Intermolecular Forces: relationships between molecules

Polarity and Shape Polar bonds do not necessarily always create a polar molecule. The shape of the molecule and the polarity of each bond is considered before stating if a molecule is considered POLAR or NON-POLAR We will watch a 5 minute explanation to help us understand this difficult concept major intermolecular forces

Intramolecular forces intra = inside Intramolecular forces: forces that bond the atoms to each other within the molecule. COVALENT BONDS!!!

Intermolecular Forces inter = between Forces affecting the relationships between molecules London Forces (Dispersion Forces) AKA van der Waals forces Dipole-Dipole Interactions Ion-dipole forces Hydrogen Bonding

London (dispersion) Forces aka van Der Waals forces weakest intermolecular force occurs between all types of molecules It is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles

London Dispersion forces in a molecule These instantaneous dipoles may be induced and stabilized as an ion or a polar molecule approaches the non-polar molecule.

Dipole-Dipole Forces Dipole = polar molecule Molecules with dipoles will change their direction so that their oppositely charged ends are near to one another. The electrostatic attraction between the ends is dipole-dipole force

Orientation of Polar Molecules in a Solid Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11.2

Dipole - Dipole Interactions

Ion Dipole Forces (the reason why ionic compounds dissolve in water) The force of attraction between an ion and a polar molecule. NaCl breaks up because the ion dipole with water is stronger than the attraction of Na+ to Cl-

Polar molecules can interact with ions: Ion - Dipole Interactions

The Hydrogen Bond

A hydrogen bond is the strongest form of dipole-dipole interaction. A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to the small, highly electronegative atoms, F, O, or N. F—H O—H N—H

A molecule containing this combination of atoms, will be attracted to another molecule with the same atom combination. A strong attractive force between the two molecules which is called a hydrogen bond is formed. hydrogen bond covalent bond covalent bond

water has the lowest molar mass water has the highest heat of fusion water has the highest melting point water has the highest boiling point water has the highest heat of vaporization The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group 17.

Hydrogen bonding in water animation hydrogen bonding animation Water exhibits these unusual properties because of hydrogen bonding between water molecules.

Why do we care about intermolecular forces? The forces that act on molecules determine their physical properties Strong intermolecular forces between molecules increase melting and boiling points because it is these forces that are broken when substances change state.

Polarity and Boiling Point: The polarity of the molecules determines the forces of attraction between the molecules in the liquid state. Polar molecules are attracted by the opposite charge effect (the positive end of one molecule is attracted to the negative end of another molecule). Molecules have different degrees of polarity as determined by the functional group present. The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point.

What is boiling point? Boiling point is the temperature at which a liquid turns into a gas (or vapour). For a liquid to turn into a gas, intermolecular forces must be broken. The stronger the intermolecular forces, the higher the boiling point of a substance.

The evidence for hydrogen bonding Many elements form compounds with hydrogen - referred to as "hydrides". If you plot the boiling points of the hydrides of the Group 4 elements, you find that the boiling points increase as you go down the group. The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater.

The evidence for hydrogen bonding If you repeat this exercise with the hydrides of elements in Groups 5, 6 and 7, something odd happens.

Solubility and Intermolecular Forces Like dissolves like Polar solutes dissolve in polar solvents Nonpolar solutes dissolve in nonpolar solvents Molecules with similar intermolecular forces will mix freely

Ionic Solute with Polar Solvent

Ionic Solute with Nonpolar Solvent

Nonpolar Solute with Nonpolar Solvent

Nonpolar Solute with Polar Solvent

Comparison of the Properties of Substances with Ionic, Covalent, Metallic or Intermolecular Bonds Bond strength Strong Very strong Variable strength, generallymoderate Weak Hardness Moderate to high Insulators in solids and liquid states Low to moderate; ductile, malleable Crystal soft and somewhat plastic Electrical conductivity Conducts by ion transport, but only when liquid or dissociated Low Good conductors; conducts by electron transport Insulators in both solid and liquid states Melting point Very high Generally high Solubility Soluble in polar solvents Very low solubility Insoluble except in acids or alkalis by chemical reaction Soluble in organic solvents Examples Most minerals Diamond, oxygen, hydrogen, organic molecules Cu, Ag, Au, other metals Organic compounds

Boiling Point of Various Material (˚C) Noble gas Helium neon argon He Ne Ar -269 -246 -186 Nonpolar covalent hydrogen oxygen methane chlorine H2 O2 CH4 Cl2 -253 -183 -164 -34 polar covalent ammonia hydrogen fluoride water NH3 HF H2O -33 19.5 100 ionic potassium chloride sodium chloride magnesium oxide KCl NaCl MgO 771 1413 2826 metallic copper iron tungsten Cu Fe W 2567 2750 5660