanged between system and surroundings includes expansion work and electrical work, 214... 

Finally, (c) is represented by any cyclic process occurring within an isolated system. With the system returning to its initial state, and with no opportunity for transfer of energy between system and surroundings, all energy changes in the process must add to zero. 

FIGURE 7.17 (a) In an exothermic process, heat escapes into the surroundings and increases their entropy, (b) In an endothermic process, the entropy of the surroundings decreases. The red arrows represent the transfer of heat between system and surroundings, and the green arrows indicate the entropy change of the surroundings. 

When calculating entropy changes, be careful about the sign of q, use the appropriate temperatures, and sum the changes for system and surroundings. 

In your own words, define the terms system and surroundings. Use an example. 

A classmate is having difficulty understanding how the concepts of system, insulated system, and surroundings are related to exothermic and endothemic reactions. Write a note to explain to your classmate how the concepts are related. Use diagrams to help clarify your explanation. 

The thermodynamic changes for reversible, free, and intermediate expansions are compared in the first column of Table 5.1. This table emphasizes the difference between an exact differential and an inexact differential. Thus, U and H, which are state functions whose differentials are exact, undergo the same change in each of the three different paths used for the transformation. They are thermodynamic properties. However, the work and heat quantities depend on the particular path chosen, even though the initial and final values of the temperature, pressure, and volume, respectively, are the same in all these cases. Thus, heat and work are not thermodynamic properties rather, they are energies in transfer between system and surroundings. 

Proved that in a reversible process net entropy change for the system and surroundings is zero. 

Already, you should be thinking to yourself But the particles in solids really don t move that mnch and you are certainly correct. They do move or translate in the liquid state of that same solid, however, and don t forget about rotation and vibration, which we will see in subsequent chapters can be very important in solids. But along this line of thinking, we can simplify the First Law of Thermodynamics, which in general terms can be written for a closed system (no transfer of matter between the system and surroundings) as... 

Figure 3.2 Final state of system and surroundings for the three paths (a)-(c) discussed in the text.

Work can be defined as the energy transferred between the system and surroundings. It is often expressed as a vector force acting through a vector displacement on the system boundaries ... 

ASt = total entropy change of system and surroundings Qi = heats from heat reservoirs except the surroundings at To ASi = entropy change of these reservoirs... 

It is informative to consider the same process and the same system and surroundings with the exception that the piston is made a part of the system. Part of the boundary is now defined as the upper surface of the piston rather than the lower surface. Equation (2.15) takes the form... 

The force exerted by the substance within the cylinder on the lower force of the piston under these conditions is the product of the pressure exerted by the substance on the surface of the piston and the area of the piston. Moreover, the product of the area and the differential displacement of the piston is equal to the differential change of volume. The integral J F dh is then equal to P dV. This relation is the only change that is made in Equation (2.15) or a similar equation for quasistatic processes. The frictional effects or the collisions result in a temperature increase either of the surroundings, or of both the system and surroundings as the case may be, or the effects may be interpreted in terms of heat, as discussed above. 

If the multi-criteria evaluation of energy system is introduced in this analysis, indicators which are reflecting all potential interaction of the system and surrounding must be also recognized. In this respect, the... 

Because it takes some practice to be able to use the recipes for calculating entropy changes in the system and surroundings, a few simple examples are presented here. 

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