The study of chemistry is all about chemical reactions.
Chapter 4 classifies the major types of chemical reactions,
and also includes interesting and important information
about water, the solvent of life on planet earth.
Chapter 4 Outline
4.1 Types of Chemical Reactions: An Accounting of Reactants and Products
4.2 The Role of Water as a Solvent
4.3 Some Important Aqueous Ionic Reactions; Writing Ionic Equations
4.4 Oxidation-Reduction (Redox) Reactions
4.5 Reversible Reactions: An Introduction to Chemical Equilibrium
Breakdown, Discussion & Help With Sample Problems: What You Should Know
4.1 Types of Chemical Reactions: An Accounting of Reactants and Products
Chemical reactions can be classfied according to the kinds and numbers of reactant substances and product substances. This section contains extensive explanations for, and numerous examples of, the three main reaction types: combination, decomposition, and displacement reactions.
4.2 The Role of Water as a Solvent
The importance of water in earth-bound processes cannot be over-emphasized. Section 4.2 contains several paragraphs that aim to describe various aspects of water's properties, including its polar nature and solubilizing ability. The Sample Problems in 4.2 are two illustrations of using the concepts of molarity and concentration...and should be mastered by all CHEM 200 students.
click here for a few words about SAMPLE PROBLEM 4.2 (page 140)
click here for a few words about SAMPLE PROBLEM 4.3 (page 144)
4.3 Some Important Aqueous Ionic Reactions; Writing Ionic Equations
In section 3.3, CHEM 200 students learned the mechanics of balancing chemical equations. There are several classifications of chemical reactions, though, that need to be balanced, including ionic reactions, precipitation reactions and acid-base reactions. Section 4.3 includes material for all of these reactions.
A key component in developing sufficient understanding of precipitation reactions is the comprehension of the data with Table 4.1: Solubility Rules for Ionic Compounds in Water. Many ions in an ionic reaction remain dissolved in water...only when the ionic compounds that form in these reactions are insoluble do they "fall" out of solution. Table 4.1 enables students to make sense out of these reactions.
An additional important part of 4.3 is the description of neutralization reactions. Neutralization reactions are those when an acid reacts with a base...forming a salt and water. It is important for students to understand that the fragment being transferred from the acid to the base is simply a proton...hence the name proton transfer reaction for acid-base reactions.
Appropriate definitions for acids and bases are therefore needed. Silberberg (p 148) defines an acdid as a substance that produces protons (H+ ions) when dissolved in water...while a base is a substance that produces hydroxide ions (-OH) when dissolved in water.
Finally, an additional important concept discussed in this section is the acid-base titration. A titration is an experiment in which a solution of known concentration is used to determine the concentrations of another solution through a monitored reaction. CHEM 201 students normally carry out at least one acid-base titration per semester...an experiment in which a known amount and concentration of acid (or base) is titrated vs. a known amount of acid (or base) of unknown concentration. In the experiment, the careful student can determine the concentration of the acid (or base).
Sample Problem 4.6 illustrates the concept of acid-base titrations...
click here for a few words about SAMPLE PROBLEM 4.6 (page 151)
4.4 Oxidation-Reduction (Redox) Reactions
In redox reactions, the key chemical event is the net movement of electrons from one reactant to the other. Oxidation is the loss of electrons, and reduction is the gain of electrons. Conversely, an oxidizing agent is a species that oxidizes one of the other reactants. Therefore, an oxidizing agent gets reduced. Similarly, a reducing agent is a species that reduces one of the other reactants. Therefore a reducing agent gets oxidized.
In order to fully grasp the important concepts surrounding redox reactions, CHEM 200 students must be able to comprehend the information presented in Table 4.3 (p 155): Rules for Assigning an Oxidation Number. Initially, the rules within Table 4.3 are perhaps best memorized.
Balancing redox reactions (pages 157-159) is yet another topic in the general area of balancing chemical reactions. In essence, CHEM 200 students learn that, when balancing redox equation, in addition to balancing atoms and charge...it is also important to balance the number of electrons that are transferred.
There are at least two methods for balancing redox reactions.
One of these is the oxidation number method, which is outlined on page 157 of Silberberg's text.
Pay particular attention to Sample Problem 4.9, which asks students to balance two different redox reactions using the oxidation number method.
click here for a few words about SAMPLE PROBLEM 4.9 (page 157-8)
Another technique often utilized to balance redox reactions is referred to as the half-reaction method (page 159). The half-reaction method is discussed in further detail in Chapter 20.
Finally, electrons can be titrated in a process referred to as a redox titrations, in a fashion analogous to protons (in acid-base titrations, section 4.3).
click here for a few words about SAMPLE PROBLEM 4.10 (page 161-2), which is concerned with concentration determinations doing redox titrations
4.5 Reversible Reactions: An Introduction to Chemical Equilibrium
In this short section, CHEM 200 students are introduced to the concept of equilibrium.
In essence, a chemical reaction should not be thought of as a one-way left-to-right process. Instead, at the very same time that a reaction is proceeding from the reactant (i.e. left) side of the arrow to the product (i.e. right) side of the arrow, at least some small fraction of "product" moves back from right-to-left and forms "reactant".
A reaction mixture has reached dynamic equilibrium when no further change occurs in the amounts of reactants and products, because the forward and reverse reactions are occurring at the same speed.
exercises such as numbers 4.14, 4.52, 4.66, 4.81, and 4.89 are all straightforward problems of the type that are often found on CHEM 200 exams