C. Reactions Main

Introduction

Types of Reactions

Stoichiometry

Limiting Reagents and Theoretical Yield

Product- and Reactant-Favored Reactions

Potential-Energy Curves

k, The Equilibrium Constant

Le Chatlier's Principle

Rates of Reaction

Hess' Law

Catalysts

Practice Problems



Types of Reaction

Chemists have sorted reactions into several types to make them easier to classify and understand. We'll go through each of the major categories below.

Replacement reactions: These reactions occur when one atom or group takes the place of another atom or group, as in the following example:

Zn + Cu(NO3)2 Cu + Zn(NO3)2

In this case, the Zn replaced Cu. Note that the subscripts each atom may change during replacement; for instance, the following reaction is still classified as single-replacement:

Ca + 2 HCl H2 + CaCl2

If we ignore the amount of atoms in each molecule, we can see that the same type of substitution occurred. The general format for substitutions is:

AB + C AC + B

Double-replacement: As the name implies, these are reactions in which parts of two molecules switch places, as in the following reaction:

CaCl2 + 2 AgNO3 2 AgCl + Ca(NO3)2

In this example, we can say that Ca and Ag switched places, or that Cl and NO3 did. The structure for double-replacement reactions is:

AB + CD AC + BD

Neutralization reactions: A special type of double-replacement reactions, these reactions involve an acid and a base reacting to form water and a salt:

HCl (acid) + NaOH (base) H2O + NaCl

These reactions have the format shown below (note that strong bases always have the group "OH", while strong acids have the group "H"; these combine to make water):

AH + BOH H2O + AB

Addition reactions: In these reactions, a molecule or atom is added to another molecule or atom to form a more complex molecule. The reaction below is an example:

HCl + C6H12 C6H13Cl

Addition reactions have the general pattern of:

A + B AB (more reactants can be used if needed)

Synthesis reactions: These reactions can be thought of as a sub-type of formation reactions. In synthesis reactions, a compound is formed from chemical elements only, as in the following example:

2 H2 + O2 2 H2O

These reactions have the same format as addition reactions, but the reactants may only be elements:

A (element) + B (element) AB (again, more reactants are often present)

Formation reactions: A sub-type of synthesis reactions, formation reactions require that the coefficient on the product be 1. Fractional coefficients are allowed on reactants to make this law apply. Changing the above example to a formation reaction would be:

H2 + 1/2 O2 H2O

The general format for formation reactions is identical to addition and synthesis reactions, but the reactants must be elements and the product must have coefficient of 1: A (element) + B (element) 1 AB (more reactants can be present)

Combustion reactions: In these reactions, an organic, or carbon-containing, molecule reacts with oxygen to produce carbon dioxide and water. If other elements are present in the organic compound, they usually appear as products in their elemental state (N2, for instance). The energy changes for these reactions are usually largely negative (they generate a lot of heat!). An everyday example is the combustion of heptane, one of the major ingredients in gasoline:

C7H16 + 11 O2 8 H2O + 7 CO2    ΔH = -large

The structure of combustion reactions is usually the following:

A + O2 H2O + CO2

where A is a carbon-containing, hydrogen-containing molecule. If A has more elements, than they will appear as products as well. Coefficients will not usually be 1.

Decomposition reactions: These reactions can be thought of as the reverse of addition reactions; a large molecule breaks apart into simpler molecules, often its elements. Sometimes, oxygen or another compound is also a reactant. An example is the decomposition of sugar:

C6H12O6 6 H2O + 6 C

The general format for decomposition reactions is:

ABC (large molecule) AB + C (products may be elemental)

Net Ionic Equations: When an ionic substance is placed in water, it usually breaks apart into molecules or atoms: one positively charged, one negatively charged, as salt does in the reaction below:

NaCl(s) (in water) Na+(aq) + Cl-(aq)

When a reaction such as the one below occurs in water,

NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

it is more accurate to write each aqueous group as dissolved ions, as shown here:

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3- (aq) AgCl(s) + Na+ + NO3-

Notice that the Na and NO3 ions do not contribute anything to the reaction--they stay exactly the same both before and after. These types of ions, which do not participate in a reaction, are called spectator ions. They should be eliminated from the reaction, since they are not involved:

Ag+(aq) + Cl-(aq) AgCl(s)

This simplified reaction is called the net ionic equation, or "net equation" for short. It is where we got the name for this web site; we couldn't resist a pun like that! The general form of a net ionic equation is hard to illustrate, but the only ions in the reaction must be those that are somehow affected by it.

Oxidation-Reduction Reactions: These reactions, called "redox" for short, involve the transfer of electrons between species. You can often recognize these reactions by changes in the charges of species (although not always). These reactions will be discussed in detail in the "Redox Reactions" chapter, but we'll give you an example here:

Cl2(g) + Zn(s) 2 Cl- (aq) + Zn2+(aq)

You'll notice that each chlorine atom has gained an electron, giving it a negative charge. To facilitate this, the zinc has lost two electrons. Again, there is no general format for a redox reaction; just look for changes in charges, and you should be fine until the redox chapter.

Well, those are the main groups of chemical reactions. Some sample problems are below: first, identify the type of each reaction. Then, rewrite the aqueous reactions below in full ionic form and as net ionic equations.

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