Balancing equations by inspection

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. 

It is stated in Section 6.3 of the text that to balance equations by inspection you start "with the most complicated molecule." What does this mean Why is it best to do this ... 

Most of the equations in the remainder of this chapter can be balanced by inspection. The following procedure demonstrates how to master balancing equations by inspection using a step-by-step approach. The equation for the decomposition of water (see Figure 1.7) will be used as an example. 

CHq + NH3 + O2 HCN + H2 O Balance the equation by inspection, one element at a time. Carbon and nitrogen are in balance when there are equal numbers of molecules of CH4, NH3, and HCN. Hydrogen and oxygen, however, are not in balance. There are seven hydrogen atoms on the left but only three on the right. The coefficient for HCN cannot be changed without unbalancing C and N. Thus, to balance H, we must multiply the coefficient for H2 O by 3 ... 

The equation is balanced. Although we could have balanced this equation by inspection, the detailed procedure reveals its redox nature. 

With simple reactions it is usually possible to balance the stoichiometric equation by inspection, or by trial and error calculations. If difficulty is experienced in balancing complex equations, the problem can always be solved by writing a balance for each element present. The procedure is illustrated in Example 2.3. 

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. 

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. 

Balance each of the following redox equations by inspection. Write the balanced half-reactions in each case. 

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

A completely different method for balancing complex redox equations by inspection has been described by D. Kolb. J. Chem. Ed. 1981,58, 642. For some challenging problems in balancing redox equations, see R. Stout, J. Chem. 

Balance the equation by inspection, starting with the most complicated molecule(s). Determine what coefficients are necessary to ensure that the same number of each type of atom appears on both reactant and product sides. Do not change the identities (formulas) of any of the reactants or products. 

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. 

Step 6. Add the two half-reactions together and balance the final equation by inspection. The electrons on both sides must cancel. 

Step 3 Balance the equation by inspection, starting with the most complicated molecule. 

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

Notice that the carbon and hydrogen atoms are not balanced. There are two carbon atoms on the left and one on the right, and there are six hydrogens on the left and two on the right. We need to find the correct numbers of reactants and products so that we have the same number of all types of atoms among the reactants and products. We will balance the equation "by inspection" (a systematic trial-and-error procedure). 

This equation is not balanced, but by inspection it is easily balanced ... 

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

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

We can now balance the remainder of the equation by inspection. Notice that there are 4 moles of K on the left, but only 1 mole of on the right. The balanced equation is ... 

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

Why is a systematic method for balancing oxidation-reduction reactions necessary Why can t these equations be balanced readily by inspection ... 

Step 4 Add the two half-equations together and balance the final equation by inspection. The electrons on both sides must cancel. If the oxidation and reduction half-reactions contain different numbers of electrons, we need to multiply one or both half-reactions to equalize the number of electrons. 

In this chapter, 1 show you how to balance equations by using a method called balancing by inspection, or as 1 call it, fiddling with coefficients. You take each atom in turn and balance it by adding appropriate coefficients to one side or the other. 

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