intermolecular forces - university of nairobi personal
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Intermolecular Forces
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Particles in a solid – animation
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Particles in a liquid – animation
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Particles in a gas – animation
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• Covalent compounds may be gases, liquids orsolids.
• Compounds that have low molecular weight andno dipole moments, such as methane (CH4) andcarbon dioxide (CO2), are gases. The forces thatact between such molecules are very weak.
• The forces that act between molecules are calledintermolecular forces.
• These interactions increases significantly as themolecular weight and the size of the moleculeincreases.
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• They also increase with increasing polarity of themolecule.
• The greater the attractive forces betweenmolecules, the higher is the boiling point of thecompound
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van der Waals forceDipole–dipole interaction
Hydrogen bonds
Three types of intermolecular forces are important fororganic compounds :
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Dipole–Dipole InteractionMolecules with dipole moments (polar molecules)tend to orient themselves in the liquid and solidphases so that the negative end of the molecule isfacing the positive end of another molecule.
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Dipole – Dipole Interaction
As a result of such dipole-dipole interactions CH3I isliquid at room temperature while CH4 is gas at roomtemperature.
Polar Molecule
Intramolecular forces of attraction
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Nonpolar molecules have lower boiling point thanpolar molecules.
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van der Waals force
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Gas (b.p. -162°C)
Liquid ( b.p. 69°C)
Solid ( m.p. 36°C)
Intermolecular forces act to attract even non-polarmolecules.
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van der Waals Forces
The boiling point of a compound increases with the increase in van der Waals force.
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As the number of carbon and hydrogen atomsincrease, the additive effects of these weakintermolecular becomes significant, as evidenced bythe increase in boiling and melting points frommethane to hexane to icosane.
Boiling Points of Alkanes
Straight chain
Branched chain
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b.p. 36.1°C
b.p. 28°C
b.p. 9.5°C
van der Waals forces depend on the surface of thecompounds interacting.
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Hydrogen Bonding
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Examples
An especially strong type of dipole-dipoleinteraction occurs between moleculescontaining a hydrogen atom bonded to fluorine,oxygen or nitrogen. Each of these latterelements are the most electronegative and hasunshared valence electrons.
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In the liquid state, the molecules of any of thesecompounds have strong attractions for one another.
Intermolecular forces of attraction
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Intramolecular forces of attraction
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H-bonding is substantially stronger than most dipole-dipole attractions. The reason for this is due to thesize of the atoms involved. A hydrogen atom is smallcompared to other atoms and can occupy a positionclose to the unshared electrons of the electronegativeatom.Atoms larger than hydrogen can’t occupy such aposition; consequently dipole-dipole interactionsbetween other atoms are weaker.
Hydrogen bonds are not all the same strength. An O ---- HO hydrogen bond is stronger than an N ---- HNhydrogen bond.
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This is because oxygen is more electronegative thannitrogen; therefore, the O-H group is more polar andhas a more positive H. This more positive H is morestrongly attracted by a negative centre.
Boiling point 69°C -6.3°C
StrongH-bonding Weaker
H-bonding
Boiling point increases with molecular weight becauseof increased van der Waals attractions. As a resultethanol (CH3CH2OH) has a higher boiling point thanmethanol (CH3OH).
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H-bonding may form between two differentcompounds; such as between CH3OH and H2O orbetween CH3NH2 and H2O.
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A compound that can form H-bonding with watertends to be more soluble in water than acompound that can’t.
N
H
H
H3C
O
H
H
H-bondingH-bonding
Intermolecular forces of attraction
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Like Dissolves Like
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Nonpolar compound dissolves in nonpolar solvent
Polar compound dissolves in polar solvent
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Nonpolar
Polar
Oil (nonpolar) and water (polar) are immiscible.