Intermolecular Forces
Molecular Compounds The simplest molecule is H 2 : Increased electron density draws nuclei together The pair of shared electrons constitutes a covalent bond.
Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kj to vaporize 1 mole of water (inter) 930 kj to break all O-H bonds in 1 mole of water (intra) Generally, intermolecular forces are much weaker than intramolecular forces. Measure of intermolecular force boiling point melting point ΔH vap ΔH fus ΔH sub 12.2
A Molecular Comparison of Gases, Liquids and Solids Converting a gas into a liquid or solid requires the molecules to get closer to each other: cool or compress. Converting a solid into a liquid or gas requires the molecules to move further apart: heat or reduce pressure. The forces holding solids and liquids together are called intermolecular forces.
Intermolecular Forces The covalent bond holding a molecule together is an intramolecular force. The attraction between molecules is an intermolecular force. Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kj/mol vs. 431 kj/mol for HCl). When a substance melts or boils the intermolecular forces are broken (not the covalent bonds). When a substance condenses intermolecular forces are formed.
Intermolecular Forces in Solutions
Intermolecular Forces
Intermolecular Forces
Intermolecular Forces Dipole-Dipole Forces There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble. If two molecules have about the same mass and size, then dipoledipole forces increase with increasing polarity.
Intermolecular Forces London Dispersion Forces Weakest of all intermolecular forces. It is possible for two adjacent neutral molecules to affect each other. The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). For an instant, the electron clouds become distorted. In that instant a dipole is formed (called an instantaneous dipole).
Intermolecular Forces London Dispersion Forces
Intermolecular Forces London Dispersion Forces One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom). The forces between instantaneous dipoles are called London dispersion forces. Polarizability is the ease with which an electron cloud can be deformed. The larger the molecule (the greater the number of electrons) the more polarizable.
Intermolecular Forces Ion-Induced Dipole: An ion induces a dipole moment in an adjacent molecule or atom. Interaction between an ion (e.g. Na + ) and a dipole (e.g. water).
Intermolecular Forces Polarizability & Periodic Table Polarizability increases down a group of atoms or ions because size increases & larger electron clouds are more easily distorted Polarizability decreases from left to right across a period because the effective nuclear charge holds the electrons more tightly Cations are less polarizable than parent atom because they are smaller, whereas anions are more polarizable because they are larger
Intermolecular Forces London Dispersion Forces London dispersion forces increase as molecular weight increases. London dispersion forces exist between all molecules. London dispersion forces depend on the shape of the molecule. The greater the surface area available for contact, the greater the dispersion forces. London dispersion forces between spherical molecules are lower than between sausage-like molecules.
Polar Forces
Electronegativity Polarity refers to a separation of positive and negative charge. In a nonpolar bond, the bonding electrons are shared equally: H 2, Cl 2 : In a polar bond, electrons are shared unequally because of the difference in Z eff. HCl:
Chlorine - Cl2
Adding Dipole Moments
We know a COVALENT bond comes from sharing the bonding pair of electrons
F F Shared pair (bonding pair)
F F The nucleus of each atom pulls on the bonding pair.
F F Both atoms have equal pull, so the bonding pair is shared equally.
H Cl If two different atoms share a bond, one will pull more strongly on the bonding electrons.
H Cl
H Cl
H Cl
H Cl
H Cl
H Cl
H Cl
H Cl
H Cl The bonding electrons carry negative charge.
H Cl The closer they get to the chlorine atom, the more negative it gets. The farther they get from the hydrogen, the more positive it gets.
_ + H Cl But the charge is only partial. Hydrogen has not lost the electrons as in the formation of an ion.
H Cl There is an unequal sharing of electrons.
δ H Cl δ + The partial charge is denoted by a + or and the Greek letter delta, δ
δ + δ The partial charge is denoted by a + or and the Greek letter delta, δ
δ + δ
A polar bond is a bond in which the bonding electron pair is shared unequally. A polar molecule is a molecule with regions of partial negative (δ ) and partial positive (δ +) charge.
The degree of sharing (equal to unequal) is determined by the electronegativity difference between the two atoms.
F F Two atoms of equal electronegativity will share the bond equally
2.1 H Cl 3.0 Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.
δ + H Cl δ Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.
δ + H Cl δ This is a polar bond: The bonding pair is, on average, closer to one atom.
δ + H Cl δ Is a polar bond a covalent bond?
0.8 K F 4.0 Two atoms with a large difference in electronegativity will result in a loss of an electron, resulting in a full charge.
K F
K F
K F
K F
K F
K F
K + F _ positive ion negative ion
A polar bond in a molecule may make the entire molecule polar, with one end slightly positive and the other end slightly negative.
δ +H Cl δ Hydrogen chloride is an example of a polar molecule.
The SHAPE of the molecule determines whether its polar bonds make the molecule polar
Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid
In CF 4 the fluorines are symmetrically arranged around the carbon.
The fluorines all pull on the valence electrons in opposite directions, effectively cancelling out the polarity of the bonds
CF 4 has four polar bonds but it is a non-polar molecule: There is no partial charge on the molecule.
The two hydrogens of water are not symmetrically positioned around the oxygen.
δ + δ δ + The O-H polarities do not cancel, and the molecule carries a partial charge.
δ + δ δ + δ Water is a liquid instead of a gas because the partial positives and negatives attract each other.
δ + δ δ + δ This attraction holds the molecules together, forming a liquid rather than the spread out molecules of a gas.
Intermolecular Forces Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction
Dispersion Forces Intermolecular Forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction
separated Cl 2 molecules instantaneous dipoles Dispersion forces among nonpolar molecules
Dispersion Forces Continued Intermolecular Forces Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass.
Hydrogen Bond Intermolecular Forces The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H B or A H A A & B are N, O, or F
Intermolecular Forces Hydrogen Bonding Special case of dipole-dipole forces. By experiments: boiling points of compounds with H- F, H-O, and H-N bonds are abnormally high. Intermolecular forces are abnormally strong. H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). Electrons in the H-X (X = electronegative element) lie much closer to X than H. H has only one electron, so in the H-X bond, the δ+ H presents an almost bare proton to the δ- X. Therefore, H-bonds are strong.
Intermolecular Forces Hydrogen Bonding Special case of dipole-dipole forces. By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high. Intermolecular forces are abnormally strong. H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). Electrons in the H-X (X = electronegative element) lie much closer to X than H. H has only one electron, so in the H-X bond, the δ+ H presents an almost bare proton to the δ- X. Therefore, H-bonds are strong.
H-Bonding Occurs when Hydrogen is attached to a highly electronegative atom. N-H N- O-H N- F-H N- N-H O- O-H O- F-H O- N-H F- O-H F- F-H F- δ + δ - Requires Unshared Electron Pairs of Highly Electronegative Elements
Structure of Ice Observe the orientation of the Hydrogen Bonds HB-ice
Why is the hydrogen bond considered a special dipole-dipole interaction? Decreasing molar mass Decreasing boiling point
What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH 4 CH 4 is nonpolar: dispersion forces. S SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules. O O
SAMPLE PROBLEM PROBLEM: Drawing Hydrogen Bonds Between Molecules of a Substance Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bonds between them. O (a) C 2 H 6 (b) CH 3 OH (c) CH 3 C NH 2 PLAN: Find molecules in which H is bonded to N, O or F. Draw H bonds in the format -B: H-A-. SOLUTION: (a) C 2 H 6 has no H bonding sites. (b) H (c) H H C O H O H N CH 3 C H H CH 3 C N H O H O C H H H CH 3 C O N H H H H O N CH 3 C
Hydrogen Bonds A hydrogen bond is an intermolecular force in which a hydrogen atom covalently bonded to a nonmetal atom in one molecule is simultaneously attracted to a nonmetal atom of a neighboring molecule The strongest hydrogen bonds are formed if the nonmetal atoms are small and highly electronegative e.g., N, O, F
Hydrogen Bonding Effects Solid water is less dense than liquid water due to hydrogen bonding Hydrogen bonding is also the reason for the unusually high boiling point of water The structures of proteins, substances essential to life, are determined partly by hydrogen bonding Proteins
Hydrogen Bonding Effects Many organic acids can form dimers due to hydrogen bonding Certain organic molecules can also form an intramolecular hydrogen bond
Interaction of Water and Oil What do you know about oil and water? They don t mix Why? Because water is polar and oil is nonpolar Water molecules exert their attractive forces on other water molecules Oil remains insoluble and floats on the surface of the water as it is less dense
Boiling Points of Liquids and Melting Points of Solids Energy is used to overcome the intermolecular attractive forces in a substance, driving the molecules into a less associated phase The greater the intermolecular force, the more energy is required leading to Higher melting point of a solid Higher boiling point of a liquid
Factors Influencing Boiling and Melting Points Strength of the attractive force holding the substance in its current physical state Molecular mass Larger molecules have higher m.p. and b.p. than smaller molecules as it is more difficult to convert a larger mass to another phase Polarity Polar molecules have higher m.p. and b.p. than nonpolar molecules of similar molecular mass due to their stronger attractive force
Melting and Boiling Points Selected Compounds by Bonding Type