Chemistry Essentials for Dummies

Chapter 7. Chemical Reactions

Types of Reactions

Several general types of chemical reactions can occur based upon the identity of the reactants and products and/or which bonds are broken and made. The more common reactions are combination, decomposition, single displacement, double displacement, combustion, and reduction-oxidation (redox) reactions. I describe them all here.

Combination reactions: Coming together

In combination reactions, two or more reactants form one product. The reaction of sodium and chlorine to form sodium chloride,

and the burning of coal (carbon) to give carbon dioxide,

are examples of combination reactions.

Decomposition reactions: Breaking down

In decomposition reactions, a single compound breaks down into two or more simpler substances (elements and/or compounds). Decomposition reactions are the opposite of combination reactions. The decomposition of water into hydrogen and oxygen gases,

and the decomposition of hydrogen peroxide to form oxygen gas and water,

are examples of decomposition reactions.

Single displacement reactions: Kicking out another element

In single displacement reactions, a more active element displaces (kicks out) another less active element from a compound. For example, if you put a piece of zinc metal into a copper (II) sulfate solution, the zinc displaces the copper, as this next equation shows (in case you’re wondering, Chapter 5 explains why copper (II) sulfate is named the way it is):

The notation (aq) indicates that the compound is dissolved in water — in an aqueous solution.

Because zinc replaces copper in this case, it’s said to be more active. If you instead place a piece of copper in a zinc sulfate solution, nothing will happen.

Using the activity series

Table 7-1 shows the activity series of some common metals. Notice that because zinc is more active in the table, it will replace copper, just as the earlier equation shows.

Table 7-1. The Activity Series of Common Metals

Activity

Metal

Most active

Alkali and alkaline earth metals

Al (aluminum)

Zn (zinc)

Cr (chromium)

Fe (iron)

Ni (nickel)

Se (selenium)

Pb (lead)

Cu (copper)

Ag (silver)

Least active

Au (gold)

Writing ionic and net-ionic equations

Take a look at the reaction between zinc metal and a copper (II) sulfate solution. I’ve written this reaction as a molecular equation, showing all species in the neutral form.

However, these reactions normally occur in an aqueous (water) solution. When you dissolve the ionically bonded CuSO4 in water, it breaks apart into ions (atoms or groups of atoms that have an electrical charge due to the loss or gain of electrons). The copper ion has a 2+ charge because it lost two electrons. It’s a cation, a positively charged ion. The sulfate ion has a 2- charge because it has two extra electrons. It’s an anion, a negatively charged ion. (Check out Chapter 5 for a more complete discussion of ionic bonding.)

Here’s an equation that shows the ions separately. Equations in this form are called ionic equations because they show the reaction and production of ions. Notice that the sulfate ion, SO42-, doesn’t change in the reaction:

REMEMBER. Ions that don’t change during the reaction and are found on both sides of the equation in an identical form are called spectator ions. Chemists often omit the spectator ions and write the equation showing only those chemical substances that are changed during the reaction. This is called the net-ionic equation:

Double displacement reactions: Trading places

In single displacement reactions (see the preceding section), only one chemical species is displaced. In double displacement reactions, or metathesis reactions, two species (normally ions) are displaced. Most of the time, reactions of this type occur in a solution, and either an insoluble solid (in precipitation reactions) or water (in neutralization reactions) will be formed.

Precipitation reactions: Forming solids

The formation of an insoluble solid in a solution is called precipitation. For instance, if you mix a solution of potassium chloride and a solution of silver nitrate, a white insoluble solid forms in the resulting solution. Here are the molecular, ionic, and net-ionic equations for this double-displacement reaction:

The white insoluble solid that forms (AgCl) is silver chloride. You can drop out the potassium cation and nitrate anion spectator ions, because they don’t change during the reaction and are found on both sides of the equation in an identical form. (See the earlier section “Writing ionic and net-ionic equations” for details on spectator ions.)

To write these equations, you have to know something about the solubility of ionic compounds:

If a compound is soluble, it will not react at all and you can represent it by the appropriate ions or use (aq).

If a compound is insoluble, it will precipitate (form a solid).

Table 7-2 gives the solubilities of selected ionic compounds (see Chapter 5 for info on the names of ionic compounds).

Table 7-2. Solubilities of Selected

Ionic

Compounds

Water Soluble

Water Insoluble

All chlorides, bromides, iodides ...

Except those of Ag+, Pb2+, Hg22+

All compound of NH4+

Oxides

All compounds of alkali metals

Sulfides

All acetates

Most phosphates

All nitrates

Most hydroxides

All chlorates

All sulfates ...

Except PbSO4, BaSO4, and SrSO4

To use Table 7-2, take the cation of one reactant and combine it with the anion of the other reactant (and vice versa), keeping the neutrality of the compounds. This allows you to predict the possible products of the reaction. Then look up the solubilities of the possible products in the table. If the compound is insoluble, it’ll precipitate. If it’s soluble, it’ll remain in solution.

Neutralization reactions: Forming Water

Besides precipitation (see the preceding section), the other type of double-displacement reaction is the reaction between an acid and a base. This double-displacement reaction, called a neutralization reaction, forms water.

Take a look at the mixing solutions of sulfuric acid (auto battery acid, H2SO4) and sodium hydroxide (lye, NaOH). Here are the molecular, ionic, and net-ionic equations for this reaction:

To go from the ionic equation to the net-ionic equation, you let the spectator ions (those that don’t really react and that appear in an unchanged form on both sides on the arrow) drop out. Then reduce the coefficients in front of the reactants and products down to the lowest common denominator.

Combustion reactions: Burning

Combustion reactions occur when a compound, usually one containing carbon, combines with the oxygen gas in the air. This process is commonly called burning. Heat is the most useful product of most combustion reactions.

Here’s the equation that represents the burning of propane:

Combustion reactions are also a type of redox reaction.

Redox reactions: Exchanging electrons

Redox reactions, or reduction-oxidation reactions, are reactions in which electrons are exchanged. The following three reactions are examples of other types of reactions (such as combination, combustion, and single-replacement reactions), but they’re also all redox reactions — they all involve the transfer of electrons from one chemical species to another:

Redox reactions are involved in combustion, rusting, photosynthesis, respiration, the movement of electrons in batteries, and more. I talk about redox reactions in some detail in Chapter 8.