Chemical equations requirements

Balancing a chemical equation requires an understanding of the Law of Conservation of Mass, which says that mass cannot be created or destroyed. The amount of mass in the reactants will be the amount of mass in the products. The credit for this discovery is given to Antoine Lavoisier, who took very careful measurements of the quantities of chemicals and equipment that he used. Conservation of mass also holds true when balancing equations. The number of atoms of each element in the reactants will be equal to the number of atoms of each element in the products. A useful mnemonic device for conservation of mass is What goes in, must come out. ... 

Learning Goal Performing Calculations based on a chemical equation requires a facility for relating the number of atoms of an element to a corresponding number of moles of that element and ultimately to their mass in grams. Such calculations involve the use of conversion factors. This type of calculation was first described in Chapter 1. Some examples follow. 

A moment s reflection on Eq. (7.64) tells us that this independence of the values of the enthalpies of the elements must be correct for all chemical reactions. If, in the expression for the AH of a reaction, we replace the molar enthalpy of every compound by the expression in Eq. (7.64), then it is clear that the sum of the enthalpies of the elements composing the reactants must be equal to the sum of the enthalpies of the elements composing the products. The balanced chemical equation requires this. Therefore the enthalpies of the elements must drop out of the expression. We are left only with the proper combination of the enthalpies of formation of the compounds. This conclusion is correct at every temperature and pressure. 

In other words, writing and balancing a chemical equation requires two main steps ... 

Balancing a chemical equation requires something of a trial-and-error approach. You may find that you change the coefficient for a particular reactant or product, only to have to change it again later in the process. In general, it will facilitate the balancing process if you do the... 

Strategy In calculations of this kind, the procedure is to break the overall process down into a sequence of steps such that their sum is the chemical equation required. 

In order to obtain an improved yield of the desired product, an excess over the proportion required by the chemical equation of one (or more) of the reactants is often used. In a given preparation, the selection of the reagent to be employed in excess will depend upon a number of factors these include its relative cost and ease of removal after the reaction, and... 

Mechanisms. Mechanism is a technical term, referring to a detailed, microscopic description of a chemical transformation. Although it falls far short of a complete dynamical description of a reaction at the atomic level, a mechanism has been the most information available. In particular, a mechanism for a reaction is sufficient to predict the macroscopic rate law of the reaction. This deductive process is vaUd only in one direction, ie, an unlimited number of mechanisms are consistent with any measured rate law. A successful kinetic study, therefore, postulates a mechanism, derives the rate law, and demonstrates that the rate law is sufficient to explain experimental data over some range of conditions. New data may be discovered later that prove inconsistent with the assumed rate law and require that a new mechanism be postulated. Mechanisms state, in particular, what molecules actually react in an elementary step and what products these produce. An overall chemical equation may involve a variety of intermediates, and the mechanism specifies those intermediates. For the overall equation... 

Theoretical Oxygen and Air for Combustion The amount of oxidant (oxygen or air) just sufficient to burn the carbon, hydrogen, and sulfur in a fuel to carbon dioxide, water vapor, and sulfur dioxide is the theoretical or stoichiometric oxygen or air requirement. The chemical equation for complete combustion of a fuel is... 

Stoichiometric The exact quantity of reactants required to ctrmpletely react according to a particular chemical equation. If... 

Interpolation methods based on N chemical shifts require the use of the general equations.Those reported in the previous edition (76AHCSl,p. 29, see also 82JOC5132) have been slightly modified for the present purpose. We call / x the observed average property, and the property of the individual tautomers (A or B), / ma and / mb a corresponding property that can be measured (in a model compound or in the solid state) or calculated theoretically, and P and / b the correction factors defined as P = -... 

The term titrimetric analysis refers to quantitative chemical analysis carried out by determining the volume of a solution of accurately known concentration which is required to react quantitatively with a measured volume of a solution of the substance to be determined. The solution of accurately known strength is called the standard solution, see Section 10.3. The weight of the substance to be determined is calculated from the volume of the standard solution used and the chemical equation and relative molecular masses of the reacting compounds. 

Some redox reactions, particularly those involving oxoanions, have complex chemical equations that require special balancing procedures. We meet examples and see how to balance them in Chapter 12. 

X 10 4 M HCl(aq) with phenolphthalein indicator to see how much NaOH was left unreacted. They found that 30.2 mL of HCl(aq) was required to reach the stoichiometric point, (a) Write the balanced chemical equation for the reaction of S02 and water, (b) What amount of NaOH (in mol) had reacted with the SO, (c) What was the concentration of sulfur dioxide in the air, in parts per million ... 

A note on good practice The chemical equations for elementary reaction steps are written without the state symbols. They differ from the overall chemical equation, which summarizes bulk behavior, because they show how individual atoms and molecules take part in the reaction,. We do not use stoichiometric coefficients for elementary reactions. Instead, to emphasize that we are depicting individual molecules, we write the formula as many times as required. 

Another key requirement of chemical equations (when presented in formulae, see below for consideration of word equations), is that they should be balanced . This is considered further below, and relates to conservations that are expected during chemical processes (of matter, charge, energy). 

For students to make sense of the basic grammar of chemical equations they need to appreciate the concept of the chemical reaction. This, in turn, requires an understanding of the notion of chemical snbstance. Although these are basic concepts in chemistry, they are known to present difficulties to many learners. 

A chemistry scheme of work based on the syllabus requirements was developed for the year in collaboration with all teachers who would be teaching the subject. The content to be covered in each of the topics was detailed on a weekly basis. Each of the topics had to be covered within a stipulated time frame followed by a cotmnon topic test that was to be administered on a particular week. In general, students were only required to learn and remember the chemical equations as well as the ionic equations, where relevant, with no specific emphasis on understanding the triplet relationship. 

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 ... 

A chemical synthesis requires the proper amounts of starting materials for a successful outcome. Just as a cake recipe provides the amounts of ingredients needed for successful baking, a balanced chemical equation is a chemical recipe for successful synthesis. For example, how many grams of hydrogen do we need to produce 68 g of ammonia To find out, we begin with the balanced chemical equation N2 + 3 H2 2 NH3... 

Either method requires the balanced chemical equation ... 

C22-0030. Outline the requirements for a balanced nuclear equation, and explain how they differ from those for a balanced chemical equation. 

The application of the time-independent Schrodinger equation to a system of chemical interest requires the solution of a linear second-order homogeneous differential equation of the general form... 

The balanced equation expresses quantities in moles, but it is seldom possible to measure out quantities in moles directly. If the quantities given or required are expressed in other units, it is necessary to convert them to moles before using the factors of the balanced chemical equation. Conversion of mass to moles and vice versa was considered in Sec. 4.5. Here we will use that knowledge first to calculate the number of moles of reactant or product, and then use that value to calculate the number of moles of other reactant or product. 

Arts. The chemist can put in as little as is weighable or as much as the vessel will hold. For example, the fact that a reactant has a coefficient of 2 in the balanced chemical equation does not mean that the chemist must put two moles into the reaction vessel. The chemist might decide to add the reactants in the ratio of the balanced chemical equation, but even that is not required. And even in that case, the numbers of moles of each reactant might be twice the respective coefficients or one-tenth those values, etc. The equation merely states the reacting ratio. 

The net ionic equation, like all balanced chemical equations, gives the ratio of moles of each substance to moles of each of the others. It does not immediately yield information about the mass of the entire salt, however. (One cannot weigh out only Ba2+ ions.) Therefore, when masses of reactants are required, the specific compound used must be included in the calculation. The use of net ionic equations in stoichiometric calculations will be more important after study of molarity (Chap. 10). 

Hence, from the balanced chemical equation. 0.699 mol of Ag is required ... 

In more complex cases when several reactions are occurring simultaneously in the system under observation, calculations of the composition of the system as a function of time will require the knowledge of a number of independent composition variables equal to the number of independent chemical equations used to characterize the reactions involved. 

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