Qualitative chemistry

Because the ratios of atoms of different elements are so important in chemistry, we need to know how to determine the numbers of the different types of atoms, ions, or molecules present in a sample. Knowing the types of atoms is fundamental to qualitative chemistry—understanding the properties of substances, for instance. Knowing the numbers of atoms is fundamental to quantitative chemistry—the calculation of the values of these properties. 

Qualitative chemistry is an area of chemistry concerned with identifying substances. In Activity 9.1 you will perform a qualitative analysis to detect the presence of certain ions that, in turn, may reveal an art forgery. The ions could come from paints that were not available at the time of the artwork. In this qualitative analysis, metal ions (cations) and nonmetal ions (anions) are reacted with solvents and with each other. Then the cations and anions present are identified by the products produced. In addition, flame tests and pH determinations are used to identify ions. Qualitative analysis is an engaging opportunity for you to develop experience with chemical change and review solubility principles. Nowadays, however, most of the time a chemist analyzes a substance to detect ion content using quantitative analytical computerized instruments. 

Stoichiometry is the branch of chemistry that deals with the amounts of products produced from certain amounts of reactants. Most of the chemistry discussed so far in this book has dealt with what is present (qualitative chemistry). The next step is to examine how much is present (quantitative chemistry). You may want to refer to Appendix 1, Mathematical Skills Review, in the back of this book. 

Prc- mincntly an experimentalist, Scheele became the greatest master of qualitative chemistry in the era between Boyle and Lavoisier. By so doii he met a crying need of the chemistry of his day for more information about elements, compounds, and chemical change. He employed the simplest kinds of apparatus and since his work dealt mainly with the isolation of new substances and the examination of their properties, he was not concerned, like his contemporary, Black, with the use of the balance in following Ixis observations. 

In their qualitative chemistry, conjugated dienes behave very much like alkenes They readily react with electrophiles in addition reactions. Just as in the case of alkenes. this addition proceeds to give the most stable intermediate. For conjugated dienes, this normally turns out to be a re.sonance stabilized allylic cation ... 

It is often customary to date the emergence of histochemistry from the publication in 1936 of Lison s Histochimie animale. Rapid advances in the qualitative chemistry of organic compounds during the early decades of this century made the 1930 s ripe for the application of color tests and allied methods to tissue sections for the localization of known compounds or at least their functional groups. As early as 1850, however, histologists were attempting to identify specific substances, initially metals, in situ (Pearse, 1951). 

Shaw s career illustrates the contrast between the social situation of the chemist within (or more accurately, without) the English scientific world when compared to France. He promoted himself as an indispensable intermediary between the proponents of chemical knowledge, and the society physicians who were his patrons [33]. As Golinski has noted, Shaw s choices of works for translation and publication did not emphasise a predominantly Newtonian stance. Instead, they took for the most part what might be termed a chemical position that followed the Baconian inductive method and emphasised a qualitative chemistry, often founded on systems of elements [33]. Bfis entrepreneurial role served to bring him to the attention of the establishment, and enabled him to achieve similar social heights, at which point his interest in chemistry apparently had served its purpose and he published no more. 

The purpose of this chapter is to provide an introduction to tlie basic framework of quantum mechanics, with an emphasis on aspects that are most relevant for the study of atoms and molecules. After siumnarizing the basic principles of the subject that represent required knowledge for all students of physical chemistry, the independent-particle approximation so important in molecular quantum mechanics is introduced. A significant effort is made to describe this approach in detail and to coimnunicate how it is used as a foundation for qualitative understanding and as a basis for more accurate treatments. Following this, the basic teclmiques used in accurate calculations that go beyond the independent-particle picture (variational method and perturbation theory) are described, with some attention given to how they are actually used in practical calculations. 

Although a separation of electronic and nuclear motion provides an important simplification and appealing qualitative model for chemistry, the electronic Sclirodinger equation is still fomiidable. Efforts to solve it approximately and apply these solutions to the study of spectroscopy, stmcture and chemical reactions fonn the subject of what is usually called electronic structure theory or quantum chemistry. The starting point for most calculations and the foundation of molecular orbital theory is the independent-particle approximation. 

This venerable book was written in 1935, shortly after the birth of modern quantum mechanics. Nevertheless, it remains one of the best sources for students seeking to gain an understanding of quantum-mechanical principles that are relevant in chemistry and chemical physics. Equally outstanding jobs are done in dealing with both quantitative and qualitative aspects of the subject. More accessible to most chemists than Landau and Lifschitz. 

A superb treatment of applied molecular orbital theory and its application to organic, inorganic and solid state chemistry. Perhaps the best source for appreciating the power of the independent-particle approximation and its remarkable ability to account for qualitative behaviour in chemical systems. 

Quantum chemical methods, exemplified by CASSCF and other MCSCF methods, have now evolved to an extent where it is possible to routinely treat accurately the excited electronic states of molecules containing a number of atoms. Mixed nuclear dynamics, such as swarm of trajectory based surface hopping or Ehrenfest dynamics, or the Gaussian wavepacket based multiple spawning method, use an approximate representation of the nuclear wavepacket based on classical trajectories. They are thus able to use the infoiination from quantum chemistry calculations required for the propagation of the nuclei in the form of forces. These methods seem able to reproduce, at least qualitatively, the dynamics of non-adiabatic systems. Test calculations have now been run using duect dynamics, and these show that even a small number of trajectories is able to produce useful mechanistic infomiation about the photochemistry of a system. In some cases it is even possible to extract some quantitative information. 

Chapter XI is devoted to Qualitative Organic Analysis. The subject b discussed in moderate detail and this, coupled with the various Sections and Tables of Physical Constants of Organic Compounds and their Derivatives in Chapters III and IV, will provide a satisfactory course of study in this important branch of chemistry. No attempt has been made to deal with Quantitative Organic Analysb in this volume. 

The dawn of the nineteenth century saw a drastic shift from the dominance of French chemistry to first English-, and, later, German-influenced chemistry. Lavoisier s dualistic views of chemical composition and his explanation of combustion and acidity were landmarks but hardly made chemistry an exact science. Chemistry remained in the nineteenth century basically qualitative in its nature. Despite the Newtonian dream of quantifying the forces of attraction between chemical substances and compiling a table of chemical affinity, no quantitative generalization emerged. It was Dalton s chemical atomic theory and the laws of chemical combination explained by it that made chemistry an exact science. 

The term theoretical chemistry may be defined as the mathematical description of chemistry. The term computational chemistry is generally used when a mathematical method is sufficiently well developed that it can be automated for implementation on a computer. Note that the words exact and perfect do not appear in these definitions. Very few aspects of chemistry can be computed exactly, but almost every aspect of chemistry has been described in a qualitative or approximately quantitative computational scheme. The biggest mistake a computational chemist can make is to assume that any computed number is exact. However, just as not all spectra are perfectly resolved, often a qualitative or approximate computation can give useful insight into chemistry if the researcher understands what it does and does not predict. 

An area of great interest in the polymer chemistry field is structure-activity relationships. In the simplest form, these can be qualitative descriptions, such as the observation that branched polymers are more biodegradable than straight-chain polymers. Computational simulations are more often directed toward the quantitative prediction of properties, such as the tensile strength of the bulk material. 

This qualitative theory still provides the most widely used means for describing reactions in organic chemistry. Two principal modes of electronic interaction in organic molecules are recognised the inductive and mesomeric effects. 

A.I. Vogel, "Vogel s Textbook of Practical Organic Chemistry, including Qualitative Organic Analysis", 4th Edition, p. 325, Longman, London and New York, (1978). 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

Analytical chemistry is often described as the area of chemistry responsible for characterizing the composition of matter, both qualitatively (what is present) and quantitatively (how much is present). This description is misleading. After all, almost all chemists routinely make qualitative or quantitative measurements. The argument has been made that analytical chemistry is not a separate branch of chemistry, but simply the application of chemical knowledge. In fact, you probably have performed quantitative and qualitative analyses in other chemistry courses. For example, many introductory courses in chemistry include qualitative schemes for identifying inorganic ions and quantitative analyses involving titrations. 

You will come across numerous examples of qualitative and quantitative methods in this text, most of which are routine examples of chemical analysis. It is important to remember, however, that nonroutine problems prompted analytical chemists to develop these methods. Whenever possible, we will try to place these methods in their appropriate historical context. In addition, examples of current research problems in analytical chemistry are scattered throughout the text. 

In Section lA we indicated that analytical chemistry is more than a collection of qualitative and quantitative methods of analysis. Nevertheless, many problems on which analytical chemists work ultimately involve either a qualitative or quantitative measurement. Other problems may involve characterizing a sample s chemical or physical properties. Finally, many analytical chemists engage in fundamental studies of analytical methods. In this section we briefly discuss each of these four areas of analysis. 

The purpose of a qualitative, quantitative, and characterization analysis is to solve a problem associated with a sample. A fundamental analysis, on the other hand, is directed toward improving the experimental methods used in the other areas of analytical chemistry. Extending and improving the theory on which a method is based, studying a method s limitations, and designing new and modifying old methods are examples of fundamental studies in analytical chemistry. 

Current research in the areas of quantitative analysis, qualitative analysis, and characterization analysis are reviewed biannually (odd-numbered years) in Analytical Chemistry s Application Reviews. ... 

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