Chemical equations, representation

Chemical equations Representations of chemical reactions by the formulas of reactants and products... 

These examples suggest that the learner is not always aware which aspects of our use of symbolic representation in chemistiy are intended to be significant. This is an area where further work would be useful, as clearly teachers need to do more to induct learners into the intended symbolism we use in teaching the subject. In the next section, these issues will be explored further in the particular context of learning about chemical equations. 

One of the central forms of representation in chemistry, and so in teaching and leanting of chemistry, is the use of chemical equations. These are so ubiquitous, and again so familiar to the expert (the chemist, the science teacher) that the abstract and complex nature of the representations, and so the leanting challenges, may not be readily apparent. 

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. 

Given this context, the use of chemical symbols, formulae and equations can be readily misinterpreted in the classroom, because often the same representations can stand for both the macroscopic and sub-microscopic levels. So H could stand for an atom, or the element hydrogen in an abstract sense H2 could mean a molecule or the substance. One common convention is that a chemical equation represents molar quantities, so in Example 9 in Table 4.1,... 

When interpreting the chemical equation for the reaction between aqueous sodium hydroxide and dilute nitric acid, 20% of students appeared to hold the view that Na+ and NO3" ions (submicroscopic and symbolic representations) had reacted in aqueous solution to produce aqueous sodium nitrate. It was not apparent to these students that the net chemical reaction had only involved removal of H+ and OH in aqueous solution to produce molecules of H2O. 

The representation in Fig. 8.1 lb is similar to one noted by Yarroch (1985) who investigated students understanding of balancing chemical equations. His results showed that while all the students were able to balance the four chemical equations presented to them, 42% of them could not constract sub-micro diagrams consistent with the symbohc representation of the balanced equation. 

A model is one of the main outcomes of ary scientific enquiry and hence is a major contributor to philosophy of science. A model may be defined as a simplified representation of a phenomenon (an object, system, event, process) or idea produced for the specific purpose of providing an explanation of that entity, the most important outcomes of which are the production of successful predictions of how it will behave under a range of circumstances (Gilbert, Boulter, Elmer, 2000). Entities can be modelled at the three levels at the macroscopic, by representing some of the aspects of the entity that can be seen at the sub-microscopic, by representing the ideas produced to explain the constitution and behaviour of the particles that constitute the entity and at the symbolic, by representing the symbols created to simplify the reference to such particles (as, for instance, chemical formulae and chemical equations). 

The capacity to construct a representation in any appropriate mode and submode for a given purpose. For example, being able to represent the working of an oil refinery in terms of a diagram of its component parts to an explanation of what takes place in terms of molecular transformations and the chemical equations for these ... 

Some tasks in the Test of Gained Knowledge required students to connect observations about the macro course of chemical reactions with their notations in the submicro and/or symbolic types of representation. The results indicate that most students were able to rearticulate the information about reactants and products of a chemical reaction from the textual description of chemical reaction into the form of word chemical equation (textual description of macros word equation of macro Task 8.2, f(o/ )=89.82% Task 9.1, f(o/ )=87.61%). This action corresponds to the first step in learning to write down chemical equation in the LON approach. It can easily be explained, because teachers described the learning process to be very efficient to this point, as is illustrated below ... 

Using models in learning about ehemieal equations has proved a successful tool, espeeially by students with diffieulties in eoneept understanding. By eounting the number of atoms in partiele representations they better understood the meaning of a balanced equation. Some students still have problems with balaneing chemical equations when models are not avail-... 

Students ability to connect observations at the macroscopic level with their descriptions using the submicro and symbolic types of representation improved as a consequence of the LON teaching approach. Teachers attributed the improvement to the consistent use of all three types of representation and to the use of visible models as a tool for bridging the gap between macroscopic observations and symbolic notations of chemical equations. 

The reduction half-reaction does not include a solid conductor of electrons, so an inert platinum electrode is used in this half-cell. The platinum electrode is chemically unchanged, so it does not appear in the chemical equation or half-reactions. However, it is included in the shorthand representation of the cell. 

EQUATION (chemical) A representation of a chemical reaction, using the symbols of the elements to represent the actual atoms and molecules taking part in a reaction. For example, a classical, but simplified, overall reaction for the deflagration of gunpowder is as follows ... 

Many conditions are required for a chemical reaction to proceed. Conditions such as heat, light, and pressure must be just right for a reaction to take place. Furthermore, the reaction may proceed very slowly. Some reactions occur in a fraction of a second, while others occur very slowly. Consider the difference in the reaction times of gasoline igniting in a car s cylinder versus the oxidation of iron to form rust. The area of chemistry that deals with how fast reactions occur is known as kinetics (Chapter 12). Finally, not all reactions go to completion. The amount of product produced based on the chemical equation is known as the theoretical yield. The amount actually obtained expressed as a percent of the theoretical is the actual yield. In summary, it s best to think of a chemical equation as an ideal representation of a reaction. The equation provides a general picture of the reaction and enables us to do theoretical calculations, but in reality reactions deviate in many ways from that predicted by the equation. 

Chemical equation A shorthand representation showing the reactants and products of a chemical reaction and their relative quantities. 

We distinguish between the chemical reaction (the actual process) and the chemical equation, which is a symbolic representation of the reaction in terms of chemical formulas. 

Chemical equation a representation of a chemical reaction showing the relative numbers of reactant and product molecules. (3.6)... 

This description of the reaction of CH4 with Oj based on experimental observations. By this we mean experiments have shown that when one CH4 molecule reacts with two O2 molecules, one CO2 molecule and two H2O molecules are formed. Chemical equations are based on experitnental observations. Special conditions required for some reactions are indicated by notation over the arrow. Figure 3-1 is a pictorial representation of the rearrangement of atoms described by this equation. 

In this generalized representation, a represents the coefficient of substance A in the balanced chemical equation, b is the coefficient of substance B, and so on. For example, in an earlier equation given for the decomposition of N2O5, a = 2, A represents N2O5, r = 4, C represents NO2, and so on. 

We recognize that both NaCH3COO and NaCN are salts of strong bases and weak acids. The anions in such salts hydrolyze to give basic solutions. As we have done before, we first write the appropriate chemical equation and equihbrium constant expression. Then we complete the reaction summary, substimte the algebraic representations of equilibrium concentrations into the equilibrium constant expression, and solve for the unknown concentration(s). 

Polymeric materials, both as end products and intermediates, are an ever-increasing segment of the chemical industry. Representation of polymer mixtures by equations of state especially developed for this task is a fairly mature area (see the review of... 

Chemieal reaetions occur in nature, and somse also can be performed in a laboratory setting. Chemical Equations are linear representations of how these reaetions oeeur. Combination reactions occur when two separate reactants are... 

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