Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Reactions in Aqueous Solutions
Ways to Classify Reactions

Objective

· To learn various classification schemes for reactions.

So far in our study of chemistry we have seen many, many chemical reactions—and this is just Chapter 7. In the world around us and in our bodies, literally millions of chemical reactions are taking place. Obviously, we need a system for putting reactions into meaningful classes that will make them easier to remember and easier to understand.

In Chapter 7 we have so far considered the following “driving forces” for chemical reactions:

· Formation of a solid

· Formation of water

· Transfer of electrons

We will now discuss how to classify reactions involving these processes. For example, in the reaction

solid (a precipitate) is formed. Because the formation of a solid when two solutions are mixed is called precipitation, we call this a precipitation reaction .

Notice in this reaction that two anions ( and ) are simply exchanged. Note that was originally associated with in and that was associated with in . In the products these associations are reversed. Because of this double exchange, we sometimes call this reaction a double-exchange reaction or double-displacement reaction . We might represent such a reaction as

So we can classify a reaction such as this one as a precipitation reaction or as a double-displacement reaction. Either name is correct, but the former is more commonly used by chemists.

In this chapter we have also considered reactions in which water is formed when a strong acid is mixed with a strong base. All of these reactions had the same net ionic equation:

The ion comes from a strong acid, such as or , and the origin of the ion is a strong base, such as or . An example is

We classify these reactions as acid—base reactions . You can recognize this as an acid—base reaction because it involves an ion that ends up in the product water.

The third driving force is electron transfer. We see evidence of this driving force particularly in the “desire” of a metal to donate electrons to nonmetals. An example is

where each lithium atom loses one electron to form , and each fluorine atom gains one electron to form the ion. The process of electron transfer is also called oxidation—reduction. Thus we classify the preceding reaction as an oxidation—reduction reaction.

Chemistry in Focus Oxidation—Reduction Reactions Launch the Space Shuttle

Launching into space a vehicle that weighs millions of pounds requires unimaginable quantities of energy—all furnished by oxidation—reduction reactions.

Notice from Fig. 7.8 that three cylindrical objects are attached to the shuttle orbiter. In the center is a tank about feet in diameter and feet long that contains liquid oxygen and liquid hydrogen (in separate compartments). These fuels are fed to the orbiter’s rocket engines, where they react to form water and release a huge quantity of energy.

Figure 7.8.

(Adapted from Chem. Eng. News, September 19, 1988, pg. 9. © American Chemical Society)

For launch, the Space Shuttle Orbiter is attached to two solid-fuel rockets (left and right) and a fuel tank (center) that supplies hydrogen and oxygen to the orbiter’s engines.

Note that we can recognize this reaction as an oxidation—reduction reaction because is a reactant.

Two solid-fuel rockets feet in diameter and feet long are also attached to the orbiter. Each rocket contains million pounds of fuel: ammonium perchlorate and powdered aluminum mixed with a binder (“glue”). Because the rockets are so large, they are built in segments and assembled at the launch site as shown in Fig. 7.9. Each segment is filled with the syrupy propellant (Fig. 7.10), which then solidifies to a consistency much like that of a hard rubber eraser.

Figure 7.9.

(Adapted from Chem. Eng. News, September 19, 1988, pg. 9. © American Chemical Society)

The solid-fuel rockets are assembled from segments to make loading the fuel more convenient.

Figure 7.10.

Orbital ATK

A mix bowl mixing propellant for a rocket motor.

The oxidation—reduction reaction between the ammonium perchlorate and the aluminum is represented as follows:

It produces temperatures of about and million pounds of thrust in each rocket.

Thus we can see that oxidation—reduction reactions furnish the energy to launch the space shuttle.

See Problem 7.52

An additional driving force for chemical reactions that we have not yet discussed is formation of a gas. A reaction in aqueous solution that forms a gas (which escapes as bubbles) is pulled toward the products by this event. An example is the reaction

for which the net ionic equation is

Note that this reaction forms carbon dioxide gas as well as water, so it illustrates two of the driving forces that we have considered. Because this reaction involves that ends up in the product water, we classify it as an acid—base reaction.

Consider another reaction that forms a gas:

How might we classify this reaction? A careful look at the reactants and products shows the following:

Note that in the reactant zinc metal, exists as uncharged atoms, whereas in the product it exists as . Thus each atom loses two electrons. Where have these electrons gone? They have been transferred to two ions to form . The schematic for this reaction is

This is an electron transfer process, so the reaction can be classified as an oxidation—reduction reaction.

Another way this reaction is sometimes classified is based on the fact that a single type of anion has been exchanged between and . That is, is originally associated with in and ends up associated with in the product . We can call this a single-replacement reaction in contrast to double-displacement reactions, in which two types of anions are exchanged. We can represent a single replacement as