OZ 1

• Electronegativity


  The further up and right in the periodic table excluding Group 8, the more electronegative an element is
• Molecules with an even distribution of charge are non-polar, while molecules with uneven charge distribution are polar
• The Pauling scale quantifies how electronegative an element is, with fluorine having the highest value of 4
• In a polar bond, electrons are pulled towards the more electronegative atom, creating a partial positive and partial negative charge denoted by Delta symbols
• Polar bond

  Bonds become polar when atoms attached to it have different electronegativities
• Carbon dioxide, despite having polar bonds, is a non-polar molecule due to its symmetrical structure and even charge distribution
• A polar bond is more polar the greater the difference in electronegativity between the atoms
• Fluorine is the most electronegative element in the periodic table
• Atoms with the same electronegativity have non-polar bonds
• Instantaneous dipoles, also known as Van der Waals forces, exist between atoms and molecules as weak forces
• Water is a polar molecule due to its uneven charge distribution
• The ability for an atom to attract electrons towards itself in a covalent bond is called electronegativity
• A difference of zero in electronegativity values indicates a purely covalent bond
• Formation of an instantaneous dipole - induced dipole


  Occurs as electrons in a molecule move to one end when near another molecule, creating a temporary dipole
• Instantaneous dipole - induced dipole force

  • Iodine molecules forming a crystal structure held together by these weak forces
• Permanent dipole - permanent dipole

  Stronger than instantaneous dipole - induced dipole forces
• Boiling a liquid breaks the weak instantaneous dipole - induced dipole forces between molecules, not the covalent bonds within the molecules
• Intermolecular forces and bonds

  1. Hydrogen bonding

  2. Permanent dipole - permanent dipole

  3. Instantaneous dipole - induced dipole
• Branching in molecules weakens the instantaneous dipole - induced dipole forces, lowering the boiling points compared to straight-chain hydrocarbons
• Interaction of neighboring molecules

  Electrons move away due to repulsion, creating a delta negative and a delta positive which are attracted to each other while the molecules are nearby
• Molecules with branching have lower boiling points compared to molecules with no branching
• Instantaneous dipole - induced dipole

  The weakest type of force where any molecule or atom that has electrons can form a dipole when near another atom or molecule
• More energy is needed to break larger molecules with more instantaneous dipole - induced dipole forces, leading to higher boiling points
• Instantaneous dipole - induced dipole forces are weaker than covalent bonds and exist between molecules, not within atoms
• Molecules with no branching have a lower boiling point


  • Due to the absence of branching, molecules have a lower boiling point
• Molecules with permanent dipole - permanent dipole interactions also have instantaneous dipole-induced dipole interactions
• Water molecules exhibit hydrogen bonding, permanent dipole - permanent dipole interactions, and instantaneous dipole-induced dipole interactions
• Hydrogen bonding

  • Strongest type of intermolecular force, occurs with very electronegative elements like nitrogen, oxygen, or fluorine
• Classic example of permanent dipole - permanent dipole interactions

  • Water
• Testing polar molecules

  Using a charged rod to observe water bending towards it as a test for polarity
• Permanent dipole - permanent dipole interactions

  • Stronger than instantaneous dipole-induced dipole interactions, exist in molecules with permanent polarity
• Ice expands when freezing due to the ordered structures formed by hydrogen bonds, making it less dense and causing it to float on water
• Delta negative part on one molecule is attracted to the Delta positive on another


  Interaction between molecules with permanent dipole - permanent dipole interactions
• Formation of hydrogen bonds

  Lone pair on nitrogen, oxygen, or fluorine in one molecule forms a bond with hydrogen in another molecule
• Formation of ice

  Hydrogen bonds between water molecules push them further apart, making ice less dense and causing it to float on water
• Water, with two hydrogen bonds per molecule, has stronger intermolecular forces leading to a smaller temperature change during evaporation compared to hexane and ethanol
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• Substances with weaker intermolecular bonds evaporate more readily and draw more heat from the surroundings, resulting in a larger drop in temperature
• Boiling points of hydrogen halides

  • HF has the highest boiling point due to hydrogen bonding, followed by HCl, HBr, and HI with increasing mass leading to higher boiling points
• Strength of intermolecular bonds investigation

  Measure temperature change as different solvents evaporate to determine the strength of their intermolecular bonds