Macroscopic world

Computer simulations act as a bridge between microscopic length and time scales and tlie macroscopic world of the laboratory (see figure B3.3.1. We provide a guess at the interactions between molecules, and obtain exact predictions of bulk properties. The predictions are exact in the sense that they can be made as accurate as we like, subject to the limitations imposed by our computer budget. At the same time, the hidden detail behind bulk measurements can be revealed. Examples are the link between the diffiision coefficient and... 

That s fine in the macroscopic world, but how does the concept of temperature translate to the microscopic world ... 

Bodner, 1992) macroscopic world of chemistry molecular world of chemistry symbolic world of chemistry... 

A sutmnaty of the above shows various terms used for eaeh type of representation first (maero level, maeroscopic level, macroscopic world), second (sub-micro level, microscopic level, submicro level, submicroscopic level, molecular world, atomic world), and third (symbolic level, sy mbolic world, representational chemistry, algebraic system). In onr view, the system of terminology shonld be both as brief as possible and avoid any possible ambiguities of meaning. Conseqnently, sub-micro and snb-microscopic fall foul of our first criterion for they perhaps imply that snch a level can be seen through an optical microscope. For those reasons, we have decided to nse macro, submicro, symbolic for the individual types and triplet relationship to cover all three. The triplet relationship is a key model for chemical edncation. However, the authors in this book have been fiee to decide for themselves which conventions to use. Nevertheless, it is our intention to promote the terms macro, submicro, symbolic in all subsequent work and to discuss the value of the triplet relationship in chemical education. 

To elucidate the distinction between the sub-microscopic and macroscopic worlds, Table 1.1 lists some observable chemical phenomena (the macroscopic level) along with the corresponding explanatory models that we use at the sub-microscropic level. 

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. 

When students do make an attempt to relate between the three levels of representation, several unexpected trends in their reasoning are revealed. In the majority of explanations given by students about chemical reactions in a review imdertaken by Andersson (1986), there was a clear extrapolation of physical attributes and changes from the macroscopic world to the particle or submicroscopic one. So, when wood bums, wood molecules are also said to bum. If metallic copper is bright reddish-brown, atoms of copper are also imagined to be reddish-brown in colour (Ben-Zvi, Eylon, Silberstein, 1986). One reason for such extrapolation of physical attributes of substances to the particulate level is the tendency of students to assume that the atoms, ions and molecules in a substance are veiy small portions of the continuous substance. 

Kinetics provides the frame vork for describing the rate at which a chemical reaction occurs and enables us to relate the rate to a reaction mechanism that describes how the molecules react via intermediates to the eventual product. It also allows us to relate the rate to macroscopic process parameters such as concentration, pressures, and temperatures. Hence, kinetics provides us with the tools to link the microscopic world of reacting molecules to the macroscopic world of industrial reaction engineering. Obviously, kinetics is a key discipline for catalysis. 

The units we use in daily life, such as kilogram (or pound) and meter (or inch) are tailored to the human scale. In the world of quantum mechanics, however, these units would lead to inconvenient numbers. For example, the mass of the electron is 9.1095 X J0 31 kg and the radius of the first circular orbit of the hydrogen atom in Bohr s theory, the Bohr radius, is 5.2918 X 10 11 m. Atomic units, usually abbreviated as au, are introduced to eliminate the need to work with these awkward numbers, which result from the arbitrary units of our macroscopic world. The atomic unit of length is equal to the length of the Bohr radius, that is, 5.2918 X 10 n m, and is called the bohr. Thus 1 bohr = 5.2918 X 10"11 m. The atomic unit of mass is the rest mass of the electron, and the atomic unit of charge is the charge of an electron. Atomic units for these and some other quantities and their values in SI units are summarized in the accompanying table. 

Haick H, Cahen D (2008) Making contact connecting molecules electrically to the macroscopic world. Prog Surf Sci 83 217-261... 

As mentioned already, quantum and classical mechanics are fundamentally incompatible in many ways, yet the macroscopic world is well-described by classical dynamics. Physicists have struggled with this quandary ever since the laying of the foundations of quantum theory. It is fair to say that, even today, not everyone is satisfied with the state of affairs - including many seasoned practitioners of quantum mechanics. 

A production chemist is interested primarily in the macroscopic world, not the microscopic one of atoms and molecules. Even a chemistry student working in the laboratory will not be weighing out individual atoms and molecules, but large numbers of them in grams. There must be a way to bridge the gap between the microscopic world of individual atoms and molecules, and the macroscopic world of grams and kilograms. There is—it is called the mole concept, and it is one of the central concepts in the world of chemistry. 

The mole (mol) is the amount of a substance that contains the same number of particles as atoms in exactly 12 grams of carbon-12. This number of particles (atoms or molecules or ions) per mole is called Avogadro s number and is numerically equal to 6.022 x 1023 particles. The mole is simply a term that represents a certain number of particles, like a dozen or a pair. That relates moles to the microscopic world, but what about the macroscopic world The mole also represents a certain mass of a chemical substance. That mass is the substance s atomic or molecular mass expressed in grams. In Chapter 5, the Basics chapter, we described the atomic mass of an element in terms of atomic mass units (amu). This was the mass associated with an individual atom. Then we described how one could calculate the mass of a compound by simply adding together the masses, in amu, of the individual elements in the compound. This is still the case, but at the macroscopic level the unit of grams is used to represent the quantity of a mole. Thus, the following relationships apply ... 

Distinguish between the view of matter and energy in the macroscopic world (the world of everyday experience) and the view of matter and energy in the quantum mechanical world. 

Thermodynamics is concerned with energy and the way energy is transferred. It is a science of the macroscopic world but its effects are applied even at the microscopic scale. The first law introduces the basic thermodynamic concepts of work, heat and energy and can be defined as follows Energy can neither be created or destroyed in a system of constant mass, although it may be converted from one form to another. 

Polymers are real and all around us. We can look at giant molecules on a micro or atomic level or on a macroscopic level. The PET bottles we have may be composed of long chains of poly(ethylene terephthate) (PET) chains. The aramid tire cord is composed of aromatic polyamide chains. Our hair is made up of complex bundles of fibrous proteins, again polyamides. The polymers you study are related to the real world in which we live. We experience these large molecules at the macroscopic level everyday of our lives and this macroscopic behavior is a direct consequence of the atomic-level structure and behavior. Make pictures in your mind that allow you to relate to the atomic and macroscopic worlds. 

D) In the macroscopic world, wave and particle properties are mutually exclusive. At the atomic level, objects exhibit characteristic properties of both waves and particles. The amplitude of a wave is the measure of the magnitude of the maximum disturbance in the medium during one wave cycle, and is measured in units depending on the type of wave. 

Instead of gravity, electromagnetic forces are the major players in the nanoworld. This is a different environment, which along with the effects of quantum mechanics, means that materials can have different properties in nanotechnology than they have in the macroscopic world. 

Forces in the nanoworld can appear strange because they do not exist in the same proportion as forces in the macroscopic world. Gravitational forces are insignificant to molecules because this force is weak unless large masses are involved. Electrical interactions, as in the electric current driving the atomic movement in ZettFs motor, are tremendously more important than gravity on this scale. 

We cannot extrapolate our knowledge of everyday macroscopic world to the world of subatomic dimensions. The Heisenberg uncertainty principle, the wave character of particle motion and quantization of energy become important when the masses of the particles become comparable to Planck s constant h. 

The results you obtain for Problem 1.5 should show that, since the joule (J) is a macroscopic base unit of energy, property values on the microscopic scale have extremely small magnitudes. We now explore this idea further in Worked Problem 1.3 and in Problem 1.6 to give more practice in manipulating numbers in scientific form and, more importantly, to provide further insight into size differences in the microscopic and macroscopic worlds. 

The uncertainty principle shows that the classical trajectory of a particle, with a precisely determined position and momentum, is really an illusion. It is a very good approximation, however, for macroscopic bodies. Consider a particle with mass I Xg, and position known to an accuracy of 1 pm. Equation 2.41 shows that the uncertainty in momentum is at least 5 x 10 29 kg m s-1, corresponding to a velocity of 5 x 10 JO m s l. This is totally negligible for any practical purpose, and it illustrates that in the macroscopic world, even with very light objects, the uncertainty principle is irrelevant. If we wanted to, we could describe these objects by wave packets and use the quantum theory, but classical mechanics gives essentially the same answer, and is much easier. At the atomic and molecular level, however, especially with electrons, which are very light, we must abandon the idea of a classical trajectory. The statistical predictions provided by Bom s interpretation of the wavefunction are the best that can be obtained. 

When trying to manipulate fluids on the micron scale, formidable problems have to be overcome [301], Miniature pumps, valves, switches, and new analytical tools have to be developed. To illustrate this we discuss the fundamental problem of transporting a liquid through a capillary tube. In the macroscopic world we would apply a pressure AP between the two ends. According to the law of Hagen-Poiseuille the volume of liquid V transported per time t is (assuming laminar flow)... 

For macroscopic objects the adhesion force is often small compared to the load. For microscopic bodies this can be different. The reason is simple the weight of an object sliding over a surface usually decreases with the third power of its diameter (or another length characterizing its size). The decrease of the actual contact area and hence the adhesion force follows a weaker dependence. For this reason, friction between microbodies is often dominated by adhesion while in the macroscopic world we can often neglect adhesion. 

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