Chemistry 100
Bettelheim, Brown, Campbell & Farrell
Ninth Edition
Introduction to General,
Organic and Biochemistry
Chapter 4
Chemical Reactions
Chemical Reactions
In a chemical reaction, one set of chemical substances
called reactants are converted to another set of
substances called products. The atoms comprising
reactants are rearranged to form products, but no
atoms are created or destroyed.
In this chapter we discuss three aspects of chemical
reactions:
1) Mass relationships (stoichiometry).
2) Types of reactions.
3) Heat (energy) gain or loss resulting from reactions.
Formula Weight and the Mole
The Formula Weight of a substance is the total weight
of all the atoms comprising the chemical formula of
that substance.
FWt. = ? For C6H6 FW = 6(At. Wt. C) + 6(At. Wt. H)
FWt. = 6(12.011) + 6(1.008) = 78.114 amu
If the substance is a covalent molecule the FWt. is
called a Molecular Weight.
If the FWt. is expressed in grams/mole it is called a
Molar Mass.
One molar mass of any substance contains one
Avogadro’s Number of units = 6.02214199 x 1023units.
Formula Weight and the Mole
One Mole is defined as the number of atoms contained
in exactly 12 grams of carbon-12.
One Mole of any substance contains exactly this same
number, Avogadro’s Number, of formula units.
Moles reacted have an exact 1:1 relationship to the
coefficients found in a balanced chemical equation of
formula units.
We now have a relationship between the formula units
in a chemical equation and something we can measure
on a chemical balance, i.e. grams.
This is called stoichiometry.
Molar Mass and Reacting Weights
If we have a balanced chemical equation we can now
compute the grams of one substance that will react
with another:
How much hydrogen and oxygen must react to
produce 65.00 grams of water?
Balanced Eqn:
Stoichiometry:
2 H2 + O2
1 mole
2 moles
2 H2O
2 moles
Stoichiometry
Using the equality: 1 mole = xxxx grams
We can create a conversion factor that will
convert any mass to moles for a given molar mass.
We can create a conversion factor that will
convert any number of moles of a substance to a mass
that can be weighed.
Using the equality: 1 mole = 6.02214199 x 1023 units
We can convert any number of moles into the
number of formula units it contains.
We can then determine from the formula for the
substance how many atoms of each element it contains.
Balancing Chemical Equations
When one has an equation in words such as;
butane + oxygen yields carbon dioxide + water
We must write the chemical equation:
C4H10(g) + O2(g) Æ CO2(g) + H2O(g)
This equation tells us the substances, their composition
and physical state. But it doesn’t tell us how much of
each reacts. The equation must be balanced!
First: Start with an element that occurs in only one
substance on each side of the equation. If more than
one choose the one nearest the center of its period in
the Periodic Chart. This case choose carbon: C
Balancing Chemical Equations
C4H10(g) + O2(g) Æ 4 CO2(g) + H2O(g)
Now choose the other element bound to carbon:
C4H10(g) + O2(g) Æ 4 CO2(g) + 5 H2O(g)
Now balance the oxygens, by adjusting O2(g)
C4H10(g) + 6.5 O2(g) Æ 4 CO2(g) + 5 H2O(g)
Make all coefficients whole numbers: multiply Eqn. by 2.
2 C4H10(g) + 13 O2(g) Æ 8 CO2(g) + 10 H2O(g)
It is now possible to calculate the reacting masses of the
substances in the chemical equation.
Limiting Reactant
Chemists rarely react exactly the reacting masses
required in a balanced chemical equation.
For many reasons it it common to have more of one
substance than the equation requires. This reactant is
in excess and not all of it will react.
But this one is not as important as the lesser reactant.
The reaction stops when the lesser reactant is used up!
This reactant is called the Limiting Reactant!
The amount of the limiting reactant determines
the actual amounts of the substances that react!
Limiting Reactant
If we have a balanced chemical equation and for whatever
reason have an excess of oxygen, we can still compute the
grams of one substance that will react with another:
How much hydrogen must react with unlimited
oxygen to produce 65.00 grams of water?
Limiting Reactant
In a chemical equation theoretically any reactant may
be the Limiting Reactant.
In practice, however, chemists are usually limited by
other constraints. For example:
Butane burns cleanly to carbon dioxide and water in
excess oxygen.
But if butane is in excess the reaction is not clean and
carbon monoxide (a poison!), and carbon (soot!) will
form along with some carbon dioxide and water.
In reality, chemistry, and chemists, must allow for
these vagaries of nature!
Limiting Reactant and Theoretical Yield
When a chemist reacts two or more reactants, one of
which is the limiting reactant, the maximum yield
possible, the theoretical yield, may be calculated.
Rarely does a reaction, or a chemist, actually produce
this amount of material in purified form. This lesser
amount is called the actual yield.
The Percent Yield is the percent of the theoretical
yield this actual yield represents. It represents the
efficiency of the reaction together with the prowess of
the chemist.
actual yield (100) = percent yield
theoretical yield
Reactions Between Ionic Compounds
Ionic compounds or salts consist of both positive and
negative ions. When an ionic compound dissolves in
water, it dissociates to aqueous ions.
KCl(s)
H2O
K+(aq) +
Cl-(aq)
When aqueous solutions of two different ionic compounds
or salts are mixed, the ions intermingle. If two of the ions
combine to form a water-insoluble compound, a
precipitate will form, a reaction has occurred. If this
doesn’t happen, no physical change will be observed.
spectator ions
K+(aq)
Ag+(aq)
+
+
Cl -(aq)
NO 3-(aq)
K+(aq) + NO 3-(aq)
+ AgCl (s)
a precipitate
Reactions Between Ionic Compounds
The equation that expresses only the reaction that
occurs is called the net ionic equation.
Ag+(aq) +
Cl-(aq)
AgCl(s)
net ionic equation = all spectator ions omitted
Reactions Between Ionic Compounds
In general, ions in solution react with each other
when one of the following has happened:
Two ions form a compound that is insoluble in water.
Two ions react to form a gas that escapes from the
reaction mixture as bubbles.
H+(aq)
+
HCO3-(aq)
CO2(g) + H2O
the gas formed excapes from the solution
An acid neutralizes a base (Chapter 8)
H +(aq)
+
OH -(aq)
H 2O
One of the ions can oxidize another (Section 4.7)
Oxidation – Reduction Reactions
Oxidation: the loss of electrons by a species.
Reduction: the gain of electrons by a species.
Oxidation-reduction (redox) reaction: any reaction in
which electrons are transferred from one species to
another.
An alternative definition of an oxidation-reduction
reaction is:
Oxidation: the gain of oxygen or loss of hydrogen by a
species.
Reduction: the loss of oxygen or gain of hydrogen by a
species.
Oxidation – Reduction Reactions
Oxidation-reduction (redox) reaction: any reaction in
which electrons are transferred from one species to
another:
a gain of two electrons
Cu2+(aq) +
Zn2+(aq) +
Zn(s)
Cu(s)
a loss of two electrons
What is observed is the blue copper solution gradually
fades to colorless and the zinc diminishes and what
remains gets copper plated.
Using the Alternative RedOx Definition:
CH4(g) + 2 O2(g)
Æ
CO2(g) + 2 H2O(g)
Carbon oxidized
Oxygen Reduced
Heats of Reaction
In almost all chemical reactions, heat is either given off
or absorbed:
Exothermic reaction: one that gives off heat
Endothermic reaction: one that absorbs heat
Heat of reaction: the heat given off or absorbed in a
chemical reaction
For example: The combustion (oxidation) of carbon
liberates 94.0 kcal per mole of carbon oxidized.
C(s) + O2(g) Æ
CO2(g) + 94.0 kcal/mole C
Heat of combustion: the heat given off in a combustion
reaction; all combustion reactions are exothermic.