Symbolizing Chemical Reactions

Let us begin as usual by expressing the symbolic chemical reaction... 

The preceding demonstration, All Things Being Equal, was intended to illustrate three significant subtleties concerning chemical reactions. The first is that although we often symbolize chemical reactions with neat little arrows pointing from reactants to products. 

Measurements of the emf of galvanic cells can be used to advantage to extract thermodynamic information concerning the characteristics of chemical reactions. As already stated, corresponding to the symbolic chemical reaction one may specify an equilibrium parameter Kx = Oy If cell can... 

If a themiodynamic system includes species that may undergo chemical reactions, one must allow for the fact that, even in a closed system, the number of moles of a particular species can change. If a chemical reaction (e.g. N2 + 3H2 INHg) is represented by the symbolic equation... 

Disconnection An analytical operation, which breaks a bond and converts a molecule into a possible starting material. The reverse of a chemical reaction. Symbol and a curved line drawn through the bond being broken. Called a dislocation by some people. 

FGI Functional Group Interconversion The operation of writing one functional group for another so that disconnection becomes possible. Again the reverse of a chemical reaction. Symbol with FGI written over it. 

The circle m a hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds Robinson s symbol is a convenient time saving shorthand device but Kekule type formulas are better for counting and keeping track of electrons especially m chemical reactions... 

Symbols used as subscripts to denote a chemical reaction or process ... 

The amount of energy contained in the bonds of a chemical is called the free energy of that chemical (given the symbol fi). To understand how free energy and entropy are related, consider the following chemical reaction ... 

So far in this chapter our discussion has focused on thermochemistry, the study of the heat effects in chemical reactions. Thermochemistry is a branch of thermodynamics, which deals with all kinds of energy effects in all kinds of processes. Thermodynamics distinguishes between two types of energy. One of these is heat (q) the other is work, represented by the symbol w. The thermodynamic definition of work is quite different from its colloquial meaning. Quite simply, work includes all forms of energy except heat. 

The painting, "Equilibrium," shows arrows pointing in opposite directions. The same symbols are used to denote a chemical reaction in equilibrium. 

The "ordinary chemical reactions" discussed to this point involve changes in the outer electronic structures of atoms or molecules. In contrast nuclear reactions result from changes taking place within atomic nuclei. You will recall (Chapter 2) that atomic nuclei are represented by symbols such as... 

This idea—that each molecular substance has a characteristic heat content—provides a good explanation of the heat effects found in chemical reactions. Chemists symbolize heat content by H. Since the heat effect in a reaction is the difference between the U s of the products and the H s of the reactants, the heat of reaction is called AH, the Greek letter A (delta) signifying difference. We can see what AH means in terms of an example. Consider reaction (/) ... 

In the combustion reaction as carried out in the calorimeter of Figure 7-2, the volume of the system is kept constant and pressure may change because the reaction chamber is sealed. In the laboratory experiments you have conducted, you kept the pressure constant by leaving the system open to the surroundings. In such an experiment, the volume may change. There is a small difference between these two types of measurements. The difference arises from the energy used when a system expands against the pressure of the atmosphere. In a constant volume calorimeter, there is no such expansion hence, this contribution to the reaction heat is not present. Experiments show that this difference is usually small. However, the symbol AH represents the heat effect that accompanies a chemical reaction carried out at constant pressure—the condition we usually have when the reaction occurs in an open beaker. 

Charles, Jacques, 57 Charles law, 58 Chemical bonding, see Bonding Chemical bonds, see Bond Chemical change, 38 Chemical energy, 119 Chemical equations, see Equations Chemical equilibrium, law of, 152 Chemical formulas, see Formula Chemical kinetics, 124 Chemical reactions, see Reactions Chemical stability, 30 Chemical symbols, 30 not from common names, 31 see inside back cover Chemotherapy, 434 Chlorate ion, 360 Chloric acid, 359 Chlorides chemistry of, 99 of alkali metals, 93,103 of third-row elements, 103 Chlorine... 

During a chemical reaction, the change A//, in the number of moles for each substance is proportional to i/, with the proportionality constant the same for all components. This proportionality constant is given the symbol and is called the extent of the reaction. It is related to the number of moles reacted by... 

The growth of a child, the production of polymers from petroleum, and the digestion of food are all the outcome of chemical reactions, processes by which one or more substances are converted into other substances. This type of process is a chemical change. The starting materials are called the reactants and the substances formed are called the products. The chemicals available in a laboratory are called reagents. In this section, we see how to use the symbolic language of chemistry to describe chemical reactions. 

A balanced chemical equation symbolizes both the qualitative and the quantitative changes that take place in a chemical reaction. The stoichiometric coefficients tell us the relative numbers of moles of reactants and products taking part in the reaction. 

In a balanced chemical equation (commonly called a chemical equation ), the same number of atoms of each element appears on both sides of the equation, chemical equilibrium A dynamic equilibrium between reactants and products in a chemical reaction, chemical formula A collection of chemical symbols and subscripts that shows the composition of a substance. See also condensed structural formula empirical formula,- molecular formula structural formula. 

Consider the examples of some of the forms of chemical equations (and related representations) met in school and college (i.e. middle and senior high school) science and chemistiy classes that are shown in Table 4.1. For the purposes of this chapter half-equations (Example 11) and symbolic representations of processes such as ionisation (Example 10) will be included under the generic heading of chemical equations . Table 4.1 does not include examples of chemical reactions and reaction schemes that include stmctural formulae, as are commonly nsed in organic chemistiy. 

Both of these forms of representation are symbohc although words are more familiar symbols to students. Both of these forms present difficulties to learners. Although word equations may seem a more direct way for novices to represent chemical reactions, they may sometimes make more demands on learners. So, for example in completing equations, non-systematic names - such as ammonia - may not provide strong clues to the elements present, and there is a need to learn, recall and apply such rules as -ate implies oxygen present, etc. 

To be able to explain chemical reactions, students will have to develop mental models of the submicroscopic particles of the substances that undergo rearrangement to produce the observed changes. However, students have difficulty in understanding submicroscopic and symbolic representations as these representations are abstract and carmot be directly experienced (Ben-Zvi, Eylon, Silber-stein, 1986, 1988 Griffiths Preston, 1992). As a result, how well students understand chemistry depends on how proficient they are in making sense of the invisible and the untouchable (Kozma Russell, 1997 p. 949). 

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