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Unit 3: Chemical Reactions
Overview:
 Balancing chemical reactions
 Equilibria and the concept of reactions not going to completion
 Terminology of solutions
 Dissolution of ionic solids in solvents such as water to give electrically conducting solutions
 Dissolution of molecular solids in water to give non-conducting solutions
 Concept of weak and strong acids according to the extent of ionization
 Concept of net ionic reactions as applied to precipitation and gas-forming reactions
 Oxidation-reduction reactions and the concept of oxidation numbers

Learning Outcomes:
 Understand information conveyed in a balanced chemical reaction
 Be able to balance chemical reactions – remember, must know reactants and products to do
this
 Understand that not all reactions go to completion but result in a “dynamic equilibrium”
 Understand nature of chemical reactions in solution and the terms solution, solute and solvent
 Understand that when an ionic solid dissolves in water the solid is fully dissociated into
separate ions and that such solutions are electrical conductors.
 Understand that aqueous solutions of most molecular compounds do not conduct electricity
are called non-electrolytes.
 Understand that some molecular compounds when dissolved in water break apart into ions. In
some instances they break apart completely (e.g. HCl to produce H+ + Cl-). These are called
strong electrolytes. In other cases, e.g. acetic acid (CH3COOH) we develop an equilibrium
between the neutral CH3COOH and the ions H+ and CH3COO-. This is an example of a weak
electrolyte.
 Know the electrolyte solubility guidelines.
 Be able to predict the outcome of precipitation reactions using the solubility guidelines
 Be able to write precipitation reactions as net ionic reactions and understand the concept of
spectator ions.
 Know the common strong and weak acids and bases.
 Understand the Arrhenius definition of acids and bases
 Understand the Brönsted-Lowry definition of acids and bases
 Understand the idea that a strong acid dissociates 100% into H3O+ in water whereas a weak
acid only partially dissociates.
 Know how to write a net ionic reaction for the reaction of a strong acid (e.g. HCl) and a
strong base (e.g. NaOH)
 Understanding how non-metal and metal oxides act as acids and bases respectively
 Be able to write net ionic reactions for gas forming reactions
 Be able to identify oxidation-reduction reactions and to recognize oxidizing and reducing
agents
 Understanding the connection between oxidation-reduction reactions and electron transfer
 Know the rules for determining the oxidation number for an element in a compound and be
able to use them to determine oxidation numbers
 Understand the role of oxidation numbers in identifying an oxidation-reduction reaction.
 Be able to balance oxidation-reduction reactions
Readings: Sections 3.1 – 3.10
Assignment: From Chapter 3: # 2, 6, 10, 20, 24, 26, 30, 38, 42, 46, 50, 70
Introduction:
Now that we know something about atoms and molecules, it's time to see what kinds of
reactions they can undergo and how we can describe these reactions. This unit will cover
chemical equations, the nature of solutes in aqueous solutions and a few types of common
reactions.
A balanced chemical equation neatly summarizes a reaction. Consider the equation,
2 H2 (g) + O2 (g) → 2 H2O (l). From this, don't know everything about the reaction, but we do
know the reactants, the products, the states of the reactants and products and the stoichiometry of
the reaction. The stoichiometry is an essential component of the equation if we are to perform
calculations such as determining the mass of water formed from a mixture of hydrogen and
oxygen. Remember that atoms are not created or destroyed in reactions, but they are traded.
This means that the number of each type of atom on the reactant side of the equation must equal
the number of each type on the product side. In our example, four hydrogen atoms on the lefthand side means there must be four hydrogen atoms on the right-hand side. If you know the
reactants and products, you will be able to balance many chemical reactions by inspection, but be
aware that some reactions require a more rigorous approach to balancing their equations.
As long as the reactions proceed to completion (so that at least one of the reactants is
completely consumed), we can easily calculate the amount of products formed. Some reactions
do not proceed to completion. We say that the reactants and products are in chemical
equilibrium. In this case, while some products are formed, reactants will also be present, even
after an infinite amount of time. Calculating the amounts of products formed is more involved
and will be covered in another unit in this course.
Since many reactions occur in solution, and often the solvent is water, it is worth
knowing something about the nature of dissolved solutes in aqueous solutions. Ionic compounds
such as NaCl and some covalent compounds such as HCl dissolve in water to completely
produce ions. These are known as strong electrolytes. Other compounds such as acetic acid
(CH3COOH) dissolve in water, but only a small fraction of the acetic acid exists as the ions,
CH3COO- and H3O+. Compounds that do not completely ionize in water are weak electrolytes.
Other compounds such as ethanol and glucose dissolve in water without producing ions. These
are non-electrolytes. In the previous unit, you tried to develop a mental image of what atoms and
molecules looked like, now you should try the same thing, but with the molecules and ions as
they exist dissolved in water.
For the first time in this course, some solubility guidelines are provided to assist you in
determining when a precipitate forms as a result of a reaction. Many factors determine whether a
compound will be soluble, and these factors will be discussed in subsequent units, but for now
you can simply apply the guidelines. Note the exceptions to the guidelines, and be aware that
other exceptions may exist that are not listed in your book.
Even though we are only in unit three, it is time to discuss acids and bases. Most people
have an idea of the properties of an acid or base, but their chemical nature remained a mystery
until relatively recently. Today, even the word, acid, has more than one meaning. The
Arrhenius and the Bronsted-Lowry definitions describe acids (and bases) in terms of chemical
species. The Bronsted-Lowry definition of an acid is fairly comprehensive and is worth knowing
at this point. By this definition, an acid is a species that donates a proton (H+), and a base is a
species that accepts a proton. Acids and bases, when mixed, neutralize each other. Consider
HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq). Note that this can also be described by the
net ionic equation, H+(aq) + OH- (aq) → H2O (l). Before leaving this unit, be sure to study the
tables listing common strong and weak acids and bases. It is important that you are able to
identify which compounds are strong and weak acids and which are strong and weak bases.
The final section in this unit deals with redox reactions. Be able to define reduction and
oxidation. From here, you should be able to identify which compounds should be good
oxidizing and reducing agents. You will notice that a set of rules helps you determine the
oxidation number of atoms within molecules or ions. Once you have mastered these concepts,
you will be able to balance redox equations. Just as acid-base reactions are very common, so are
redox reactions. Consider that through redox reactions, plants oxidize water to form oxygen, and
then animals reduce oxygen to form water. This should emphasize the importance of redox
reactions.
Concluding Remarks:
This unit introduces some general types of reactions such as acid-base and redox
reactions. Such reactions are neatly described by their balanced chemical equations. And since
many of these reactions occur in water as the solvent, we have considered the nature of the
species in solution. Remember that some compounds simply dissolve in water to produce free
molecules while other compounds (electrolytes) dissolve to produce ions. It may appear that you
should memorize many of the reactions presented in this unit, but instead make an attempt to
learn a few of them. Later, as you learn more theory, you will be able to more accurately predict
which reactions will occur and the products of those reactions.