Intermolecular Forces

Intermolecular Forces

Overview of Intermolecular Forces

  • Intermolecular forces (IMFs) are the attractions between neighboring molecules.

    • Example of strong bonds: Covalent bond

    • Example of weak interactions: Intermolecular attraction

Breaking of Intermolecular Forces

  • Intermolecular forces are broken when molecules boil.

  • The stronger the intermolecular forces, the higher the boiling point (BP).

Types of Intermolecular Forces

  • Three broad types of intermolecular forces are identified:

    1. London forces

    2. Dipole-dipole forces

    3. Hydrogen bonds

  • The order of increasing strength of these forces is as follows:

    • London forces < Dipole-dipole forces < Hydrogen bonds

Hydrogen Bonding

  • Hydrogen bonding is characterized by strong dipole attractions between hydrogen (H) and highly electronegative elements: fluorine (F), oxygen (O), or nitrogen (N).

    • The statement "Hydrogen is FON" helps in remembering the elements involved in hydrogen bonds.

Identification of Hydrogen Bonding

  • Example assignment: Identify which molecule would exhibit hydrogen bonding from given options (H2O, HF, etc.).

  • Hydrogen bonding involves the partial positive and negative charges denoted by $ $.

Effects of Hydrogen Bonding in Water

  • Hydrogen bonding in water causes:

    1. Capillary action: Observed in a glass tube with water forming a concave meniscus due to adhesion and cohesion.

    2. High surface tension: Water exhibits significant surface tension due to hydrogen bonding.

    3. High boiling point: The boiling point of water is around 100extC100^{ ext{C}}, significantly higher due to hydrogen bonding.

    4. Low vapor pressure: Water exhibits comparatively low vapor pressure, as hydrogen bonds stabilize liquid water.

    5. Flotation of ice: Ice is less dense than liquid water due to stable hydrogen bonds, which is why ice floats.

Bond Types in Water

  • Questions regarding bond types:

    1. When water is boiled, covalent bonds are broken.

    2. When electrolyzing water to form hydrogen and oxygen, covalent bonds are also broken.

Dipole-Dipole Forces

  • Defined as attractions between polar molecules characterized by partial positive ($ $) and negative ($ $) charges.

    • Example assignment: Identify dipole-dipole intermolecular forces in given molecules (e.g., HCl, etc.).

London Dispersion Forces (LDFs)

  • Definition: LDFs arise due to the constant movement of electrons forming instantaneous dipoles that can induce dipoles in neighboring molecules.

  • They are present in all molecules and are the only IMF in nonpolar molecules.

  • The strength of LDFs increases with molecular size and surface area.

Influencing Factors of London Dispersion Forces

  • The more electrons a molecule possesses, the more polarizable its electron cloud, leading to stronger dispersion forces.

  • Boiling point increases with stronger LDFs, as seen in examples comparing C8H18 with C4H10.

Crude Oil Separation by Boiling Point

  • Crude oil is a mixture of hydrocarbons, which can be separated by boiling points:

    • Example boiling points:

    • Butane & Propane: 20extC20^{ ext{C}} to 150extC150^{ ext{C}}

    • Petrol: 200extC200^{ ext{C}}

    • Kerosene: 300extC300^{ ext{C}}

    • Diesel: 370extC370^{ ext{C}}

    • Fuel Oil: 400extC400^{ ext{C}}

    • Higher boiling points correlate with larger, more complex molecules.

Effects of Molecular Shape on LDFs

  • Shape influences LDFs:

    • Example with neoprentane (2,2-Dimethylpropane) vs. n-pentane, where the shape causes differing boiling points despite equal molecular weight.

Intermolecular Force Types in Covalent Molecules

  • For determining types of IMFs in covalent molecules:

    • Assess if the molecule is polar and whether hydrogen is bonded to N, O, or F.

Types of IMFs and Their Strengths

  • Hydrogen Bonds (strongest)

  • Dipole-Dipole Forces

  • London Dispersion Forces (weakest)

Solubility Concepts

  • "Like dissolves like" principle:

    • Polar substances dissolve in polar (hydrophilic) solvents.

    • Nonpolar substances dissolve in nonpolar (hydrophobic) solvents.

  • Example of immiscibility: Oil (nonpolar) and water (polar).

Practical Examples of Solubility

  • Oil does not mix with water due to the differing polarities.

  • Conclusion: Polar (hydrophilic) substances are soluble in water, while nonpolar (hydrophobic) substances are not.