Chemical Reactions: Bonding
and Changes in Physical
Properties - Part 2
Dr. Pedro M. Pereyra
Holy Trinity
2006, 2007, 2008, 2012 ©
Version 2 Rev 1
Expectations
Matter and its changes
•
•
•
•
explain how different elements combine to form covalent
and ionic bonds using the octet rule
demonstrate an understanding of the formation of ionic
and covalent bonds and explain the properties of the
products
demonstrate an understanding of the relationship between
the type of chemical reaction (e.g., synthesis,
decomposition, single and double displacement) and the
nature of the reactants
relate the reactivity of a series of elements to their position
in the periodic table (e.g., compare the reactivity of metals
in a group and metals in the same period; compare the
reactivity of non-metals in a group).
Dr. Pedro M. Pereyra ©) 2006
Index
•
•
•
•
•
Bonding Continuum
Types of Compounds & Bond properties
Molecular and Ionic Compounds
Intermolecular Forces & Interactions
Physical properties arising from
intermolecular interactions
Dr. Pedro M. Pereyra ©) 2006
Bonds & Bond Formation
Bonds and Bond Formation
Applications and extension of the concept of
Electronegativity (÷)
Lewis Dot Diagrams
• Types of Bonds:
•
‚
‚
‚
‚
Ionic,
Pure Covalent, Polar Covalent and Coordination bonding
Network bonding
Metallic Bonding
Electronegativity (÷)
• Difference in Electronegativity (Ä÷) and % Ionic Character
• Bonding Continuum
•
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonds and Bond Formation
•
Bonds - binding atoms together
Linus Pauling's definition of the chemical bond: "whatever
is convenient to the chemist to define as a bond"
•
•
Source: http://www.iupac.org/general/about.html
Some other less “convenient” definitions:
‚
In general a bond is established between atoms by the balance of the
electrostatic (e.g. charge attraction or repulsions) and electromagnetic
forces (e.g. those responsible fro the structure of the electrons in an atom)
acting between the valence electrons and the residual nuclear attractions
(e.g electron affinity) of the atoms involved.
• A covalent bond is considered when a pair of electrons share the
valence shell of the combining atoms
• A coordination bond is considered when a pair of electrons is donated by
only one of the combining atoms.
• An ionic bond is considered when the available valence electrons of one
atom are donated to the valence shell of an other atom forming a
positively and a negatively charged species that are predominantly
attracted by electrostatic forces
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonds and Bond Formation
Bonding and the Concept of Valence
•
•
•
Valence, also known as valency or valency number, is a
measure of the number of chemical bonds formed by the
atoms of a given element.
The concept was developed in the middle of the
nineteenth century in an attempt to rationalize the
formulae of different chemical compounds.
Although it has fallen out of use in higher level work with
the advances in the theory of chemical bonding, it is still
widely used in elementary studies where it provides a
heuristic introduction to the subject.
•
Source: http://en.wikipedia.org/wiki/Valence_(chemistry)
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonds and Bond Formation
Bonding and the Concept of Valence
•
The number of bonds formed by a given element was originally
thought to be a fixed chemical property and in fact, in many
cases, this is a good approximation.
‚
•
However it soon became apparent that, for many elements, the
valence could vary between different compounds.
‚
•
For example, in virtually all of their compounds, carbon forms four bonds, oxygen two
and hydrogen one.
One of the first examples to be identified was phosphorus, which sometimes
behaves as if it has a valence of three and sometimes as if it has a valence of five.
One method around this problem is to specify the valence for
each individual compound
‚
although it removes much of the generality of the concept, this approach has given
rise to the idea of oxidation numbers (used in Stock nomenclature) and to lambda
notation in the IUPAC nomenclature of inorganic chemistry.
•
Source: http://en.wikipedia.org/wiki/Valence_(chemistry)
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Bonds
•
Ionic Bonding
When an atom, generally a metal atom, loses one or more
of its electrons, a positive ion or Cathion is formed.
Na 6 Na+ + eG
‚
•
When an atom, generally a non-metal atom, gains one or
more electrons, a negative ion or Anion is formed.
Cl 2 + 2eG 6 2ClG
‚
•
Just as with the opposite poles of a magnet, when positive
and negative ions approach one another a very strong
force of attraction is formed.
•
•
Source: http://www.rjclarkson.demon.co.uk/found/found3.htm
Kind: group of individuals that share features: a group or class of individuals connected by shared characteristics
Microsoft® Encarta® Reference Library 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Type: a category of things or people whose members share some qualities
Microsoft® Encarta® Reference Library 2004. © 1993-2003 Microsoft Corporation. All rights reserved.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Bonds
•
Ionic Bonding
This is called an ionic bond, though more accurately it is a
strong ionic force of attraction.
+
‚
Na + + ClG 6 Na GCl or NaCl
•
•
•
•
•
Source: http://www.rjclarkson.demon.co.uk/found/found3.htm
In strict terms, an ionic bond refers to the electrostatic
attraction experienced between the electric charges of a
cation and an anion, in contrast with a purely covalent
bond where electrons are shared between atoms.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Bonds
Covalent Bonding
•
covalent bond is a chemical bond that involves sharing pairs of
electrons between atoms in a molecule
•
•
http://www.wordreference.com/definition/covalent%20bond
The following kind of covalent bonding are recognized:
‚
Single covalent bond: when a single covalent bond is established between two
atoms
•
•
‚
Double covalent bond: when two covalent bonds are established between two
atoms perpendicular to each other. Two possibilities are recognized:
•
•
‚
One sigma bond and one pi bond are formed
Two pi bonds are formed
Triple covalent bond: when three covalent bonds are established between two
atoms.
•
•
Along the axis of encounter: Sigma bond ( ó).
Perpendicular to the axis of encounter: Pi bond ( ð).
Usually formed when one sigma bond and two pi bond are. established
coordinate bond or dative bond is a covalent bond in which
both electrons are provided by one of the atoms.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Bonds
Ionic - Covalent Bonds and %Ionic Character
•
•
In practice, it is preferable to consider the amount of ionic
character of a bond rather than referring to purely ionic or
purely covalent bonds.
The relationship was proposed (L.Pauling) for the
estimation of ionic character of a bond between atoms A
and B:
•
•
•
•
where ÷A and ÷B. are the Pauling electronegativities of atoms A and B.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Electronegativity and % Ionic Character
Electronegativity (÷)
•
"Electronegativity is the power of an atom when in a
molecule to attract electrons to itself." The
electronegativity will depend upon a number of factors
including other atoms in the molecule, the number of
atoms coordinated to it, and the oxidation number for the
atom. There are a number of ways to produce a set of
numbers which represent electronegativity scales. The
Pauling scale is perhaps the most famous.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Electronegativity and % Ionic Character
Difference in Electronegativity (Ä÷) and % Ionic
Character
•
Pauling noticed that the bond energy E(AB) in a
molecule AB is always greater than the mean of
the bond energies E(AA) + E(BB) in the homonuclear
species AA and BB.
•
E (AB) > (E(AA) + E(BB))/2
•
•
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Electronegativity and % Ionic Character
Difference in Electronegativity (Ä÷) and % Ionic
Character
•
His argument was that in an "ideal" covalent bond, E(AB)
should equal this mean, and that the "excess" bond
energy is caused by electrostatic attraction between the
partially charged atoms in the heteronuclear species AB.
In effect, he was saying that the excess bond energy
arises from an ionic contribution to the bond (% Ionic
Character) .
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Electronegativity and % Ionic Character
Difference in Electronegativity (Ä÷) and % Ionic
Character
•
He managed to treat this ionic contribution by the
equation
•
E (AB) = [E(AA) + E(BB)]/2 + 96.48(÷A -÷B)2
• in which E(AB is expressed in kJ mol-1 and ÷A -÷B represents the
difference in "electronegativity" between the two elements, whose
individual electronegativities are given the symbols ÷A and ÷B.
• Using this equation, Pauling found that the largest electronegativity
difference was between Cs and F. Pauling set F arbitrarily at 4.0 (today,
the value for F is set to 3.98) and this gives a scale in which the values
for all other elements are less than 4 but still with a positive number.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
% Ionic Character vs Ä÷
Calculation of % Ionic Character in terms of Pauling's Electronegativity and in terms of Hannay & Smyth:
(
)
2
IC(Dc) := 1 - e - .25 ×Dc
Dc := 0, .1.. 4
2
ICHS(Dc) := 0.16Dc + 0.035Dc
% Ionic Character vs Electronegativity
Dc =
1.2
1
0.8
IC(Dc) =
ICHS(Dc) =
2.497•10
-3
0
0.016
9.95•10
-3
0.033
0.3
0.022
0.051
0.4
0.039
0.07
0
0.1
0.2
0
0.5
0.061
0.089
IC( Dc)
0.6
0.086
0.109
ICHS( Dc) 0.6
0.7
0.115
0.129
0.8
0.148
0.15
0.9
0.183
0.172
1
0.221
0.195
1.1
0.261
0.218
1.2
0.302
0.242
1.3
0.345
0.267
1.4
0.387
0.293
1.5
0.43
0.319
0.4
0.2
0
0.5
1
1.5
2
2.5
3
3.5
4
Dc
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonding Continuum
Pure vs Polar Covalent vs Ionic Bonding
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonding Continuum
Pure vs Polar Covalent vs Ionic Bonding
•
Pure covalent bonds are considered to have very little if no
ionic character at all.
‚
‚
‚
%Ionic Character corresponding to electronegativity differences between 0 and 0.50
Charge density distribution is symmetrical.
For example Hydrogen gas (H2)
The term polarity refers to
the presence in the same
particle (e.g. molecule or
atom) of a positively
charged and a negatively
charged regions
The term charge density
refers to the volume in
space around and between
atoms where the bonding
electrons are concentrated
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonding Continuum
Pure vs Polar Covalent vs Ionic Bonding
Covalent bond is interpreted as a bonf formed by “sharing” a pair
of electrons between two atoms.
• It actually represents the formation of an energy level common to
the two bonding atoms known as a Molecular Orbital (MO).
•
‚
•
Every MO has two possible states
• A high energy state where the two electrons have unpaired spins (88) also
known as anti-bonding state
• A low energy state where the two electrons have a paired spins (89) also
known as bonding state
For atoms to bond covalently the bonding state of the bonding
MO must exist at a lower energy level than the energy level of
the atomic orbitals where the electrons originated from
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonding Continuum
Pure vs Polar Covalent vs Ionic Bonding
•
•
As we learned before the electron of hydrogen atoms exist in an energy
level with the value of -13.6eV
The bonding Molecular Orbital of the hydrogen molecule has a value of
-17.6eV. A state 4eV lower in energy favouring the formation og
hydrogen gas.
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonding Continuum
Pure vs Polar Covalent vs Ionic Bonding
•
Polar covalent bonds are considered to have some to
considerable ionic character due to the difference in
electronegativity of the bonding atoms
‚
‚
‚
%Ionic Character corresponding to electronegativity differences larger then 0.50 and
up to about 1.6
Charge density distribution is asymmetrical due to the electrostatic effect of the atom
with higher electronegativity. A MO is present because such an energy level is still of
lower energy than the atomic orbital supplying the electrons
For example
• Carbon Monoxide gas (CO) with Ä÷ = 1.0, or Hydrogen Chloride (HCl) with Ä÷ =
0.8
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Bonding Continuum
Pure vs Polar Covalent vs Ionic Bonding
•
Ionic bonds are considered to have considerable ionic character
due to the difference in electronegativity of the bonding atoms
‚
‚
‚
‚
%Ionic Character corresponding to electronegativity differences larger then 1.60.
Charge density distribution is biased to the atom or group with highest
electronegativity duue to the strong electrostatic effect.
The energy level of the MO is higher or equal to that of the atomic orbital of the atom
with the strongest electronegativity
For example Lithium Chloride.
Noticed the charge density concentrated in the
chlorine atom only
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Na
Cl
Types of Compounds
Types of Compounds
Tetrahedron of Material Type
Just like we considered before a bonding
continuum between ionic and molecular
compounds, we can incorporate the other
bonding types to build a tetrahedron of
material types known erroneously as Laing
Tetrahedron.
http://www.meta-synthesis.com/webbook/38_laing/tetrahedra.html
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Compounds
Tetrahedron of Material Type
Dr. Pedro M. Pereyra ©) 2006
Types of Compounds
Tetrahedron of Material Type
We have become
empirically familiar with
some of the concepts
related to the continuum
between molecular and
network compounds when
we observed the drastic
changes in physical
properties of sulfur as it
was heated, changing from
a molecule (S8)to polymeric
chains (Sn) and back to
molecules. (S2 6 S8)
We are also familiar with
this molecular dimensionality in plastics and
biological macromolecules
S8 6 S n 6 S 2 6 S 8
160EC
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
600EC
20EC
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Compounds
Tetrahedron of Material Type
The range of ionic to network
bonding compounds is
associated with many
ceramics and refractory oxides
and the range of metallic to
ionic bonding compounds is
associated with many known
alloys
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Compounds
Tetrahedron of Material Type
Dr. Pedro M. Pereyra ©) 2006
Types of Compounds
Molecular compounds
Molecules are made of atoms covalently bonded and with a
definite geometry defined by fix average bond lengths and bond
angles
• The geometries are defined by
•
‚
‚
‚
•
the balance of attractive and repulsive forces among the atoms making up the
molecule.
The number and type of covalent bonds made by each atom
The number of unused electrons remaining in the valence shell after bonding
(electron pairs or free radicals). When assessing the possible geometries these
electrons count as a bond (for every unused pair or for every free radical)
The following basic geometries are recognized:
‚
‚
‚
‚
‚
‚
‚
Linear (180E)
Triangular Planar (120E)
Tetrahedral (109.5E)
Triangular bipyramidal (120E and 90E)
Square bipyramidal (90E)
Pentagonal bipyramidal (72E and 90E)
Hexagonal bipyramidal (60E and 90E)
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Types of Compounds
Molecular compounds
Atoms with valence electrons in “s” and “p” orbitals can only
contain up to 4 pairs of electrons or 4 bonds; their geometries
are restricted to linear, triangular planar, and tetrahedral.
• Atoms with “d” valence electrons can also contain electrons in “s”
and “p” type orbitals allowing for higher order geometries
•
BeBr2 ; 180E Linear
BF3 ; 120E Triangular
Planar
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
CH4 ; Tetrahedral
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Net Polarity
Polar Molecular compounds
Molecules with polar covalent bonds exhibit a collective property
known and Net Dipole Moment
• In general, each polar bond will contribute to the total, or net
polarity of the molecule in a predictable way determined by the
average geometry of the molecule.
• In Beryllium Bromide’s case the strong polarity arising from the
difference in electronegativity (Ä÷ = 1.3) between Beryllium and
Bromine is cancelled by the overall symmetrical geometry of the
molecule giving rise to a Net Dipole Moment of zero
•
BeBr2 ; 180E
Linear
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Net Polarity
Polar Molecular compounds
•
In a similar way Boron Trifluoride and Methane will display also a
Net Dipole Moment of zero.
BF3 ; 120E Triangular
Planar
CH4 ; Tetrahedral
Symmetrical Molecules will display a
Net Dipole Moment of zero
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Net Polarity
Polar Molecular compounds
•
•
•
•
Some molecules, as mentioned before, will
contain also unbonded or lone pairs of
electrons in their valence shell. These
electrons will contribute to the overall
geometry of the molecule.
Water and ammonia are two important
molecules that display a geometry
influenced by the valence electron pairs
In the case of water, Oxygen has two
electron pairs, that together with the two
bonded Hydrogen requires a tetrahedral
geometry.
In the case of ammonia, Nitrogen has one
electron pair and is bonded to three
Hydrogen requiring also a tetrahedral
geometry
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Angle between Hydrogens = 104.5E
Angle between Hydrogens = 109.5E
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Net Polarity
Polar Molecular compounds
•
The resultant molecules are not totally
symmetric and the polarity of their bonds
becomes enhanced resulting in a definite
and significant Net Dipole Moment
Angle between Hydrogens = 104.5E
The Net Dipole
Moment gives
water a strong
net polarity that
determines
much of its
unique physical
properties
The H-O dipoles cancel
in their horizontal, but not
in their vertical
components
Reactions Types Mechanisms
Bonding
Intermolecular Interactions
Physical properties
Bond types
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Application Exercises
We are now in a position to predict some of the fundamental
structural properties of simple molecular compounds by
applying what we have learned about bonding and bonds in a
systematic manner:
• We start with the compositional formula of the compound and:
•
‚
‚
•
Determine the electronegativity difference (Ä÷) for each atom pair involved in the
compound’s bonds.
Using Lewis Dot diagrams, determine the bonding pattern among the atoms
making up the compound
With this information we can determine
‚
‚
‚
The types of bonds, their polarity, as well as, their % Ionic Character (%IC).
The number and kind of bonds in the compound, and
The number of electron pairs or free radicals not involved in bonding
As a last step we can now determine the most likely geometry
of the arrangement of the atoms around the central atom of
the compound and if the molecule will or not display a net
polarity
• For example: CO2
•
Application Exercises
From the Ä÷ we can determine that the
bonds between Carbon and Oxygen must
be polar covalent, indicating that it is a
molecular compound.
Application Exercises
From the Ä÷ we can determine that the
bonds between Carbon and Oxygen must
be polar covalent, indicating that it is a
molecular compound.
From the Lewis Dot bonding diagram we can infer that
carbon is linked to oxygen by double bonds
Application Exercises
From the Ä÷ we can determine that the
bonds between Carbon and Oxygen must
be polar covalent, indicating that it is a
molecular compound.
From the Lewis Dot bonding diagram we can infer that
carbon is linked to oxygen by double bonds
Because carbon has no other electrons in its valence shell we can distribute the two
Oxygens 180E apart, thus as far as possible from each other; giveing the moleclule a linear
geometry.
Because of this, the polarity of the Carbon-Oxygen bonds is cancelled giving the molecule a
net polarity of zero
Application Exercises
Intermolecular Forces
and Bulk Physical Properties
Intermolecular interactions
Effects of ionic and polar bonding
•
Van der Waals Forces(§) (<0 - 10 kJ/mol)
‚
‚
‚
•
Dispersion or London Forces (London 1930)
• Induced dipole-induced dipole interactions in particles with no or very weak
permanent polarity
Dipole-dipole interactions (Keesom 1912)
• In particles with a permanent net-dipole
Ion-dipole and ion-induced dipole interactions
H-bonding (~24 kJ/mol)
(§)There's a bit of a problem here with modern syllabuses. The majority of the syllabuses talk as if dipole-dipole interactions were quite distinct
from van der Waals forces. Such a syllabus will talk about van der Waals forces (meaning dispersion forces) and, separately, dipole-dipole
interactions. All intermolecular attractions are known collectively as van der Waals forces. The various different types were first explained by
different people at different times. Dispersion forces, for example, were described by London in 1930; dipole-dipole interactions by Keesom in
1912. http://www.chemguide.co.uk/atoms/bonding/vdw.html
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Dispersion Forces
The dynamic nature of electrons in all atoms and molecules
(particles) allows for the momentary polarization of the particle
creating a weak dipole, even if the particle shows no or only a
very weak net polarity.
• Neighboring particles within a critical distance will be influenced
by this momentary polar force allowing for the formation, on the
neighboring particles, of an induced-dipole.
• In its turn, so long as the particles are close to each other, this
induced-dipoles will reinforce the induce-dipoles in all the
particles involved.
• The larger the particle the stronger this induced-dipole
interactions are going to be. Shape of the particle also
influences dispersion force effects.
•
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Dispersion Forces
•
The dynamic nature of electrons in all atoms and molecules (particles)
allows for the momentary polarization of the particle creating a weak
dipole, even if the particle shows no or only a very weak net polarity.
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Dispersion Forces
•
•
Neighboring particles within a critical distance will be influenced by this
momentary polar force allowing for the formation, on the neighboring
particles, of an induced-dipole.
In its turn, so long as the particles are close to each other, this
induced-dipoles will reinforce the induce-dipoles in all the particles
involved.
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Dispersion Forces and Physical Properties
•
The effect of dispersion forces at the bulk level translates into important
physical behaviors such as the trends in the melting or boiling point of
pure non-metal elements and molecular substances, as well as, their
behaviour in mixtures. For example: Noble Gases
b.p. of Noble gases
0
-50
-100
-150
-200
-250
-300
Helium
Neon
Argon
Krypton
Xenon
Radon
Helium
Neon
Argon
Krypton
Xenon
Radon
-269
-246
-186
-152
-108
-62
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Dispersion Forces and Physical Properties
•
The effect of dispersion forces at the bulk level translates into important
physical behaviors such as the trends in the melting or boiling point of pure nonmetal elements and molecular substances, as well as, their behaviour in
mixtures. For example: Hydrocarbons are particularly good examples of the
effects of dispersion forces on boiling and melting points as a function of
molecular size and carbon chain.
300
200
100
0
-100
-200
m.p. (C)
Methane CH4
Butane C4H10
Heptane C7H16
Dacane C10H22
Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
Pentane C5H12
Hexane C6H14
Heptane C7H16
Octane C8H18
Nonane C9H20
Dacane C10H22
Tetradecane C13H28
b.p. (C)
Ethane C2H6
Pentane C5H12
Octane C8H18
Tetradecane C13H28
Propane C3H8
Hexane C6H14
Nonane C9H20
-182
-182
-190
-138
-125
-95
-91
-57
-53
-30
3
-161
-88
-44
-0.5
36
68
98
126
151
174
254
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Dispersion Forces and Physical Properties
•
The effect of dispersion forces is strongly dependent of the number of
electrons present in the molecule. This is shown below for molecules
with the same number of carbons and hydrogens -an therefore the
number of electrons. As is evident, the dispersion effect on melting and
boiling points is barley affected by the shape of the molecule
100
50
0
-50
-100
-150
m.p. (C)
2,2 Dimethylpentane
2,2 Dimethylpentane
2,3 Dimethylpentane
2,4 Dimethylpentane
2 Methylhexane
2,3 Dimethylpentane
b.p. (C)
2,4 Dimethylpentane
2 Methylhexane
-124
-124
-119
-118
79
90
80
90
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
dipole-dipole interactions
•
•
•
Dipole-dipole interaction forces work on similar principles
as dispersion force interactions.
They are stronger due to the permanent net-dipole nature
of the molecules due to the presence of an element in the
molecule with a high electronegativity (e.g. F, Cl, O, etc.)
The effect of the net-dipole sometimes enhances the
attraction among molecules, however, the combined effect
of all dispersion forces in a molecule can sometimes be
more significant.
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
dipole-dipole interactions
•
Case study: monocarbon chloride compounds
CH4
CH3Cl
CH2Cl2
CHCl3
CCl4
m.p.°C
-182
-91
-75
-63
-23
b.p.°C
-161
-26
40
61
77
100
50
0
-50
-100
-150
-200
CH4
CH3Cl
CH2Cl2
CHCl3
m.p
m.p
b.p
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
-182
-161
Bond properties
-91
-26
CCl4
b.p
-75
40
-63
61
-23
77
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
Ion - Dipole and Ion-Induced dipole
•
•
Ions are permanently charged particles that will interact
strongly with polar substances
The solubilization of many ionic compounds in polar
solvents like water is a classic example of ion-dipole
interactions
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
hydrogen bonding
•
•
•
Hydrogen bonding or H-bonding is a unique type of intermolecular
interaction somewhere between dipole-dipole interactions and
coordination bonding.
The term H-bonding was coined by Huggins in 1912 and widely
publicized by Pauling’s “Nature of the Chemical Bond” and published in
1930. The idea however was first discussed by Nerst in 1891 working
with hydroxides
H-bonding is unique to a limited number of compounds containing
Oxygen, Nitrogen or Fluorine Hydrogen bonds
‚
‚
•
O-H, N-H, or F- H.
The interaction is established between a pair of “active” non-bonding
valence electrons in the strong electronegative element (F, O, or N) and
the Hydrogen bonded to a similar strong electronegative element:
‚
e.g: N: ----- H - N
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
http://www.hbond.de/
Reactions Types Mechanisms
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Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
hydrogen bonding
•
•
Evidence of H-bonding is easily demonstrated by comparing the boiling
points of the “hydrides” of the group 4, 5, 6, and 7 elements
The hydrides of the group 4 elements follow the expected increase of
boiling point with increasing molecular size due to dispersion force
effects
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
hydrogen bonding
•
When the “hydrides” of the group 5, 6, and 7 elements are compared,
however, the hydrides of Fluorine, Oxygen and Nitrogen depart
dramatically from the expected pattern by showing very elevated boiling
points
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
hydrogen bonding
•
The strong electronegativity of these three elements (F, O, and N)
coupled with the presence of “active” unoccupied (lone) electron pairs in
their valence shell, lead to a very strong dipole that makes the Hydrogen
atom strongly positive.
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
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Intermolecular Interactions
Bonding
Bond types
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Intermolecular interactions
hydrogen bonding
•
•
This polarization is
further enhanced as the
hydride interacts with
other similar hydrides;
effectively allowing the
electrons from two strong
electronegative atoms to
nearly coordinate with
one Hydrogen atom
In Nitrogen and Oxygen
containing hydrides the
effect is the formation of
extensive intermolecular
bonding networks
See article Ultrafast h-bond
dynamics (AAAS)
Reactions Types Mechanisms
Intermolecular Interactions
Bonding
Bond types
Physical properties
http://www.chemguide.co.uk/atoms/bonding/hbond.html#top
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Application Exercises
Let us consider the following pairs of compound and the respective melting and
boiling point data
a.
b.
c.
d.
•
NO2 and NO
H2O and H2S
O2 and O3
CH4 and SiH4
Which difference in
meting point and
boiling points are
best explained by:
‚
‚
‚
‚
dispersion force effects
only?
dipole interactions only?
H-bonding only?
a combination of
effects?
Other Physical Properties
Physical Properties
arising from intermolecular interactions
In some of the previous discussions we learned how dispersion
forces determine the melting and boiling point of many
compounds.
• The presence of a strong dipole, H-bonds and ions will enhance
or decrease the effects of dispersion forces
• H-bonds contribute greatly to the “adhesivness” of intermolecular
interactions, particularly in mixtures containing compounds
capable of forming H-bonds between each other
•
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
arising from intermolecular interactions
•
Other physical properties affected by intermolecular bonds are
‚
solubility
‚
surface tension and flash point
viscosity and fluidity
density
elasticity and plasticity
torsional and tensor strength, etc.
‚
‚
‚
‚
•
The shape and geometry of a compound plays also a very
important role in
‚
‚
‚
•
melting/fusion point
viscosity and fluidity
density
The size of the particles play a very important role in
‚
‚
solubility
viscosity and fluidity
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
Solubility
•
•
•
•
•
Solubility is a physical property that arises from the
intermolecular interactions between at least two different
substances.
Solutions are considered homogenous mixtures made of a
solute that dissolves in a solvent.
Solutes can be gases, liquids, or solids.
Solvents can also be gases, liquids or solids.
Most solutions encountered are made of a solid or liquid solute
dissolved in water which functions as a solvent.
Reactions Types Mechanisms
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Physical properties
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Dr. Pedro M. Pereyra ©) 2006
Physical Properties
Solubility
Substances that are not soluble or partially soluble will form two
separate “phases”; e.g. air and water
• The space where the two phases meet is known as the
“interphase”. This is a realm with unique physical properties
important in chemical reactions and intermolecular interactions.
• “Miscible” is an other term used to denote the fact that a
substance mixes with an other to form a solution;
•
‚
‚
its is normally used in the context of liquids or gases mixing with a liquid solvent
phase. e.g. ethanol is miscible in water.
or to indicate only partial solubility
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Dr. Pedro M. Pereyra ©) 2006
Physical Properties
expressing the extent of solubility of substances
•
•
•
The solubility of solids is measured most often in terms
of the number of grams of solute that dissolve at a given
temperature (usually 25EC) in a fix volume of solution
(usually 100mL).
The solubility of gases is measured in terms of the mL
of gas dissolved in a liter of solution.
The most commonly referred solubility is with respect to
water, however, many compounds are not soluble in
water and their solubility is assessed with respect to
other solvents.
‚
Reactions Types Mechanisms
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Bond types
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Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general description of solubility in terms of
vapourization, mixing and ionization energies
•
•
Solubilization of a molecular solute in a molecular solvent
can be considered to be equivalent as the dispersion of the
solute’s molecules among the solvent’s molecules.
To disperse the solute’s molecules one would need to apply
energy to break the intermolecular forces holding the solute
in its original solid or liquid state.
‚
•
Energy of fusion and/or vapourization
For the molecules of solute to inter-disperse in the solvent
requires for at least some of the solvent’s molecules to
break their intermolecular interactions. It is easiest to
consider the complete vapourization of the solvent.
‚
Energy of fusion and/or vapourization
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Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general description of solubility in terms of
vapourization, mixing and ionization energies
•
•
If all the molecules are now in a “gaseous state” they can
mix
Once mixed a release of energy would bring the mixture
back to a liquid or a solid state.
‚
•
As well as, it would establish new intermolecular bonding
between solute and solvent molecules. This interactions for
a soluble solute would normally release energy
‚
•
Energy of condensation or fusion (-ÄE)
Energy of solvation (-ÄE)
The net process can result in a net amount of energy being
released (exothermic process) or some energy being still
needed to form the mixture (endothermic process.
‚
Net Energy of mixing
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general description of solubility in terms of
vapourization, mixing and ionization energies
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general description of solubility in terms of
vapourization, mixing and ionization energies
•
•
Solubilization of an ionic solute in a molecular solvent can
be considered to be equivalent as the dispersion of the
solute’s cathions and anions among the solvent’s
molecules.
To disperse the solute’s ions one would need to apply
energy to break the intermolecular forces holding the ions
together in its original solid state.
‚
•
Energy of ionic dissociation
For the molecules of solute to inter-disperse in the solvent
requires for at least some of the solvent’s molecules to
break their intermolecular interactions. It is easiest to
consider the complete vapourization of the solvent.
‚
Energy of fusion and/or vapourization
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general description of solubility in terms of
vapourization, mixing and ionization energies
•
•
If all the solvent molecules are now in a “gaseous state” and
all the ions are separated they can mix
Once mixed a release of energy would bring the mixture
back to a liquid or a solid state.
‚
•
As well as, it would establish ion- dipole or ion - induce
dipole interaction between the ions and solvent molecules.
This interactions for a soluble ionic compound would
normally release energy
‚
•
Energy of condensation or fusion (-ÄE) for the solvent
Energy of ion solvation (-ÄE)
The net process can result in a net amount of energy being
released (exothermic process) or some energy being still
needed to form the mixture (endothermic process).
‚ Net
Reactions
Types
Mechanisms
Bond types
Energy
of mixing Bonding
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general description of solubility in terms of
vapourization, mixing and ionization energies
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
effect of temperature on solubility
•
•
Different compounds and substances show different
solubility behaviours in water.
Some of these behaviours can be generalized
Reactions Types Mechanisms
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Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
effect of temperature on solubility
•
The solubility of sucrose and NaCl both increase with increasing
temperatures.
•
The increase of solubility of sucrose is greater than for NaCl.
• Analyze the properties of sucrose and NaCl and build a general
hypothesis for the solubility of solid compounds in water.
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
effect of temperature on solubility
•
The solubility of Ammonia, Oxygen and Carbon Dioxide
decreases with temperature, roughly to the same extent.
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
solubility of solids in water
solubility of gases in water
•
Hypothesis #1:
‚ The solubility of all solids in
water increases with the
temperature of the solution
• Hypothesis #2:
‚ The solubility of ionic solids
is only slightly affected by
temperature
• Hypothesis #3:
‚ The solubility of polar
molecular compounds is
greatly affected by
temperature
• Hypothesis # 4:
‚ The solubility of both nonpolar and polar gases in
water decreases with
temperature to a similar %
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
testing the hypotheses using published solubility data
•
•
Use the table of
solubilities provided in
the back of your table
of elements and
Collect the
temperature solubility
data for as many of
the above compounds
using the CRC
Handbook of
Chemistry & Physics
(library)
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
testing the hypotheses using published solubility data
•
•
Plot the data to
produce a graph
similar to the one on
the right and test the 4
hypotheses.
Collect data for 25EC
and group them
according to periodic
trends and cathion
and anion group
trends
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general qualitative behaviour of different substances in
water solutions
•
•
•
Most gases will have a reduced solubility in water with
increasing temperatures.
When a substance is considered insoluble in water it means
that their solubility is very very low , sometimes below
detection levels.
‚ All substances are miscible with each other to some
extent.
‚ Only when that extent is significant for our purposes we
say that the substance is soluble
In general substances are miscible if their intermolecular
forces are similar in nature.
‚ “Similar dissolves similar”.
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
general qualitative behaviour of different substances in
water solutions
•
Nearly all solids will have their solubility in water increased by increasing the
temperature of the solution.
• The solubility of polar molecular compounds, nitrates, and bases are the most
significantly affected by the temperature of the solution.
• All nitrates are soluble
• Most polyvalent cathion oxide/hydroxides are slightly soluble or insoluble.
• The solubility of salts is less affected by temperature, but is affected by
• the type of metal cathion-anion combination.
• Most monovalent or polyvalent cathion salts bonded to halogens are
soluble.
• Most polyvalent cathion salts bonded to polyatomic anions are insoluble
• Most monovalent cathion salts bonded to polyatomic anions are soluble
• the relative size of the alkaline metal cathion
• smaller cathions are more soluble than larger ones (Li>Na>K).
• the relative size of the halogen anion
• smaller anions are less soluble than larger ones (F<Cl<Br<I).
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
saturated solutions
•
As we learned in previous slides, a solid will display an increase
solubility as the temperature of the solution increases.
• At any given temperature only so much solute can be dissolved
in the water. We say that the solution is saturated for that given
temperature.
‚ It is important to check the saturation concentration of a given solute at a given temperature before preparing solutions.
• This phenomenon will, however, reach a limit after which no
further increase in temperature will dissolve any more of the
solid.
• If a saturated solution is allowed to cool to a lower temperature,
usually room temperature (~25EC), the excess solute will
precipitate out of solution forming a super-saturated solution.
‚
this behaviour is used to purify substances by crystallization
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006
Physical Properties
solutions applications
•
•
Isolating a substance by crystallization of
saturated solutions
Isolating and purifying molecular compounds
based on their different solubilities and adhesion
properties in a liquid, gaseous or solid phase
system
‚ chromatography
‚ electrophoresis
Reactions Types Mechanisms
Bonding
Bond types
Intermolecular Interactions
Physical properties
Bond properties
Types of Compounds
Dr. Pedro M. Pereyra ©) 2006