CHEM 121
Introduction to Fundamental Chemistry
Summer Quarter 2008 SCCC
Lecture 8
http://seattlecentral.edu/faculty/lcwest/CHE121
Forces Between Particles
Ionic Bonding (polyatomic ions or metal/nonmetal)
Covalent Bonding (nonmetals)
• Molecules
• Network Solids
Metallic Bonding
• attraction between “sea of valence electrons” and +ve
“core of atoms”
Interactions of polar molecules
• dipole-dipole interactions (H-bonding)
• dipole-ion interactions
Can there be attractive forces between nonpolar molecules?
Can there be attractive forces between homoatomic molecules
such as O2, N2, Cl2, F2, I2, At2, H2 ?
Electrons located in the orbitals of an atom or molecule are in
continual random motion. We can think of this as an electron
cloud.
In nonpolar molecules we have evenly distributed e-.
As the electrons are in constant random motion very
occasionally most of the electrons will be on one side of a
molecule.
Evenly distributed e-.
Majority of e- on left.
Temporarily one side of the molecule will be more positive than
the other. This is called a temporary dipole.
The electrons in a molecule next to the negative end of a
temporary dipole will be repelled. The neighboring molecule is
then said to have an induced dipole.
The temporary and induced dipoles will be attracted to each
other as they are oppositely charged.
The attractive forces between temporary and induced dipoles
are called dispersion forces. They are also known as Van der
Waals force and London forces.
Dispersion forces are very weak (the weakest intermolecular
forces). All substances have dispersion forces between particles.
For non-polar compounds dispersion forces are the only type of
intermolecular force present and play an important role in
determining the physical properties of these substances.
Can there be attractive forces between nonpolar molecules?
Yes there can be dispersion forces
Can there be attractive forces between homoatomic molecules
such as O2, N2, Cl2, F2, I2, At2, H2 ?
Yes there can be dispersion forces
graphite
C60 Buckminster Fullerene
C70
Crystal structure of C60
Montreal's Expo 67 American pavilion
Chemical Reactions
Chemical Equations
Types of Reactions
Redox Reactions
Decomposition Reactions
All chemical reactions involve the making and or the breaking
of chemical bonds to produce new substances.
We write chemical reactions using chemical equations.
In a chemical equation the reactants appear on the left of the
equation separated from the products by an arrow.
Reactants
→
Products
Multiple reactants and products are separated by a plus (+) sign.
Reactant 1
+
Reactant 2
→
Product
The number of atoms of each type on the reactants side of the
equation must equal the number of atoms of each type on the
products side of the equation.
H2
+
O2
→
H2O
8
2H2
+
O2
→
2H2O
9
When the number of atoms of each type on the reactants side
equals the number of atoms of each type on the products side of
the equation the reaction is balanced with respect to mass.
Similarly, the total charge of the reactants side must equal the
total charge of the products side of the equation.
S2O82-
+
2I
→
I2
+
2SO42-
8
S2O82-
+
2I-
→
I2
+
2SO42-
9
When the total charge of the reactants side equals the total
charge of the products side we say the equation is balanced with
respect to charge.
Balancing equations is usually done by “inspection”.
Balanced equations are required to be in agreement with the law
of conservation of mass.
Chemical reactions can be divided into two classes, redox
reactions and nonredox reactions.
Within each class there are several subclasses which we will
learn about as the course progresses.
In a redox equation we have two processes occurring
simultaneously. One species is being reduced while another is
being oxidized.
Reduction can be defined in several ways:
• To lose oxygen
• To combine with hydrogen
• To gain electrons
• To decrease in oxidation number
Oxidation is the opposite process of reduction and can be
defined as follows:
• To combine with oxygen
• To lose hydrogen
• To lose electrons
• To increase in oxidation number
Oxidation numbers are numbers we assign to atoms in chemical
equations to help us decide what redox processes are occurring.
The rules for assigning oxidation numbers are as follows:
•Rule 1:
The O.N. of any uncombined element is 0.
For example: Fe (0), Cl2 (0), and Ca(0)
•Rule 2:
The O.N. of a simple ion is equal to the charge on the ion.
For example: Mg2+(+2), O2-(-2), and Cl-(-1).
•Rule 3:
The O.N. of group IA and IIA elements when they are in
compounds are always +1 and +2, respectively.
e.g. Na2S (Na = +1) and Ca(NO3)2 (Ca = +2)
•Rule 4:
The O.N. of hydrogen is always +1.
e.g. HBr (H = +1) and H2SO4(H = +1)
•Rule 5:
The O.N. of oxygen is -2 except in peroxides where it is -1
e.g. MgO (O = -2), HBrO3 (O = -2), and H2O2 (O = -1)
•Rule 6:
The sum of the oxidation numbers of all atoms in a
complete compound equals zero.
e.g. MgSO4 (O.N. of Mg = +2, O.N. of O = -2, therefore
O.N. of S = +6)
Rule 7:
The sum of the O.N. of all the atoms in a polyatomic ion is equal
to the charge on the ion.
e.g. HCO3- (O.N. of H is +1, O.N. of O is -2, and O.N. of C is +4)
Rules 6 and 7 are important when we don’t have a simple rule to
identify what the O.N. of an atom is
A question will appear on the screen confer with your group.
When you have written an answer nominate a person to present
your answer to the class they should stand up and say
something distinctive (e.g. BINGO !! or if you want to be scientific
EUREKA !!)
First person to stand up gets the opportunity to present the
answer on the board (in full) to the class.
Team with most correct answers wins the prize.
What is the formula of an ionic
compound made from Cu2+ and
chlorine?
CuCl2
What is the mass of 2.0 moles of
silver?
216 g
How does this appear when properly
balanced: NaClO3 → O2 + NaCl ?
2 NaClO3 → 3 O2 + 2 NaCl
How does this appear when properly
balanced: NaClO3 → O2 + NaCl ?
2 NaClO3 → 3 O2 + 2 NaCl
When water and ethanol (the
alcohol in beer, wine, and
liquor) are mixed, what is the
strongest intermolecular force to
play a role in the mixing?
hydrogen bonding
What is the oxidation number of
S in H2SO4
6+
What is the oxidation number of
O in H2O2
1-
What is the oxidation number of
C in CH4
4+
What is the Lewis structure of
CH4
What is the Lewis structure of
SO4
2-
What is the angle between the
outer atoms in CO3
2-
What is the name of
CN
Cyanide ion
In decomposition reactions one substance is broken to form two
or more new substances.
A→B+C
Decomposition reactions may or may not be redox reactions.
e.g.
2HgO(s) → 2Hg(l) + O2(g)
In combination reactions two or more substances combine to
form a single substance.
B+C→A
Combination reactions may or may not be redox reactions.
redox: 2Mg(s) + O2(g) → 2MgO(s)
non-redox: SO3(g) + H2O(l) → H2SO4(aq)
When ionic compounds are dissolved in water to form
aqueous
solutions (aq) the compounds dissociate into their component
ions.
NaOH(aq) → Na+(aq) + OH(aq)
Similar behaviour is observed for a few covalent compounds
particular those formed between hydrogen and the halogens.
HCl(aq) → H+
(aq)+ Cl(aq)
that dissociate into ions can be
written by a total ionic
equation which indicates the
ions
present:
e.g.
HCl(aq) + NaOH(aq) → H2O(l)
+ NaCl(aq)
H+
(aq) + Cl(aq) + Na+
(aq) + OH(aq) → H2O(l) + Na+
(aq) + Cl-
and product side of the
equation. These are called
spectator ions.
H+
(aq) + Cl(aq) + Na+
(aq) + OH(aq) → H2O(l) + Na+
(aq) + Cl(aq)
Eliminating the spectator ions
from both sides results in the
net
ionic equation:
In some reactions energy is
released into the surroundings
(heat,
motion or sound).
CH4 + O2 → CO2 + 2H2O +
energy
Reactions of this type are called
exothermic.
stored in the products,
resulting in the surroundings
losing
energy.
H2O(l) + energy → H2O(g)
Reactions of this type are called
endothermic reactions.