Chemical equation by inspection

In this chapter, you learned how to balance simple chemical equations by inspection. Then you examined the mass/mole/particle relationships. A mole has 6.022 x 1023 particles (Avogadro s number) and the mass of a substance expressed in grams. We can interpret the coefficients in the balanced chemical equation as a mole relationship as well as a particle one. Using these relationships, we can determine how much reactant is needed and how much product can be formed—the stoichiometry of the reaction. The limiting reactant is the one that is consumed completely it determines the amount of product formed. The percent yield gives an indication of the efficiency of the reaction. Mass data allows us to determine the percentage of each element in a compound and the empirical and molecular formulas. 

For each reaction below, write a balanced chemical equation by inspection. 

Balancing chemical equations by inspection is a trial-and-error approach. It requires a great deal of practice, but it is very important Remember that we use the smallest whole-number coefficients. Some chemical equations are difficult to balance by inspection or trial and error. In Chapter 11 we will learn methods for balancing complex equations. 

In contrast, student performance on balancing chemical equations by inspection has been correlated with a battery of tests, including cognitive... 

Toth, Z. Balancing chemical equations by inspection. J. Chem. Educ. 74(11), 1363-1364 (1997)... 

Zoltan Toth, "Balancing Chemical Equations by Inspection," y. Chem. Educ, Vol. 74, 1997, 1363-1364. 

In Chapter 7, we learned how to balance chemical equations by inspection. Some redox reactions can be balanced in this way. However, redox reactions occurring in aqueous solutions are usually difficult to balance by inspection and require a special procedure called the half-reaction method of balancing. In this procedure, the overall equation is broken down into two half-reactions one for oxidation and one for reduction. The half-reactions are balanced individually and then added together. For example, consider tiie redox reaction ... 

If you know the reactants and products of a chemical reaction, you should be able to write an equation for the reaction and balance it. In writing the equation, first write down the correct formulas for all reactants and products. After they are written down, only then start to balance the equation. Do not balance the equation by changing the formulas of the substances involved. For simple equations, you should balance the equation by inspection. (Balancing oxidation-reduction equations will be presented in Chap. 13.) The following rules will help you to balance simple equations. 

We have seen how analytical calculations in titrimetric analysis involve stoichiometry (Sections 4.5 and 4.6). We know that a balanced chemical equation is needed for basic stoichiometry. With redox reactions, balancing equations by inspection can be quite challenging, if not impossible. Thus, several special schemes have been derived for balancing redox equations. The ion-electron method for balancing redox equations takes into account the electrons that are transferred, since these must also be balanced. That is, the electrons given up must be equal to the electrons taken on. A review of the ion-electron method of balancing equations will therefore present a simple means of balancing redox equations. 

This method of balancing equations by inspection works in many, but not all, cases. Section 11.4 presents techniques for balancing certain more complex chemical equations. 

The following sample study sheet shows a procedure that you can use to balance chemical equations. It is an approach that chemists often call balancing equations by inspection. Examples 4.1 through 4.5, which follow the study sheet, will help to clarify the process. 

For each of the following unbalanced oxidation-reduction chemical equations, balance the equation by inspection, and identify which species is undergoing oxidation and which... 

Kolb, D. (1981). Balancing complex redox equations by inspection. Journal of Chemical Education, 58, 642-645. 

Five tasks must be performed in this problem (1) Represent the reaction by a chemical equation in which the names of reactants and products are replaced with formulas. (2) Balance the formula equation by inspection. (3) Determine the limiting reactant. (4) Calculate the theoretical yield of sodium nitrite based on the quantity of limiting reactant. (5) Use... 

A mechanism is a series of simple reaction steps which, when added together, account for the overall reaction. The rate law for the individual steps of the mechanism may be written by inspection of the mechanistic steps. The coefficients of the reactants in the chemical equation describing the step become the exponents of these concentrations in the rate law for... 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

The stoichiometric coefficients in a balanced chemical equation must be chosen so that the atoms of each element are conserved. Many chemical equations can be balanced by inspection. Balancing by inspection means changing stoichiometric coefficients until the number of atoms of each element is the same on each side of the arrow. Usually, we can tell what changes need to be made by looking closely at the reaction and matching the numbers of atoms of each element on both sides of the equation. Consider the following example. 

A complex reacting system is defined as one that requires more than one chemical equation to express the stoichiometric constraints contained in element balances. In such a case, the number of species usually exceeds the number of elements by more than 1. Although in some cases a proper set of chemical equations can be written by inspection, it is useful to have a universal, systematic method of generating a set for a system of any complexity, including a simple system. Such a method also ensures the correct number of equations (R), determines the number (C) and a permissible set of components, and, for convenience for a very large number of species (to avoid the tedium of hand manipulation), can be programmed for use by a computer. 

Most equations are balanced by inspection. This means basically a trial-and-error, methodical approach to adjusting the coefficients. One procedure that works well is to balance the homonuclear (same nucleus) molecule last. Chemical species that fall into this category include the diatomic elements, which you should know H2, 02, N2, F2, Cl2, Br2, and I2. This is especially useful when balancing combustion reactions. If a problem states that oxygen gas was used, then knowing that oxygen exists as the diatomic element is absolutely necessary in balancing the equation correctly. 

You could balance the chemical equation for the reaction of magnesium with aluminum nitrate by inspection, instead of writing half-reactions. However, many redox equations are difficult to balance by the inspection method. In general, you can balance the net ionic equation for a redox reaction by a process known as the half-reaction method. The preceding example of the reaction of magnesium with aluminum nitrate illustrates this method. Specific steps for following the half-reaction method are given below. 

Question (b) is a matter of chemical kinetics and reduces to the need to know the rate equation and the rate constants (customarily designated k) for the various steps involved in the reaction mechanism. Note that the rate equation for a particular reaction is not necessarily obtainable by inspection of the stoichiometry of the reaction, unless the mechanism is a one-step process—and this is something that usually has to be determined by experiment. Chemical reaction time scales range from fractions of a nanosecond to millions of years or more. Thus, even if the answer to question (a) is that the reaction is expected to go to essential completion, the reaction may be so slow as to be totally impractical in engineering terms. A brief review of some basic principles of chemical kinetics is given in Section 2.5. 

These equations allow calculation of the effect of temperature and pressure on the partial Gibbs energy (or chemical potential). They are the partial-property analogs of two equations that follow by inspection from Eq. (10.2) ... 

More complex chemical equations than the ones you have already tried can be balanced by using a combination of inspection and trial and error. Here, however, are some steps to follow. 

Most chemical equations can be balanced by inspection—that is, by trial and error. It is always best to start with the most complicated molecules (those containing the greatest number of atoms). For example, consider the reaction of ethanol with oxygen, given by the unbalanced equation... 

The information provided by the xenon probe is obtained by inspection of the 8 = f(nxe) curve where 5 is the 29xe chemical shift of adsorbed xenon and nxe the number of xenon atoms adsorbed per gram of anhydrous solid. The most general form of the Fraissard s equation can be written 8 = 8o + 8s + 8sAS + 5xe-Xe... 

The empirical formula for a substance is the simplest ratio of the number of atoms or moles of each element. The molecular formula gives the exact number of each atom or moles of atoms in a molecule, whereas the formula unit is the empirical formula for a solid for which no discrete molecules exist. Chemical equations are balanced by inspection, ensuring that the same number of atoms of each element appears on both sides of the equation. 

An elementary reaction is,a reaction that occurs in a single step. The stoichiometric coefficients of an elementary equation give the molecularity of the reaction. The mol-ecularity is the number of molecules colliding at one time to make a reaction. There are three possible molecularities unimolecular, bimolecular, and termolecular. Since the reaction above is elementary, its molecularity is given by a + b. Chemical equations often represent multistep reactions called complex or composite reactions. There is no way to distinguish an elementary reaction from a complex reaction by inspection of the chemical equation. On the MCAT, the only way to know if a reaction is elementary is if you are told that it is elementary. 

The order of a chemical reaction cannot be determined by inspection, it must be determined experimentally. In practice, this means measuring the decomposition of the compound under controlled conditions and applying each of the rate equations in turn to see which type of equation fits the data and gives the best straight line. This is what scientists term an empirical method, and what the man in the street calls trial and error ... 

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