The Macroscopic Perspective

This coherent picture involves three levels of understanding or perspectives on the nature of chemistry macroscopic, microscopic, and symbolic. By the end of this course, you should be able to switch among these perspectives to look at problems involving chemistry in several ways. The things we can see about substances and their reactions provide the macroscopic perspective. We need to interpret these events considering the microscopic (or particulate ) perspective, where we focus on the smallest components of the system. Finally, we need to be able to communicate these concepts efficiendy, so chemists have devised a symbolic perspective that allows us to do that. We can look at these three aspects of chemistry first, to provide a reference for framing our studies at the outset. 

When we observe chemical reactions in the laboratory or in the world around us, we are observing matter at the macroscopic level. Matter is anything that has mass and can be observed. 

We are so often in contact with matter that we tend to accept our intuitive feel for its existence as an adequate definition. 

When we smdy chemistry, however, we need to be aware that some of what we observe in nature is not matter. For example, light is not considered matter because it has no mass. 

We will discuss corrosion and its prevention in detail in Chapter 13. 

---

To visualize how electrochemical cells generate electrical potential differences, consider a zinc electrode dipped into a solution of zinc sulfate. From the macroscopic perspective, nothing happens. At the molecular level, however, some of the zinc atoms of the electrode are oxidized to ions ... 

Figure 19-11 illustrates the zinc equilibrium at the molecular level. At equilibrium, the charge imbalance in the zinc strip is about one excess electron for every 10 zinc atoms. This is negligible from the macroscopic perspective but significant at the molecular level. 

A dislocated crystal has a distinct geometric character from one that is dislocation-free. From both the atomistic and continuum perspectives, the boundary between slipped and unslipped parts of the crystal has a unique signature. Whether we choose to view the material from the detailed atomic-level perspective of the crystal lattice or the macroscopic perspective offered by smeared out displacement fields, this geometric signature is evidenced by the presence of the so-called Burgers vector. After the passage of a lattice dislocation, atoms across the slip plane assume new partnerships. Atoms which were formerly across from... 

This partial differential equation is often called the Fokker-Planck equation. The great benefit of the Fokker-Planck equation is that it enables the macroscopic perspective on the solutions of an SDE. 

The Study of Chemistry The Macroscopic Perspective The Microscopic or Particulate... 

The waU of a cellulose fiber in turn exhibits a complex, laminar structure, wherein each layer consists of smaller, unidirectional fibers, or microfibrils, in the range of 5 to 50 run wide, and anywhere from 100 run to several microns long, depending on the source [33]. Each layer of microfibrils varies with respect to fibril orientation (microfibril angle). Microfibrils in turn have a composite stracture, consisting of slender cellulose crystallites, or whiskers, with diameters on the order of 5 run, which are threaded together and embedded in the microfibrils between amorphous regions of cellulose and hemicellulose [34]. Hence, cellulose can be viewed as a composite material from the nanoscale perspective (whiskers in microfibrils), the microscopic perspective (microfibrils in fibers), and the macroscopic perspective (fibers in wood). 

From the macroscopic perspective, matter is continuous rather than composed of discrete particles. As already noted, this is expressed in the application of mathematical analysis in thermodynamics and its extensions. This conception of matter is reflected here in the distributive condition, which goes back to Aristotle who took a definite stand against the discrete, atomic conception of matter. This stand might seem anachronistic today. But classical atomism of the crude kind opposed within an Aristotelian tradition up to the end of the 19th century is not what modern theories based on quantmn mechanics support. And notions of same substance still derive, as we saw, from macroscopic theory. But there is an objection to the distributive condition which doesn t hinge on appeal to parts of water smaller than the smallest part which is water and raises the issue of the nature of mixture. 

In chemical education, the micro-world is a key perspective in viewing the macroscopic world around us. Among other perspectives, such as a personal, a socio-economical perspective, a political perspective, this key chemical perspective can offer students a view on the materials and substances they use, that they are made of and that constitute their material world. A micro-world perspective enables the understanding of mat r phenomena and offers the possibihty to improve the material world within social circumstances. 

Mass can be defined as the amount of matter in an object. In our experience of the macroscopic world, all ordinary objects have mass. Electromagnetic radiation, such as visible light, does not have mass. From a physics perspective, mass can be defined as the resistance of an object to acceleration. That is, if we want an object, which has a mass, to start moving, stop moving, or change direction, we have to give it a push. Thus, by simply rearranging Newton s second law we get ... 

Protein function at solid-liquid interfaces holds a structural and a dynamic perspective [31]. The structural perspective addresses macroscopic adsorption, molecular interactions between the protein and the surface, collective interactions between the individual adsorbed protein molecules, and changes in the conformational and hydration states of the protein molecules induced by these physical interactions. Interactions caused by protein adsorption are mostly non-covalent but strong enough to cause drastic functional transformations. All these features are, moreover, affected by the double layer and the electrode potential at electrochemical interfaces. Factors that determine protein adsorption patterns have been discussed in detail recently, both in the broad context of solute proteins at solid surfaces [31], and in specific contexts of interfacial metalloprotein electrochemistry [34]. Some important elements that can also be modelled in suitable detail would be ... 

We now turn attention to a completely different kind of supercritical fluid supercritical water (SCW). Supercritical states of water provide environments with special properties where many reactive processes with important technological applications take place. Two key aspects combine to make chemical reactivity under these conditions so peculiar the solvent high compressibility, which allows for large density variations with relatively minor changes in the applied pressure and the drastic reduction of bulk polarity, clearly manifested in the drop of the macroscopic dielectric constant from e 80 at room temperature to approximately 6 at near-critical conditions. From a microscopic perspective, the unique features of supercritical fluids as reaction media are associated with density inhomogeneities present in these systems [1,4],... 

In summary, starting with any food, one may wonder about the origin of its physical properties, as a food scientist or as any curious individual. One realizes that the macroscopic physical properties depend on the properties and mutual interactions of the micro structural units that are present. These properties and mutual interactions, in turn, depend on the molecules that build up the units. Thus, foods have made us think about the macroscopic scale, and how properties on this scale can be related to properties on a molecular scale. The line of reasoning can also be reversed. In fact, food molecules yield a large variety of different possible structures. The knowledge of how food molecules build up microstructural units and determine their properties can be used to build microstructural units with specific properties, leading to desired properties of the food itself, being it from a taste or health perspective (see Chapter 12). 

In the following sections, our first-principles bottom-up methodology will be described from a physical perspective (a full description on mathematical terms can be found in Ref. [7]). Then, the use of this methodology will be illustrated on the a-2-hydro nitronyl nitroxide [8] (hereafter called a-HNN), the simplest member of the nitronyl nitroxide family (R = H in Figure 1). The macroscopic magnetic properties and crystal structure of this compound are well known (its crystal structure is stored in the Cambridge Crystallographic Database [9] with refcode name TOLKEK). 

Certain pure ionic liquids started to be regarded as nanostructured fluids, with two distinct kinds of spatial domain one ionic, the other nonpolar. Besides their impact on the properties of pure ionic liquids, these two kinds of domain offer distinct environments to solutes, and therefore the concept of a nanostructured medium offers a novel perspective to interpret the macroscopic properties of ionic liquids, in particular their solvent behavior. 

---