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Thermochemistry explores the basic principles of energy changes in chemical reactions. Calorimetry is described as a tool to measure the quantity of heat involved in a chemical or physical change. Quantitative overviews of enthalpy and the stoichiometry of thermochemical equations are provided, including the concepts of endothermic and exothermic reactions. Standard conditions are defined to allow comparison of enthalpies of reactions and determine how the enthalpy change for any reaction can be obtained. Hess"s Law, which allows the enthalpy change of any reaction to be calculated, is discussed with illustrative examples. A presentation of bond energies and bond dissociation enthalpies is offered along with the use of bond enthalpy to estimate heats of reactions.

The determination of the energy of particles is called ‘calorimetry’ and the corresponding detectors are called calorimeters. The particle energy is deposited in a calorimeter through inelastic reactions leading to the formation of particle showers. The deposited energy is measured either through the charge generated by ionisation or through scintillation or Cherenkov light. Depending on the particle type initiating a shower one distinguishes electromagnetic calorimeters from hadronic calorimeters. In this chapter the formation of showers for both cases is explained and the corresponding construction principles are discussed. For hadron calorimeters special attention is given to the different response to electromagnetically and hadronically deposited energy and the possible compensation of invisible energy. This is followed by a description of typical implementations of electromagnetic and hadronic calorimeters as well as of systems combining both types. Special emphasis is given to the discussion of the energy resolution of the different detectors and detector systems.

Equilibrium isotopic fractionations are best understood in terms of reactions that involve the transfer of isotopes between two phases or molecular species that have a common element (M). These isotopic exchange reactions may be written in one of several standard forms, such as where AM_b and BM^d represent the chemical formulas of the phases or species, AM^*b and BM^*d represent the same phases or species in which the trace isotope has replaced some or all of the atoms of element M, and a, b, c, and d are stoichiometric coefficients. In the case where all of the molecules are homogeneous, that is, where AMb and BMd are composed solely of the common isotope of M, and where AM^*_b and BM^*_d are phases or species in which the trace isotope M* has replaced all atoms of element M, then the product a × b equals c × d and represents the total number of atoms exchanged in the reaction. The concept of the isotopic exchange reaction is best shown by an example. Consider the exchange of deuterium between water and hydrogen gas. This may be written as a reaction among isotopically homogeneous molecules; that is, or, alternatively, as exchange between homogeneous and heterogeneous molecules: Much of the utility of isotopic exchange reactions is that they may be described by equilibrium constants, defined in the standard way as the quotient of the activities of the products and reactants. Thus, the equilibrium condition for equation 2.2b becomes where K is the equilibrium constant. In equation 2.3, K has a particularly high value of 3.7 at 25°C.

The important postulate that intermolecular interactions are independent of extent of deformation leads directly to the conclusion that such interactions cannot contribute to an energy of elastic deformation ΔEel at constant volume. In the earliest theories of rubberlike elasticity, it was additionally assumed that, intramolecular contributions to ΔEel were likewise nil. In this idealization that the total ΔEel is zero, the elastic retractive force exhibited by a deformed polymer network would be entirely entropic in origin. At the molecular level, this would correspond, of course, to assuming all configurations of a network chain to be of exactly the same conformational energy and thus the average configuration to be independent of temperature. Under these circumstances, the dependence of stress on temperature is strikingly simple, as shown, for example, by the equation . . . f* = υkT/V (〈r2〉i/〈r2〉0)(α – α-2) . . . . . . (9.1) . . . that characterizes a polymer network in elongation where, it should be recalled, 〈r2〉i3/2 is proportional to the volume of the network. This additional assumption that 〈r2〉0 is independent of temperature would lead to the prediction that the elastic stress determined at constant volume and elongation α is directly proportional to the absolute temperature. Such network chains would be akin to the particles of an ideal gas, which would obey the equation of state p = nRT(1/V) and thus exhibit a pressure at constant deformation (1/V) likewise directly proportional to the temperature.

As was mentioned in chapter 10, end-linking reactions can be used to make networks of known structures, including those having unusual chain-length distributions. One of the uses of networks having a bimodal distribution is to clarify the dependence of ultimate properties on non-Gaussian effects arising from limited-chain extensibility, as was already pointed out. The following chapter provides more detail on this application, and others. In fact, the effect of network chain-length distribution, is one aspect of rubberlike elasticity that has not been studied very much until recently, because of two primary reasons. On the experimental side, the cross-linking techniques traditionally used to prepare the network structures required for rubberlike elasticity have been random, uncontrolled processes, as was mentioned in chapter 10. Examples are vulcanization (addition of sulfur), peroxide thermolysis (free-radical couplings), and high-energy radiation (free-radical and ionic reactions). All of these techniques are random in the sense that the number of cross-links thus introduced is not known directly, and two units close together in space are joined irrespective of their locations along the chain trajectories. The resulting network chain-length distribution is unimodal and probably very broad. On the theoretical side, it has turned out to be convenient, and even necessary, to assume a distribution of chain lengths that is not only unimodal, but monodisperse! There are a number of reasons for developing techniques to determine or, even better, control network chain-length distributions. One is to check the “weakest link” theory for elastomer rupture, which states that a typical elastomeric network consists of chains with a broad distribution of lengths, and that the shortest of these chains are the “culprits” in causing rupture. This is attributed to the very limited extensibility associated with their shortness that is thought to cause them to break at relatively small deformations and then act as rupture nuclei. Another reason is to determine whether control of chain-length distribution can be used to maximize the ultimate properties of an elastomer. As was described in chapter 10, a variety of model networks can be prepared using the new synthetic techniques that closely control the placements of crosslinks in a network structure.

There are a variety of biopolymeric materials which exhibit rubberlike elasticity. This is perhaps to be expected when one recalls that most biopolymers are randomly coiled chains with considerable flexibility, and that they are frequently covalently cross-linked or have sufficient numbers of aggregated units to exist in network structures. One very large group of plant materials, the polysaccharides, are in this category, and they do require some elastomeric properties in their functioning. In many of these cases, however, the cross-linking is there primarily for a secondary purpose, such as preventing solubility. When swollen with water or aqueous solutions, such polysaccharides form gels which do exhibit the high deformability and recoverability that are the hallmarks of rubberlike elasticity. Not surprisingly, however, relatively few mechanical property measurements have been carried out to characterize the structures of these gels. The bioelastomers occurring in animals, including vertebrates and mammals, however, are there specifically for their rubberlike elasticity. They are vital, for example, for the functioning of skin, arteries and veins, and much of the lung and heart tissue. Since they are produced by the ribosome “factories” in the body, they are proteins. Thus, the major focus of this chapter is on those proteins specifically designed to function as bioelastomers. It is useful to summarize some general information on bioelastomers that is presented elsewhere. Even with the temporary restriction to bioelastomers which are proteins, there is an almost staggering variety of interesting materials. For example, there is elastin in vertebrates (including mammals) resilin in insects abductin in mollusks, arterial elastomer in octopuses, circulatory and locomotional proteins in cephalopods, and viscid silk in spider webs. Since they are mammals, polymer scientists and engineers who are interested in bioelastomers have focused heavily on elastin! Any materials of this type, however, are worth studying in their own right, to learn more about rubberlike elasticity and biological function. Such studies should also provide guidance on how Nature might be mimicked by synthetic chemists, to produce better nonbiological elastomers.