Balancing of chemical equations

Reactions occur because of collisions. One chemical species collides with another at the right place, transfers enough energy, and a chemical reaction occurs. Such reactions can be very fast or very slow. In the chapter on Kinetics, you can study how reactions occur and the factors that affect the speed of reactions. But in this chapter we will review the balancing of chemical equations, discuss the general types of chemical reactions, and describe why these reactions occur. 

The authors hope that, because you are preparing to take the AP exam, you have already been exposed to the balancing of chemical equations. We will quickly review this topic and point out some specific aspects of balancing equations as the different types of chemical reactions are discussed. 

The balancing of chemical equations is a common exercise in elementary stoichiometry. It is not always appreciated, however, that some chemical equations are ambiguous in that they can be balanced in more than one way. Consider, for example, the following equation ... 

The balance of chemical equations occurs often by inspection, whatever this is. The... 

How is the balancing of chemical equations related to the law of conservation of mass ... 

Use check calculation to check correct balancing of chemical equation in each step. Copy and paste columns B, C, and D into columns BS, BT, and BU. Check to see that entries in column BX are all zero for correct balancing of each chemical equation. 

Campanario, J.M. (1995). Automatic balancing of chemical equations. Computers in Chemistry, 19, 85-90. 

DEFINITION OF CHEMICAL REACTIONS AND BALANCING OF CHEMICAL EQUATIONS... 

Write and balance the chemical equation for the combustion of hexane, GfcH14, to gaseous carbon dioxide gas and gaseous water. 

Because electrons can be neither lost nor created in a chemical reaction, all the electrons lost by the species being oxidized must be transferred to the species being reduced. Because electrons are charged, the total charge of the reactants must be the same as the total charge of the products. Therefore, when balancing the chemical equation for a redox reaction, we have to balance the charges as well as the atoms. 

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. 

Calculate the reaction quotient, Q, for the cell reaction, given the measured values of the cell emf. Balance the chemical equations by using the smallest whole-number coefficients. 

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). 

Yarroch, W. L. (1985). Students understanding of chemical equation balancing Journal of Research in Science Teaching, 22, 449 59. 

It is easiest to balance a chemical equation one element at a time, starting with the elements that appear in only one substance on each side. Notice that all of the carbon atoms in propane end up in carbon dioxide molecules, and all of propane s hydrogen atoms appear in water molecules. This feature allows us to balance carbon and hydrogen easily. 

C04-0041. Several examples of chemical reasoning are introduced in this chapter. Write out the reasoning steps that you will follow in (a) balancing a chemical equation (b) identifying the limiting reactant (c) determining whether a precipitate forms and (d) computing a reaction yield. 

Data are given for all reactants, so this is a limiting reactant problem. We must balance the chemical equation and then work with a table of molar amounts. 

To balance a chemical equation, the chemical formulas for the reactants are first written on the left-hand side of the equation. For example, in the reaction between hydrochloric acid and sodium hydroxide, the chemical formulas are HCl and NaOH, respectively. Remember that if an acid starts with the prefix hydro-, it is a binary acid. That means that hydrochloric acid is made up of only two... 

Before you can balance a chemical equation, you have to know the formulas for all the reactants and products. If the names are given for these substances, you have to know how to write formulas from the names (Chap. 6). If reactants only are given, you have to know how to predict the products from the reactants. This latter topic is the subject of this section. 

The Law of Conservation of Matter provides the basis for balancing a chemical equation. It states that matter is neither created nor destroyed during an ordinary chemical reaction. Therefore, a balanced chemical equation must always contain the same number of each kind of atom on both sides of the equation. 

The oxidation state of an element in a compound is an indication of how many electrons each atom of that element has lost (positive oxidation state) or gained (negative). Since oxidation state is determined by a set of rules, rather than by experiment, its connection to the number of electrons actually transferred is rather tenuous. It is used in naming compounds and balancing some chemical equations. 

B The first task is to balance the chemical equation. There must be three moles of hydrogen for every mole of nitrogen in both products (because of the formula ofNH3) and reactants N2 (g) + 3 H2 (g) -> 2 NH3 (g). The volumes of gaseous reactants and products are related... 

We first balance the chemical equation and then calculate the value of AG° with data from Appendix D, and finally calculate the value of with the use of AG° = -RT nK. 

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. 

A proper set of chemical equations provides an aid in chemical book-keeping to determine composition as reaction proceeds. This is the role of chemical stoichiometry. On the one hand, it prescribes elemental balances that must be obeyed as constraints on reaction on the other hand, in prescribing these constraints, it reduces the amount of other information required (e g., from kinetics) to determine the composition. 

Equation 1.5-1 used as a mass balance is normally applied to a chemical species. For a simple system (Section 1.4.4), only one equation is required, and it is a matter of convenience which substance is chosen. For a complex system, the maximum number of independent mass balance equations is equal to R, the number of chemical equations or noncomponent species. Here also it is largely a matter of convenience which species are chosen. Whether the system is simple or complex, there is usually only one energy balance. 

It is not difficult to write a number of chemical equations to represent physical, thermal, and chemical reactions taking place in a gasification vessel. In theory, gasification processes can be designed so that heat release (exothermic reactions) balances the heat required by endothermic reactions. But in practice many of the above physical, thermal, and chemical reactions may take place simultaneously, making a precise prediction of the quantity and quality or composition of product gas somewhat difficult. 

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 example, we could use this reaction for determining the concentration of acetic acid, HQHjO in vinegar. A titration problem will give you information about one reactant, and ask you for information about the other reactant. In most titration reactions, information about the products will not be necessary. You only need to consider the products when we need to balance the chemical equation. 

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. 

In fact, you can use oxidation numbers to balance a chemical equation by a new method. The oxidation number method is a method of balancing redox equations by ensuring that the total increase in the oxidation numbers of the oxidized element(s) equals the total decrease in the oxidation numbers of the reduced element(s). 

A balanced hypothetical chemical equation indicating the transfer of electrons between two different oxidation states of the same element of chemical species. 2. One segment of a ping-pong (or double-displacement) enzyme mechanism. 

All reactions must satisfy ijiass conservation. The reaction A B must be an isomerization reaction because the molecular weights of A and B must be identical. Also, one should add to these relations the requirements that the number of atoms of each element must be conserved, but this is usually intuitively obvious for most reaction systems. We do this whenever we balance a chemical equation. 

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