Entropy change defined

An important question for chemists, and particularly for biochemists, is, Will the reaction proceed in the direction written J. Willard Gibbs, one of the founders of thermodynamics, realized that the answer to this question lay in a comparison of the enthalpy change and the entropy change for a reaction at a given temperature. The Gibbs free energy, G, is defined as... 

On the basis of the values of AS° derived in this way it appears that the chelate effect is usually due to more favourable entropy changes associated with ring formation. However, the objection can be made that and /3l-l as just defined have different dimensions and so are not directly comparable. It has been suggested that to surmount this objection concentrations should be expressed in the dimensionless unit mole fraction instead of the more usual mol dm. Since the concentration of pure water at 25°C is approximately 55.5 moldm , the value of concentration expressed in mole fractions = cone in moldm /55.5 Thus, while is thereby increased by the factor (55.5), /3l-l is increased by the factor (55.5) so that the derived values of AG° and AS° will be quite different. The effect of this change in units is shown in Table 19.1 for the Cd complexes of L = methylamine and L-L = ethylenediamine. It appears that the entropy advantage of the chelate, and with it the chelate effect itself, virtually disappears when mole fractions replace moldm . ... 

The second law of thermodynamics also consists of two parts. The first part is used to define a new thermodynamic variable called entropy, denoted by S. Entropy is the measure of a system s energy that is unavailable for work.The first part of the second law says that if a reversible process i f takes place in a system, then the entropy change of the system can be found by adding up the heat added to the system divided by the absolute temperature of the system when each small amount of heat is added ... 

Equation gives the total entropy change A — A jS ystem + A Fjmygmjgjggs We can define the HFC as the system and the contents of the refrigerator as the immediate surroundings ... 

In words, in any process that occurs at constant T and P, the free energy change for the system is negative whenever the total entropy change is positive that is, whenever the overall process is spontaneous. Defining a new function and imposing some restrictions provides a way to use properties of a system to determine whether a process is... 

In order to arrive ultimately at the entropy change accompanying deformation, we now proceed to calculate the configurational entropy change involved in the formation of a network structure in its deformed state as defined by a, ay, and (We shall avoid for the present the stipulation that the volume be constant, i.e., that axayag=l.) Then by subtracting the entropy of network formation when the sample is undeformed (ax = ay = az=l)j we shall have the desired entropy of deformation. As is obvious, explicit expressions will be required only for those terms in the entropy of network formation which are altered by deformation. 

Entropy can be described by considering a closed system undergoing a reversible process. The entropy change, dS, of the system is defined by the relationship... 

The limitation of the storage capacity is, as mentioned before, caused by the limitation of entropy change AS within the storage (see Figure 4). For sensible and latent heat storage (so-called direct thermal energy storage) this is defined by the specific heat... 

Response to the driving force is defined as the rate of change of the extensive parameter Xk, i.e. the flux Jk = (dXk/dt). The flux therefore stops when the affinity vanishes and non-zero affinity produces flux. The product of affinity and associated flux corresponds to a rate of entropy change and the sum over all k represents the rate of entropy production,... 

The sum is equal to zero for reversible processes, where the system is always under equilibrium conditions, and larger than zero for irreversible processes. The entropy change of the surroundings is defined as... 

Entropy is the ratio of a body s energy to its temperature according to the Clausius equality (as defined in the next section). For a reversible process, the change in entropy is defined by... 

The second law of thermodynamics defines the conditions for a feasible reaction. It states that for a reaction to be feasible, the total entropy change for a reaction system and its surroundings must be positive, that is ... 

The rate of change in di5 (defined as the time-dependent change in the entropy change within the system) can then be written as ... 

While the first law allows us to calculate the energy change associated with a given process, it says nothing about whether or not the process itself will take place spontaneously. This is the province of the second law of thermodynamics and leads to the introduction of another state function, entropy, S. The entropy change in a system which moves from state 1 to state 2 is defined by... 

In thermodynamics, entropy change is defined in a reversible process as... 

One of the approaches to calculating the solubility of compounds was developed by Hildebrand. In his approach, a regular solution involves no entropy change when a small amount of one of its components is transferred to it from an ideal solution of the same composition when the total volume remains the same. In other words, a regular solution can have a non-ideal enthalpy of formation but must have an ideal entropy of formation. In this theory, a quantity called the Hildebrand parameter is defined as ... 

Just as we can define a standard enthalpy of formation (AH°f) and a standard free energy of formation (AG°f), we can define an analogous standard entropy of formation (AS°f) as being the entropy change for formation of a substance in its standard state from its constituent elements in their standard states. Use the standard molar entropies given in Appendix B to calculate AS°f for the following substances ... 

The second law is associated with the direction of a process. It defines the fundamental property entropy, S, and states that in any real process the direction of the process corresponds to the direction in which the total entropy increases, that is, the entropy change of both the system and environment should in total result in a positive result or in equation form... 

The enthalpy and entropy changes of micellization have been calculated for benzenesulfonate and alkylammonium salts in low-polar solvents suggesting that micellization is essentially an enthalpy-driven effect. The aggregation can take place at low concentrations of surfactant and can have different aggregation numbers. The absence of well-defined critical micelle concentrations (CMC) for some systems in the low-polar solvents was observed (Kertes and Gutman, 1976). 

Likewise, the entropy changes are defined with reference to the temperature at which ASap is zero ... 

Suppose now that we decide to combine a secondfactor, in addition to the enthalpy change, AH0 in the search for an overall parameter (G) the change in which (AG°) will indicate to us whether a given reaction or process is likely to take place spontaneously. Suppose we call this second factor, the entropy change ( AS0) which will be based on the change in a function defined as entropy (S). Then, from the evidence above, it seems likely that ... 

Suppose that in a second experiment the piston is no longer frictionless, again the temperature is constant, now to achieve the same expansion more heat will be needed to overcome the frictional forces which oppose the expansion. So here q > qlev(q = <7rev + q ) but the work performed will be the same for the expansion process. qlev is a particular value of the heat absorbed by the gas and it is only this value which defines the entropy change, AS. This shows, as was seen in Frame 1, that q and w depend on the specific parameters of the experiment or path and are not state functions in contrast to the entropy change, AS. 

---