2.4 Intermolecular Forces

Intermolecular forces shape the world around us. These invisible attractions between molecules determine how substances behave, from boiling points to solubility. Understanding these forces helps explain why water boils at 100°C, while ethanol boils at just 78°C.

Dispersion, dipole-dipole, and hydrogen bonding are the main types of intermolecular forces. Their strength affects properties like melting point, surface tension, and miscibility. This knowledge is crucial for predicting how substances interact and behave in various applications.

Types and Characteristics of Intermolecular Forces

Types of intermolecular forces

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• Dispersion forces (London forces) arise from temporary fluctuations in electron distribution present in all molecules increase with molecular size and surface area (alkanes)

• Dipole-dipole interactions occur between polar molecules attract partial positive and partial negative charges stronger than dispersion forces (acetone)

• Hydrogen bonding special case of dipole-dipole interaction forms between H atom bonded to N, O, or F and another N, O, or F atom strongest intermolecular force (water, DNA base pairs)

Strengths of intermolecular forces

• Strength order hydrogen bonding > dipole-dipole > dispersion forces

• Factors affecting strength include molecular size and shape electronegativity differences number of hydrogen bond donors and acceptors

• Impact on molecular behavior leads to higher boiling and melting points influences viscosity and surface tension affects solubility and miscibility of substances

Effects and Applications of Intermolecular Forces

Effects on substance properties

• Boiling point requires more energy to overcome stronger forces results in higher boiling points for substances with stronger intermolecular forces (water vs ethanol)

• Melting point influenced by strength of forces between molecules in solid state higher melting points for substances with stronger intermolecular forces (ice vs liquid nitrogen)

• Solubility follows "like dissolves like" principle polar substances dissolve in polar solvents non-polar substances dissolve in non-polar solvents (sugar in water, oil in gasoline)

• Surface tension increases with stronger intermolecular forces water's high surface tension due to hydrogen bonding allows water striders to walk on water

Applications of intermolecular forces

• Predicting relative boiling points by comparing molecular masses and types of intermolecular forces present (methane vs ethane)

• Explaining miscibility predicts whether two liquids will mix based on polarity and intermolecular forces (water and ethanol mix, water and oil don't)

• Understanding protein folding hydrogen bonding maintains protein structure (alpha helices, beta sheets)

• Explaining capillary action compares adhesive forces between liquid and surface vs cohesive forces within liquid (water rising in plant stems)

• Predicting vapor pressure stronger intermolecular forces lead to lower vapor pressure (water vs diethyl ether)

• Explaining unique properties of water high boiling point surface tension and capillary action due to hydrogen bonding enables life on Earth