Physical change, enthalpy

This chapter introduces the first law of thermodynamics and its applications in three main parts. The first part introduces the basic concepts of thermodynamics and the experimental basis of the first law. The second part introduces enthalpy as a measure of the energy transferred as heat during physical changes at constant pressure. The third part shows how the concept of enthalpy is applied to a variety of chemical changes, an important aspect of bioenergetics, the use of energy in biological systems. 

TABLE 6.3 Substance Standard Enthalpies pf Physical Change Freezing Formula point (I<) (kj-rnpl ) Boiling point (K) AHV P° (kj-mol )... 

You just learned about the enthalpy changes that are associated with phase changes. Another type of physical change that involves a heat transfer is dissolution. When a solute dissolves in a solvent, the enthalpy change that occurs is called the enthalpy of solution, Affsoin- Dissolution can be either endothermic or exothermic. 

In section 5.1, you learned about the energy changes that accompany physical changes, chemical reactions, and nuclear reactions. You learned how to represent energy changes using thermochemical equations and diagrams. In the next section, you will determine the enthalpy of a reaction by experiment. 

The same sorts of rules apply to enthalpy changes listed for chemical changes and physical changes. Here s a summary of the rules that apply to both ... 

To avoid the use of the ambiguous term "heat" in connection with "heat content," it is customary to use the term enthalpy. At a given temperature and pressure, every substance possesses a characteristic amount of enthalpy (H), and the heat changes associated with chemical and physical changes at constant pressure are called changes in enthalpy (AH) AHT is the enthalpy of transition. Two common enthalpies of transition are AHf = 1435 cal/mole for the enthalpy of fusion (melting) of ice at 0°C, and AH, = 9713 cal/mole for the enthalpy of vaporization of water at 100°C. 

FIGURE 6.7 (a) The altitude of a location on a mountain is like a state property it does not matter what route you take between two points, the net change in altitude is the same, (b) Enthalpy is a state property if a system changes from state A to state B (as depicted highly diagrammatically here), the net change in enthalpy is the same whatever the route—the sequence of chemical or physical changes— between the two states. 

Two factors determine the spontaneity of a chemical or physical change in a system a release or absorption of heat (AH) and an increase or decrease in molecular randomness (AS). To decide whether a process is spontaneous, both enthalpy and entropy changes must be taken into account ... 

In this example the standard heat of formation of H20 is available for its hypothetical standard state as a gas at 25°C. One might expect the value of the heat of formation of water to be listed for its actual state as a liquid at 1 bar or l(atm) and 25°C. As a matter of fact, values for both states are given because they are both frequently used. This is true for many compounds that normally exist as liquids at 25°C and the standard-state pressure. Cases do arise, however, in which a value is given only for the standard state as a liquid or as an ideal gas when what is needed is the other value. Suppose that this, were the case for the preceding example and that only the standard heat of formation of liquid H20 is known. We must now include an equation for the physical change that transforms water from its standard state as a liquid into its standard state as a gas. The enthalpy change for this physical process is the difference between the heats of formation of water in its two standard states ... 

The value of AH for this process cannot be obtained readily by measurement in a calorimeter. We will show next how to calculate AH for chemical reactions and physical changes by using standard enthalpies of formation. 

Energy, Enthalpy, and Entropy Changes Involving Ideal Cases and Physical Changes... 

The designation LiCl(12H20) means that the product is a solution of 1 moll LiCl in 12 moles of H2O. The enthalpy change accompanying this proces 25°C and 1 bar is AH = -33,614 J. That is, a solution of 1 mole of LiCl in 121 of H2O has an enthalpy 33,614 J less than that of 1 mole of pure LiCl(s) 12 moles of pure H20(/). Equations for physical changes such as this are rea... 

It should be noted that control is achievable over the flow rate, the concentration and to a lesser extent over the rate constant, the latter by changing the temperature. However, it is not possible to alter the enthalpy, assuming that it has no temperature dependence (it is possible, in some circumstances, to magnify the enthalpy by careful choice of an appropriate buffer system). The changes in the concentration and the rate constant must also satisfy the requirement that the mechanism of reaction is not affected as a consequence. Changing the flow rate will have no impact on the mechanism since it is purely a physical change to the system. 

Overall, IGC appears to be a reproducible method for following the chemical and physical changes that occur when coals are heated in an inert atmosphere. Differences in the transition temperature and enthalpies of sorption can be observed for coals of various rank. PyHRMS results indicate that the minor transitions observed in the intermediate temperature region are a result of the loss of volatile matter from the coal. 

The quantity of heat transferred into or out of a system as it undergoes a chemical or physical change at constant pressure, q, is defined as the enthalpy change, Aff, of the process. 

T represents the desired function, providing the information about the stability constant Ks. AH is the enthalpy of reaction, R the gas constant and T the (absolute) temperature. The chemical shift expressed by ccomplex has to be recorded instantaneously as a relaxation signal, since only then does it reflect exclusively the chemical response (proceeding with its characteristic time constant), and can clearly be separated from other - physical — changes, brought about by the rapid temperature alternation. In Figs. 6 and 7 the quantity T, to be determined by such experiments is plotted as a function of the total standard concentration (at fixed sample concentration). These relaxational titration plots are to be seen in comparison with the well-known classical titration curves and with the derivatives (Figs. 8 and 9), which contain more detailed information about complex stabilities. 

Enthalpy (H) An extensive property of a substance that can be used to obtain the heat absorbed or released by a chemical reaction or physical change at constant pressure. It is defined as the sum of the internal energy (U) and the product of the pressure and the volume of the system (PV) H = U + PV. 

Now let us apply the idea of enthalpy changes to two common processes, the first involving a physical change, the second a chemical change. 

DSC instruments are sensitive pieces of modem equipment, having the capability to measure heat flows of the order of microwatts. This feature makes the applicability of the technique almost unlimited every physical change or chemical reaction takes place with a change of enthalpy and consequently absorption or release of heat. 

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