CHEM 121 Overview
Part 2
BALANCED CHEMICAL EQUATIONS
• A balanced chemical equation is one in which the number of
atoms of each element in the reactants is equal to the number
of atoms of that same element in the products.
• A reaction can be balanced by applying the law of conservation
of matter.
• Coefficients (in red below) are written to the left of each
reactant or product in order to achieve balance.
2 H2 (g) + O2 (g) → 2 H2O (l)
IONIC EQUATIONS
• MOLECULAR EQUATIONS
– In a molecular equation, each compound is represented
by its formula.
• TOTAL IONIC EQUATIONS
– In a total ionic equation, all soluble ionic substances are
represented by the ions they form in solution. Substances
that do not dissolve or that dissolve but do not dissociate
into ions are represented by their formulas.
NaCl (aq) = Na+ (aq) + Cl− (aq)
Na2S (aq) = 2 Na+ (aq) + S2− (aq)
Na 3PO4 (aq) = 3 Na+ (aq) + PO43− (aq)
IONIC EQUATIONS EXAMPLE
• Write the following molecular equation in total ionic and net ionic forms.
Soluble substances are indicated by (aq) after their formulas and insoluble
solids are indicated by (s) after their formulas.
BaCl2 (aq) + Na2S(aq)
BaS(s) + 2NaCl(aq)
• In total ionic form, all substances except the insoluble BaS will be written
in the form of the ions they form:
Ba2+(aq) + 2Cl-(aq) + 2Na+(aq) + S2-(aq)
BaS(s) + 2Na+(aq) + 2Cl-(aq)
• In net ionic form, all spectator ions are dropped. Both the Na+ and Clions are spectator ions because they appear on both sides of the
equation. The net ionic equation is:
Ba2+(aq) + S2-(aq)
BaS(s)
THE MOLE AND CHEMICAL EQUATIONS
• The mole concept can be applied to balanced chemical
equations and used to calculate mass relationships in chemical
reactions.
• Balanced equations can be interpreted in terms of the mole
concept and the results used to provide factors for use in factorunit solutions to numerical problems.
THE LIMITING REACTANT
– The limiting reactant present in a mixture of reactants is the
reactant that will run out first, and thus, it determines the
amount of product that can be produced.
– A useful approach to solving limiting reactant problems is to
calculate the amount of product that could be produced by each
of the quantities of reactant that are available. The reactant that
gives the least amount of product is then the limiting reactant.
REACTION YIELDS
• The amount of product calculated in the examples is
called the theoretical yield. The amount of product
actually produced is called the actual yield. These
two quantities are used to calculate the percentage
yield using the following equation:
actual yield
% yield 
 100
theoretica l yeild
OXIDATION NUMBERS
• Oxidation numbers (also called oxidation states) are
positive or negative numbers assigned to elements in
chemical formulas according to a set of rules. The term
oxidation number is abbreviated O.N.
– Rule 1: The O.N. of any uncombined element is 0.
– Rule 2: The O.N. of a simple ion is
equal to the charge on the ion.
– Rule 3: The O.N. of group IA and IIA elements when they are in
compounds are always +1 and +2, respectively.
– Rule 4: The O.N. of hydrogen is +1.
– Rule 5: The O.N. of oxygen is -2.
– Rule 6: The algebraic sum of the oxidation numbers of all atoms in a
complete compound equals zero.
– Rule 7: The algebraic sum of the O.N. of all the atoms in a
polyatomic ion is equal to the charge on the ion.
CHANGES IN STATE
• Changes in state are often accomplished by adding or removing heat
from a substance.
• Changes in state caused by adding heat to a substance are classified as
endothermic (heat in) processes.
• Changes in state caused by removing heat are classified as exothermic
(heat out) processes.
Add Heat
Give off Heat
SOLVENT & SOLUTE
• SOLVENT OF A SOLUTION
– The solvent of a solution is the substance present in the largest amount in the
solution
• SOLUTE OF A SOLUTION
– A solute of a solution is any substance present in an amount less than that of
the solvent. A solution may contain more than one solute.
• SOLUBLE SUBSTANCE
– A substance that dissolves to a
significant extent in a solvent
without stating how much actually
will dissolve.
• INSOLUBLE SUBSTANCE
– This is a term used to describe a
substance that does not dissolve to
a significant extent in a solvent.
• IMMISCIBLE
• Two liquids that do not mix.
DEGREES OF SATURATION
• A saturated solution is a solution that contains the maximum
amount possible of dissolved solute in a stable situation under
the prevailing conditions of temperature and pressure.
• A supersaturated solution is an unstable solution that contains
an amount of solute greater than the solute solubility under
the prevailing conditions of temperature and pressure.
• An unsaturated solution is a solution that contains an amount
of solute less than the amount required to form a saturated
solution under the prevailing conditions of temperature and
pressure.
Crystallization
1. A seed crystal is added to a supersaturated solution
2. Crystallization is initiated (crystal growth)
3. Crystallization is completed and purified compound can be separated from solvent.
THE SOLUTION PROCESS
• The solution process involves interactions between solvent
molecules (often water) and the particles of solute.
• An example of the solution process for an ionic solute in water:
– NaCl
– BaBr2
– Na2SO4
THE SOLUTION PROCESS (continued)
• An example of the solution process for a polar solute in water:
– Sucrose
– Ethanol
– Vitamins
MOLARITY
• The molarity of a solution expresses the number of moles of
solute contained in one liter of solution.
• The mathematical calculation of the molarity of a solution
involves the use of the following equation:
moles of solute
M
liters of solution
• In this equation, the number of moles of solute in a sample of
solution is divided by the volume in liters of the same sample of
solution.
SOLUTION PREPARATION - Dilution
• A quantity of solution with a concentration greater
than the desired concentration is diluted with an
appropriate amount of solvent to give a solution
with a lower concentration. This type of problem is
made simpler by using the following equation:
(Cc)(Vc) = (Cd)(Vd)
• In this equation, Cc is the concentration of the
concentrated solution that is to be diluted, Vc is the
volume of concentrated solution that is needed, Cd
is the concentration of the dilute solution, and Vd is
the volume of dilute solution.
SPONTANEOUS PROCESSES
• Spontaneous processes are processes that take place naturally
with no apparent cause or stimulus.
ENTROPY
• Entropy is a measurement or indication of the disorder or
randomness of a system.
• The more disorderly or mixed up a system is, the higher its
entropy.
EXOTHERMIC & ENDOTHERMIC DIAGRAMS
• The difference between endothermic and exothermic reactions
is clearly indicated by the following energy diagrams.
• In exothermic reactions, the energy is lost as the reaction occurs.
The products have less energy than the reactants.
• The reverse is true for endothermic reactions which gain energy
and cause the products to have more energy than reactants.
CHEMICAL EQUILIBRIUM
• All chemical reactions can (in principle) go in both directions and
products, located to the right of the arrow, can react to form
reactants, located to the left of the arrow. This condition is
indicated by the use of a double arrow pointing in both directions as
shown below:
H2(g) + I2(g)
2HI(g)
• When the reaction rate toward the right is equal to the reaction
rate toward the left, the reaction is said to be in a state of
equilibrium.
FACTORS THAT INFLUENCE THE POSITION OF
EQUILIBRIUM
• According to Le Châtelier's principle, the position of
equilibrium shifts in response to changes made in the
equilibrium.
• The factors that will be considered are:
– concentrations of reactants and products
– reaction temperature
– catalysts
• In general, Le Châtelier's principle predicts a shift away from
the side to which something is added and toward the side
from which something is removed.
ARRHENIUS ACIDS & BASES
• ARRHENIUS ACID
– An Arrhenius acid is any substance that provides hydrogen ions, H+,
when dissolved in water.
• ARRHENIUS BASE
– An Arrhenius base is any substance that provides hydroxide ions,
OH-, when dissolved in water.
• EXAMPLES OF AN ARRHENIUS ACID AND BASE
– HNO3 is an acid: HNO3(aq)
– KOH is a base: KOH(aq)
H+ (aq) + NO3- (aq)
K+ (aq) + OH- (aq)
BRØNSTED ACIDS & BASES
• BRØNSTED ACID
– A Brønsted acid is any hydrogen-containing substance that is
capable of donating a proton (H+) to another substance.
• BRØNSTED BASE
– A Brønsted base is any substance capable of accepting a proton
from another substance.
• EXAMPLE OF A BRØNSTED ACID AND BASE
– HNO2(aq) + H2O(l)
H3O+ (aq) + NO2-(aq)
– In this reaction, HNO2 behaves as a Brønsted acid by donating a
proton to the H2O. The H2O behaves as a Brønsted base by
accepting the proton.
CONJUGATE ACIDS & BASES
• CONJUGATE ACIDS AND BASES
– The base formed (NO2-) when a substance (HNO2) acts as a
Brønsted acid is called the conjugate base of the acid.
Similarly, the acid formed (H3O+) when a substance (H2O) acts
as a Brønsted base is called the conjugate acid of the base.
• CONJUGATE ACID-BASE PAIRS
– A Brønsted acid (such as HNO2) and its conjugate base (NO2-)
form what is called a conjugate acid-base pair.
– The same name is given to a Brønsted base (such as H2O)
and its conjugate acid (H3O+).
THE SELF-IONIZATION OF WATER
• Pure water does not contain only H2O molecules. In addition,
small but equal amounts of H3O+ and OH- ions are also present.
• The reason for this is that in one liter of pure water 1.0 x 10-7
moles of water molecules behave as Brønsted acids and donate
protons to another 1.0 x 10-7 moles of water molecules, which act
as Brønsted bases. The reaction is:
H2O (l) + H2O (l) ⇆ H3O+ (aq) + OH− (aq)
• As a result, absolutely pure water contains 1.0 x 10-7 mol/L of both
H3O+ and OH-.
• The term neutral is used to describe any water solution in which
the concentrations of H3O+ and OH- are equal.
• The equilibrium expression is:

-

H O OH 
K
3
H2O
2
THE ION PRODUCT OF WATER (continued)
• The equilibrium expression can be rearranged to give:
KH2O
2
 H O OH 

-
3
• Because the concentration of water is essentially constant,
the product of K multiplied by the square of the water
concentration is equal to another constant designated as Kw,
and called the ion product of water. The equation then
becomes:

 
K W  H3 O  OH -
• Because the molar concentration of both H3O+ and OH- in
pure water is 1.0 x 10-7, the numerical value for Kw can be
calculated:

K W  H3 O

OH   1.0  10 
-
7 2
 1.0  10
14
THE ION PRODUCT OF WATER (continued)

K W  H3 O

OH   1.0  10 
-
7 2
 1.0  10 14
• ACIDIC SOLUTION
– An acidic solution is a solution in which the concentration of H3O+ is
greater than the concentration of OH-. It is also a solution in which the
pH is less than 7.
• BASIC OR ALKALINE SOLUTION
– A basic or alkaline solution is a solution in which the concentration of
OH- is greater than the concentration of H3O+. It is also a solution in
which the pH is greater than 7.
THE pH CONCEPT
• It is often the practice to express the concentration of H3O+ in
an abbreviated form called the pH rather than to use scientific
notation.
• It is also a common practice to represent the H3O+ ion by the
simpler H+ ion.
• The pH notation is defined below, using H+ in place of H3O+:
pH = -log[H+], or in alternate form [H+]= 1x10-pH
• Or for pOH
pOH = -log[OH-], or in alternate form [OH-]= 1x10-pOH
CLASSIFICATION OF HOUSEHOLD PRODUCTS
Weak Acids
Weak Bases
NEUTRALIZATION REACTIONS
• In neutralization reactions, an acid reacts with a base to
produce a salt and water. The following are typical
neutralization reactions involving the base sodium
hydroxide, NaOH, which is also known commercially as lye.
• Reaction with hydrochloric acid:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
• The salt produced in this reaction is sodium chloride,
commonly called table salt.
• Reaction with nitric acid:
NaOH(aq) + HNO3(aq) → NaNO3(aq) + H2O(l)
• The salt produced in this reaction is sodium nitrate.
ANALYZING ACIDS AND BASES
•
•
•
•
•
•
The analysis of acid solutions to
determine the amount of acid they
contain is an important procedure done
in many laboratories.
An acid-base titration is one commonlyused method of analysis.
When a titration is done, an accuratelymeasured volume of acid is put into a
flask using a pipet.
A few drops of indicator solution is
added, then a base solution of known
concentration is carefully added from a
buret until all the acid has been reacted
(equivalence point).
The point at which all the acid has
reacted is shown by a color change
(endpoint) in the indicator.
The concentration of the base and the
volume required in the titration allow
the concentration of acid to be
determined.
BUFFERS
• Buffers are solutions with the ability to resist changing pH when
acids (H+) or bases (OH-) are added to them.
• Many useful buffers consist of a solution containing a mixture of a
weak acid and a salt of the acid (e.g. acetic acid and sodium
acetate).
• Any added acid (H+ ions) react with the anion from the salt, which
also happens to be the conjugate base of the weak acid.
C2H3O2− (aq) + H+ (aq) ⇌ HC2H3O2 (aq)
• Any added base (OH- ions) react with the nonionized weak acid.
HC2H3O2 (aq) + OH− (aq) ⇌ C2H3O2− (aq) + H2O (l)
• The buffer capacity is the amount of acid (H+) or base (OH-) that
can be absorbed by a buffer without causing a significant change in
pH.
COMPOUND FORMULAS
• A compound formula consists of the symbols of the elements
found in the compound. Each elemental symbol represents
one atom of the element. If more than one atom is
represented, a subscript following the elemental symbol is
used.
COMPOUND FORMULAS EXAMPLES
• Carbon monoxide, CO
– one atom of C
– one atom of O
• Water, H2O
– two atoms of H
– one atom of O
• Ammonia, NH3
– one atom of N
– 3 atoms of H