3.2 Patterns of Chemical Reactivity

Our discussion in Section 3.1 focused on how to balance chemical equations, given the reactants and products for the reaction. We did not try to say anything about the type of reaction or to predict the products. However, an introductory discussion now will help you feel more comfortable with the reactions used to illustrate concepts in the chapter.

Using the Periodic Table

We may identify reactants and products experimentally by their properties. However, we can often predict what will happen in a reaction if we have seen a similar reaction before. Naturally, recognizing a general pattern of reactivity for a class of substances gives you a broader understanding than merely memorizing a large number of unrelated reactions. The periodic table can be a powerful ally in this regard. For example, knowing that sodium, Na, reacts with water, H2O, to form NaOH and H2 (see Sample Exercise 3.1), we can predict what happens when potassium, K, is placed in water. Both sodium and potassium are in the same group of the periodic table (the alkali metal group—group 1A). We expect them to behave similarly, producing the same kinds of products. Indeed, this prediction is correct:

PM03010

[3.6]

In fact, all alkali metals react with water to form their hydroxide compounds and hydrogen. If we let M represent any alkali metal, we can write the general reaction as follows:

PM03011
PM03012

[3.7]

Figure 3.4 shows photographs of the reactions of sodium and of potassium with water. In the chapters ahead we will encounter reactions that are characteristic of a particular group of elements or of particular classes of compounds.

Combustion in Air

Combustion reactions are rapid reactions that produce a flame. Most of the combustion reactions we observe involve O2 from air as a reactant. Equation 3.5 and Practice Exercise 3.1(a) illustrate a general class of reactions involving the burning or combustion of hydrocarbon compounds (compounds that contain only carbon and hydrogen, such as CH4 and C2H6).

When hydrocarbons are combusted, they react with O2 to form CO2 and H2O. (When there is an insufficient quantity of O2 present, carbon monoxide, CO, will be produced. Even more severe restriction of O2 will cause the production of fine particles of carbon that we call soot. Complete combustion produces CO2. Unless specifically stated to the contrary, we will take combustion to mean complete combustion.) The number of molecules of O2 required in the reaction and the number of molecules of CO2 and H2O formed depend on the composition of the hydrocarbon. For example, the combustion of propane, C3H8, a gas used for cooking and home heating, is described by the following equation:

PM03013

[3.8]

The blue flame produced when propane burns is shown in Figure 3.5 .

Combustion of compounds containing oxygen atoms as well as carbon and hydrogen atoms (for example, CH3OH and C6H12O6) also produce CO2 and H2O. The simple rule that hydrocarbons and related compounds form CO2 and H2O when they burn in air summarizes the behavior of about 3 million compounds. Many compounds that our bodies use as energy sources, such as the sugar glucose, C6H12O6, similarly react in our bodies with O2 to form CO2 and H2O. In our bodies, however, the reactions take place in a series of steps that occur at body temperature.

SAMPLE EXERCISE 3.2

Write the balanced chemical equation for the reaction that occurs when methanol, CH3OH(l), is burned in air.

SOLUTION We first recall that when any compound containing C, H, and O is combusted, it reacts with the O2(g) in air to produce CO2(g) and H2O(l). Thus, the unbalanced equation is

PM03014

Because CH3OH has only one C atom, we can start balancing the equation using the coefficient 1 for CO2. Because CH3OH has four H atoms, we place a coefficient 2 in front of H2O to balance the H atoms:

PM03015

This gives four O atoms among the products and three among the reactants (one in CH3OH and two in O2). We can use the fractional coefficient PM03016 in front of O2 to provide four O atoms among the reactants (there are PM03016 2 = 3 O atoms in PM03018):

PM03019

Although the equation is now balanced, it is not in its most conventional form because it contains a fractional coefficient. If we multiply each side of the equation by 2, we will remove the fraction and achieve the following balanced equation:

PM03020

PRACTICE EXERCISE

Write the balanced chemical equation for the reaction that occurs when ethanol, C2H5OH(l), is burned in air.

Answer: PM03021

Combination and Decomposition Reactions

Table 3.1 summarizes two other simple types of reactions, combination and decomposition reactions. In combination reactions two or more substances react to form one product. There are many examples of such reactions, especially those in which different elements combine to form compounds. For example, magnesium metal burns in air with a dazzling brilliance to produce magnesium oxide, as shown in Figure 3.6:

PM03022

[3.9]

This reaction is employed in flares.

When a combination reaction occurs between a metal and a nonmetal, as in Equation 3.9, the product is an ionic solid. Recall that the formula of an ionic compound can be determined from the charges of the ions involved. For example, when magnesium reacts with oxygen, the magnesium loses electrons and forms the magnesium ion, Mg2+. The oxygen gains electrons and forms the oxide ion, O2–. Thus, the reaction product is MgO.

In a decomposition reaction one substance undergoes a reaction to produce two or more other substances. Many compounds undergo decomposition reactions when heated. For example, many metal carbonates decompose to form metal oxides and carbon dioxide when heated:

PM03023

[3.10]

The decomposition of CaCO3 is an important commercial process. Limestone or seashells, which are both primarily CaCO3, are heated to prepare CaO, which is known as lime or quicklime. Over 1.8 1010 kg (20 million tons) of CaO are used in the United States each year, principally in making glass, in obtaining iron from its ores, and in making mortar to bind bricks.

As a further example, the decomposition of sodium azide, NaN3, is used to inflate safety air bags in automobiles (Figure 3.7 ). The decomposition reaction rapidly releases N2(g), which inflates the air bag:

PM03024

[3.11]

The system is designed so that an impact causes the ignition of a detonator cap, which in turn causes NaN3 to decompose explosively. A small quantity of NaN3 (about 100 g) forms a large quantity of gas (about 50 L).

SAMPLE EXERCISE 3.3

Write balanced chemical equations for the following reactions: (a) The combination reaction that occurs when lithium metal and fluorine gas react. (b) The decomposition reaction that occurs when solid barium carbonate is heated. (Two products form: a solid and a gas.)

SOLUTION (a) The symbol for lithium is Li. With the exception of mercury, all metals are solids at room temperature. Fluorine occurs as a diatomic molecule (see Figure 2.15). Thus, the reactants are Li(s) and F2(g). The product will be a compound of a metal and a nonmetal, and consequently we expect it to be an ionic solid. Lithium ions have a 1+ charge, Li+, whereas fluoride ions have a 1- charge, F-. Thus, the chemical formula for the product is LiF. The balanced chemical equation is

PM03025

(b) First we must write the chemical formula for barium carbonate, BaCO3. As noted in the text, many metal carbonates decompose to form metal oxides and carbon dioxide when heated. In fact, the example given in Equation 3.10 involves CaCO3, which decomposes to form CaO and CO2. Thus, we would suspect that BaCO3 decomposes to form BaO and CO2. The fact that barium and calcium are both in the same group in the periodic table, group 2A, further supports the idea that they would react in the same way:

PM03026

PRACTICE EXERCISE

Write balanced chemical equations for the following reactions: (a) solid mercury(II) sulfide decomposes into its component elements when heated; (b) the surface of aluminum metal undergoes a combination reaction with oxygen in the air; (c) Si2H6 burns when exposed to air. (Note that Si is in the same group in the periodic table as C. The states of the reactants and products need not be given.)

Answers: (a) HgS(s) Hg(l) + S(s); (b) 4Al(s) + 3O2(g) 2Al2O3(s); (c) 2Si2H6 + 7O2 4SiO2 + 6H2O