Reversible heat

Figure 17.4 (A) Photoluminescence spectral shifts (AX,) of a solution of CdSe quantum dot aggregates during heating-cooling cycles photoluminescence spectral maxima were recorded at 298 K during cooling and 353 K during heating. Reversibility of the photoluminescence spectral shift was attained after four heating-cooling cycles. (B) Photoluminescence spectra of a solution ofCdSe...

If the system is heated reversibly, the change in the surroundings is equal and opposite in sign to that for the system, and... 

Hexagonal crystal density 1.88 g/cm decomposes on heating reversibly converts to an amorphous dUiydrate when heated at 88°C at 2 torr soluble in water (13% at 45°C), soluble in alcohols (17% in methanol and 31% in ethylene glycol by weight). 

The existence of photoreversible, but not of heat-reversible, absorbance change in irradiated poly dI dC was taken to prove that the photoproducts are entirely dimers (in contrast to those in poly C irradiations where the product is almost entirely the hydrate82a). It was possible to detect dimers of uracil as well as those of cytosine, by means of the much slower photoreversal of uracil dimers. In the acid hydrolysates of irradiated dl-dC, both uracil dimers and uracil could be identified. Enzymatic hydrolysis (snake venom phosphodiesterase) does not split pyrimidine dimers, and the products of such hydrolysis of irradiated tritium-labeled poly dl dC contained trinucleotides shown by radioactivity to contain cytosine dimers. Thymine dimers were formed in the photolysis of the poly dA dT, and were detected and assayed by the same methods. The yield of thymine dimers in irradiated poly... 

In conclusion, entropy is the physical quantity that represents the capacity of distribution of energy over the energy levels of the individual constituent particles in the system. The extensive variable entropy S and the intensive variable the absolute temperature Tare conjugated variables, whose product TdS represents the heat reversibly transferred into or out of the system. In other words, the reversible transfer of heat into or out of the system is always accompanied by the transfer of entropy. 

In a closed system a reversible process creates no entropy so that any change dS in entropy is caused only by an amount dQm of heat reversibly transferred from the surroundings as shown in Eqs. 3.8 and 3.9 ... 

Brighton, 23rd-25th April 1996, p.379-90. 42C382 HEAT REVERSION EFFECTS IN EXTRUDED PVC-U PROFILES Haworth B Stephenson R C Loughborough,University European Vinyls Corp.(UK)Ltd. 

At the same time, the heat source being at the same temperature and having lost an equal amount of heat reversibly has also lost an equal amount of entropy. 

As an example we would try to conceive of a Carnot s cycle without using ideal gas as the system. Let the container in Fig. 4.1 contain water and steam in equilibrium instead of an ideal gas. Here pressure would be 1 atmosphere if temperature was 100 °C. If heat is supplied to the system, more of water would get converted to steam. It is easy to conceive that, if heat is transferred infinitesimally slowly, heat transfer can be carried out in thermodynamically reversible manner with corresponding increase in volume. By releasing pressure, also in a reversible manner, more of water gets vaporised and volume increases further to point C (Fig. 6.6) at reduced pressure and temperature. Thereafter the system can be made to lose heat reversibly at the lower temperature, which would make some steam to condense to liquid water with reduction in volume. 

The viscosities of solutions of xanthan, unlike those of all other water-soluble polymers, do not decrease as the temperature is raised. They are unaffected by temperature between 0 °C and 100 °C. And even though it is an anionic polymer, its solution viscosity is not affected by pH or most salts, i. e., although ionic, it behaves as if it were a neutral gum. Xanthan interacts synergistically with guar gum and locust bean gum (both galactomannans) (O Sect. 4.2). An increase in solution viscosity results from its interaction with guar gum. Heat-reversible gelation results from its interaction with locust bean gum. 

Type 11 gels are heat-reversible, being held together by intermolecular bonds such as hydrogen bonds. Sometimes bridging by additive molecules can take place in these type 11 systems. Polyfvinyl alcohol) solutions gel on... 

Suppose 60.0 g hydrogen bromide, HBr(g), is heated reversibly from 300 to 500 K at a constant volume of 50.0 L, and then allowed to expand isothermally and reversibly until the original pressure is reached. Using Cp(HBr(g)) = 29.1 J... 

One mole of a monatomic ideal gas begins in a state with P = 1.00 atm and T = 300 K. It is expanded reversibly and adiabatically until the volume has doubled then it is expanded irreversibly and isothermally into a vacuum until the volume has doubled again and then it is heated reversibly at constant volnme to 400 K. Finally, it is compressed reversibly and isothermally until a final state with P = 1.00 atm and T = 400 K is reached. Calculate AS for this process. Hint There are two ways to solve this problem—an easy way and a hard way.)... 

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